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Added new type traits files.

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[SVN r9238]
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John Maddock 2001-02-17 12:25:45 +00:00
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/*
*
* Copyright (c) 1999
* Dr John Maddock
*
* Permission to use, copy, modify, distribute and sell this software
* and its documentation for any purpose is hereby granted without fee,
* provided that the above copyright notice appear in all copies and
* that both that copyright notice and this permission notice appear
* in supporting documentation. Dr John Maddock makes no representations
* about the suitability of this software for any purpose.
* It is provided "as is" without express or implied warranty.
*
* This file provides some example of type_traits usage -
* by "optimising" various algorithms:
*
* opt::copy - optimised for trivial copy (cf std::copy)
* opt::fill - optimised for trivial copy/small types (cf std::fill)
* opt::destroy_array - an example of optimisation based upon omitted destructor calls
* opt::iter_swap - uses type_traits to determine whether the iterator is a proxy
* in which case it uses a "safe" approach, otherwise calls swap
* on the assumption that swap may be specialised for the pointed-to type.
*
*/
/* Release notes:
23rd July 2000:
Added explicit failure for broken compilers that don't support these examples.
Fixed broken gcc support (broken using directive).
Reordered tests slightly.
*/
#include <iostream>
#include <typeinfo>
#include <algorithm>
#include <iterator>
#include <vector>
#include <memory>
#include <boost/timer.hpp>
#include <boost/type_traits.hpp>
#include <boost/call_traits.hpp>
using std::cout;
using std::endl;
using std::cin;
#ifdef BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION
#error "Sorry, without template partial specialisation support there isn't anything to test here..."
#endif
namespace opt{
//
// algorithm destroy_array:
// The reverse of std::unitialized_copy, takes a block of
// unitialized memory and calls destructors on all objects therein.
//
namespace detail{
template <bool>
struct array_destroyer
{
template <class T>
static void destroy_array(T* i, T* j){ do_destroy_array(i, j); }
};
template <>
struct array_destroyer<true>
{
template <class T>
static void destroy_array(T*, T*){}
};
template <class T>
void do_destroy_array(T* first, T* last)
{
while(first != last)
{
first->~T();
++first;
}
}
}; // namespace detail
template <class T>
inline void destroy_array(T* p1, T* p2)
{
detail::array_destroyer<boost::has_trivial_destructor<T>::value>::destroy_array(p1, p2);
}
//
// unoptimised versions of destroy_array:
//
template <class T>
void destroy_array1(T* first, T* last)
{
while(first != last)
{
first->~T();
++first;
}
}
template <class T>
void destroy_array2(T* first, T* last)
{
for(; first != last; ++first) first->~T();
}
//
// opt::copy
// same semantics as std::copy
// calls memcpy where appropiate.
//
namespace detail{
template <bool b>
struct copier
{
template<typename I1, typename I2>
static I2 do_copy(I1 first, I1 last, I2 out);
};
template <bool b>
template<typename I1, typename I2>
I2 copier<b>::do_copy(I1 first, I1 last, I2 out)
{
while(first != last)
{
*out = *first;
++out;
++first;
}
return out;
}
template <>
struct copier<true>
{
template<typename I1, typename I2>
static I2* do_copy(I1* first, I1* last, I2* out)
{
memcpy(out, first, (last-first)*sizeof(I2));
return out+(last-first);
}
};
}
template<typename I1, typename I2>
inline I2 copy(I1 first, I1 last, I2 out)
{
typedef typename boost::remove_cv<typename std::iterator_traits<I1>::value_type>::type v1_t;
typedef typename boost::remove_cv<typename std::iterator_traits<I2>::value_type>::type v2_t;
enum{ can_opt = boost::is_same<v1_t, v2_t>::value
&& boost::is_pointer<I1>::value
&& boost::is_pointer<I2>::value
&& boost::has_trivial_assign<v1_t>::value };
return detail::copier<can_opt>::do_copy(first, last, out);
}
//
// fill
// same as std::fill, uses memset where appropriate, along with call_traits
// to "optimise" parameter passing.
//
namespace detail{
template <bool opt>
struct filler
{
template <typename I, typename T>
static void do_fill(I first, I last, typename boost::call_traits<T>::param_type val);
};
template <bool b>
template <typename I, typename T>
void filler<b>::do_fill(I first, I last, typename boost::call_traits<T>::param_type val)
{
while(first != last)
{
*first = val;
++first;
}
}
template <>
struct filler<true>
{
template <typename I, typename T>
static void do_fill(I first, I last, T val)
{
std::memset(first, val, last-first);
}
};
}
template <class I, class T>
inline void fill(I first, I last, const T& val)
{
enum{ can_opt = boost::is_pointer<I>::value
&& boost::is_arithmetic<T>::value
&& (sizeof(T) == 1) };
typedef detail::filler<can_opt> filler_t;
filler_t::template do_fill<I,T>(first, last, val);
}
//
// iter_swap:
// tests whether iterator is a proxying iterator or not, and
// uses optimal form accordingly:
//
namespace detail{
template <bool b>
struct swapper
{
template <typename I>
static void do_swap(I one, I two)
{
typedef typename std::iterator_traits<I>::value_type v_t;
v_t v = *one;
*one = *two;
*two = v;
}
};
#ifdef __GNUC__
using std::swap;
#endif
template <>
struct swapper<true>
{
template <typename I>
static void do_swap(I one, I two)
{
using std::swap;
swap(*one, *two);
}
};
}
template <typename I1, typename I2>
inline void iter_swap(I1 one, I2 two)
{
typedef typename std::iterator_traits<I1>::reference r1_t;
typedef typename std::iterator_traits<I2>::reference r2_t;
enum{ can_opt = boost::is_reference<r1_t>::value && boost::is_reference<r2_t>::value && boost::is_same<r1_t, r2_t>::value };
detail::swapper<can_opt>::do_swap(one, two);
}
}; // namespace opt
//
// define some global data:
//
const int array_size = 1000;
int i_array[array_size] = {0,};
const int ci_array[array_size] = {0,};
char c_array[array_size] = {0,};
const char cc_array[array_size] = { 0,};
const int iter_count = 1000000;
int main()
{
//
// test destroy_array,
// compare destruction time of an array of ints
// with unoptimised form.
//
cout << "Measuring times in micro-seconds per 1000 elements processed" << endl << endl;
cout << "testing destroy_array...\n"
"[Some compilers may be able to optimise the \"unoptimised\"\n versions as well as type_traits does.]" << endl;
/*cache load*/ opt::destroy_array(i_array, i_array + array_size);
boost::timer t;
double result;
int i;
for(i = 0; i < iter_count; ++i)
{
opt::destroy_array(i_array, i_array + array_size);
}
result = t.elapsed();
cout << "destroy_array<int>: " << result << endl;
/*cache load*/ opt::destroy_array1(i_array, i_array + array_size);
t.restart();
for(i = 0; i < iter_count; ++i)
{
opt::destroy_array1(i_array, i_array + array_size);
}
result = t.elapsed();
cout << "destroy_array<int>(unoptimised#1): " << result << endl;
/*cache load*/ opt::destroy_array2(i_array, i_array + array_size);
t.restart();
for(i = 0; i < iter_count; ++i)
{
opt::destroy_array2(i_array, i_array + array_size);
}
result = t.elapsed();
cout << "destroy_array<int>(unoptimised#2): " << result << endl << endl;
cout << "testing fill(char)...\n"
"[Some standard library versions may already perform this optimisation.]" << endl;
/*cache load*/ opt::fill<char*, char>(c_array, c_array + array_size, (char)3);
t.restart();
for(i = 0; i < iter_count; ++i)
{
opt::fill<char*, char>(c_array, c_array + array_size, (char)3);
}
result = t.elapsed();
cout << "opt::fill<char*, char>: " << result << endl;
/*cache load*/ std::fill(c_array, c_array + array_size, (char)3);
t.restart();
for(i = 0; i < iter_count; ++i)
{
std::fill(c_array, c_array + array_size, (char)3);
}
result = t.elapsed();
cout << "std::fill<char*, char>: " << result << endl << endl;
cout << "testing fill(int)...\n"
"[Tests the effect of call_traits pass-by-value optimisation -\nthe value of this optimisation may depend upon hardware characteristics.]" << endl;
/*cache load*/ opt::fill<int*, int>(i_array, i_array + array_size, 3);
t.restart();
for(i = 0; i < iter_count; ++i)
{
opt::fill<int*, int>(i_array, i_array + array_size, 3);
}
result = t.elapsed();
cout << "opt::fill<int*, int>: " << result << endl;
/*cache load*/ std::fill(i_array, i_array + array_size, 3);
t.restart();
for(i = 0; i < iter_count; ++i)
{
std::fill(i_array, i_array + array_size, 3);
}
result = t.elapsed();
cout << "std::fill<int*, int>: " << result << endl << endl;
cout << "testing copy...\n"
"[Some standard library versions may already perform this optimisation.]" << endl;
/*cache load*/ opt::copy<const int*, int*>(ci_array, ci_array + array_size, i_array);
t.restart();
for(i = 0; i < iter_count; ++i)
{
opt::copy<const int*, int*>(ci_array, ci_array + array_size, i_array);
}
result = t.elapsed();
cout << "opt::copy<const int*, int*>: " << result << endl;
/*cache load*/ std::copy<const int*, int*>(ci_array, ci_array + array_size, i_array);
t.restart();
for(i = 0; i < iter_count; ++i)
{
std::copy<const int*, int*>(ci_array, ci_array + array_size, i_array);
}
result = t.elapsed();
cout << "std::copy<const int*, int*>: " << result << endl;
/*cache load*/ opt::detail::copier<false>::template do_copy<const int*, int*>(ci_array, ci_array + array_size, i_array);
t.restart();
for(i = 0; i < iter_count; ++i)
{
opt::detail::copier<false>::template do_copy<const int*, int*>(ci_array, ci_array + array_size, i_array);
}
result = t.elapsed();
cout << "standard \"unoptimised\" copy: " << result << endl << endl;
/*cache load*/ opt::copy<const char*, char*>(cc_array, cc_array + array_size, c_array);
t.restart();
for(i = 0; i < iter_count; ++i)
{
opt::copy<const char*, char*>(cc_array, cc_array + array_size, c_array);
}
result = t.elapsed();
cout << "opt::copy<const char*, char*>: " << result << endl;
/*cache load*/ std::copy<const char*, char*>(cc_array, cc_array + array_size, c_array);
t.restart();
for(i = 0; i < iter_count; ++i)
{
std::copy<const char*, char*>(cc_array, cc_array + array_size, c_array);
}
result = t.elapsed();
cout << "std::copy<const char*, char*>: " << result << endl;
/*cache load*/ opt::detail::copier<false>::template do_copy<const char*, char*>(cc_array, cc_array + array_size, c_array);
t.restart();
for(i = 0; i < iter_count; ++i)
{
opt::detail::copier<false>::template do_copy<const char*, char*>(cc_array, cc_array + array_size, c_array);
}
result = t.elapsed();
cout << "standard \"unoptimised\" copy: " << result << endl << endl;
//
// testing iter_swap
// really just a check that it does in fact compile...
std::vector<int> v1;
v1.push_back(0);
v1.push_back(1);
std::vector<bool> v2;
v2.push_back(0);
v2.push_back(1);
opt::iter_swap(v1.begin(), v1.begin()+1);
opt::iter_swap(v2.begin(), v2.begin()+1);
cout << "Press any key to exit...";
cin.get();
}

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<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<meta name="GENERATOR" content="Microsoft FrontPage 4.0">
<meta name="ProgId" content="FrontPage.Editor.Document">
<title>C++ Type traits</title>
</head>
<body bgcolor="#FFFFFF" link="#0000FF" vlink="#800080">
<h2 align="center">C++ Type traits</h2>
<p align="center"><em>by John Maddock and Steve Cleary</em></p>
<p align="center"><em>This is a draft of an article that will appear in a future
issue of </em><a href="http://www.ddj.com"><em>Dr Dobb's Journal</em></a></p>
<p>Generic programming (writing code which works with any data type meeting a
set of requirements) has become the method of choice for providing reusable
code. However, there are times in generic programming when &quot;generic&quot;
just isn't good enough - sometimes the differences between types are too large
for an efficient generic implementation. This is when the traits technique
becomes important - by encapsulating those properties that need to be considered
on a type by type basis inside a traits class, we can minimise the amount of
code that has to differ from one type to another, and maximise the amount of
generic code.</p>
<p>Consider an example: when working with character strings, one common
operation is to determine the length of a null terminated string. Clearly it's
possible to write generic code that can do this, but it turns out that there are
much more efficient methods available: for example, the C library functions <font size="2" face="Courier New">strlen</font>
and <font size="2" face="Courier New">wcslen</font> are usually written in
assembler, and with suitable hardware support can be considerably faster than a
generic version written in C++. The authors of the C++ standard library realised
this, and abstracted the properties of <font size="2" face="Courier New">char</font>
and <font size="2" face="Courier New">wchar_t</font> into the class <font size="2" face="Courier New">char_traits</font>.
Generic code that works with character strings can simply use <font size="2" face="Courier New">char_traits&lt;&gt;::length</font>
to determine the length of a null terminated string, safe in the knowledge that
specialisations of <font size="2" face="Courier New">char_traits</font> will use
the most appropriate method available to them.</p>
<h4>Type traits</h4>
<p>Class <font size="2" face="Courier New">char_traits</font> is a classic
example of a collection of type specific properties wrapped up in a single class
- what Nathan Myers termed a <i>baggage class</i>[1]. In the Boost type-traits
library, we[2] have written a set of very specific traits classes, each of which
encapsulate a single trait from the C++ type system; for example, is a type a
pointer or a reference type? Or does a type have a trivial constructor, or a
const-qualifier? The type-traits classes share a unified design: each class has
a single member <i>value</i>, a compile-time constant that is true if the type
has the specified property, and false otherwise. As we will show, these classes
can be used in generic programming to determine the properties of a given type
and introduce optimisations that are appropriate for that case.</p>
<p>The type-traits library also contains a set of classes that perform a
specific transformation on a type; for example, they can remove a top-level
const or volatile qualifier from a type. Each class that performs a
transformation defines a single typedef-member <i>type</i> that is the result of
the transformation. All of the type-traits classes are defined inside namespace <font size="2" face="Courier New">boost</font>;
for brevity, namespace-qualification is omitted in most of the code samples
given.</p>
<h4>Implementation</h4>
<p>There are far too many separate classes contained in the type-traits library
to give a full implementation here - see the source code in the Boost library
for the full details - however, most of the implementation is fairly repetitive
anyway, so here we will just give you a flavour for how some of the classes are
implemented. Beginning with possibly the simplest class in the library, is_void&lt;T&gt;
has a member <i>value</i> that is true only if T is void.</p>
<pre>template &lt;typename T&gt;
struct is_void
{ static const bool value = false; };
template &lt;&gt;
struct is_void&lt;void&gt;
{ static const bool value = true; };</pre>
<p>Here we define a primary version of the template class <font size="2" face="Courier New">is_void</font>,
and provide a full-specialisation when T is void. While full specialisation of a
template class is an important technique, sometimes we need a solution that is
halfway between a fully generic solution, and a full specialisation. This is
exactly the situation for which the standards committee defined partial
template-class specialisation. As an example, consider the class
boost::is_pointer&lt;T&gt;: here we needed a primary version that handles all
the cases where T is not a pointer, and a partial specialisation to handle all
the cases where T is a pointer:</p>
<pre>template &lt;typename T&gt;
struct is_pointer
{ static const bool value = false; };
template &lt;typename T&gt;
struct is_pointer&lt;T*&gt;
{ static const bool value = true; };</pre>
<p>The syntax for partial specialisation is somewhat arcane and could easily
occupy an article in its own right; like full specialisation, in order to write
a partial specialisation for a class, you must first declare the primary
template. The partial specialisation contains an extra &lt;…&gt; after the
class name that contains the partial specialisation parameters; these define the
types that will bind to that partial specialisation rather than the default
template. The rules for what can appear in a partial specialisation are somewhat
convoluted, but as a rule of thumb if you can legally write two function
overloads of the form:</p>
<pre>void foo(T);
void foo(U);</pre>
<p>Then you can also write a partial specialisation of the form:</p>
<pre>template &lt;typename T&gt;
class c{ /*details*/ };
template &lt;typename T&gt;
class c&lt;U&gt;{ /*details*/ };</pre>
<p>This rule is by no means foolproof, but it is reasonably simple to remember
and close enough to the actual rule to be useful for everyday use.</p>
<p>As a more complex example of partial specialisation consider the class
remove_bounds&lt;T&gt;. This class defines a single typedef-member <i>type</i>
that is the same type as T but with any top-level array bounds removed; this is
an example of a traits class that performs a transformation on a type:</p>
<pre>template &lt;typename T&gt;
struct remove_bounds
{ typedef T type; };
template &lt;typename T, std::size_t N&gt;
struct remove_bounds&lt;T[N]&gt;
{ typedef T type; };</pre>
<p>The aim of remove_bounds is this: imagine a generic algorithm that is passed
an array type as a template parameter, <font size="2" face="Courier New">remove_bounds</font>
provides a means of determining the underlying type of the array. For example <code>remove_bounds&lt;int[4][5]&gt;::type</code>
would evaluate to the type <code>int[5]</code>. This example also shows that the
number of template parameters in a partial specialisation does not have to match
the number in the default template. However, the number of parameters that
appear after the class name do have to match the number and type of the
parameters in the default template.</p>
<h4>Optimised copy</h4>
<p>As an example of how the type traits classes can be used, consider the
standard library algorithm copy:</p>
<pre>template&lt;typename Iter1, typename Iter2&gt;
Iter2 copy(Iter1 first, Iter1 last, Iter2 out);</pre>
<p>Obviously, there's no problem writing a generic version of copy that works
for all iterator types Iter1 and Iter2; however, there are some circumstances
when the copy operation can best be performed by a call to <font size="2" face="Courier New">memcpy</font>.
In order to implement copy in terms of <font size="2" face="Courier New">memcpy</font>
all of the following conditions need to be met:</p>
<ul>
<li>Both of the iterator types Iter1 and Iter2 must be pointers.</li>
<li>Both Iter1 and Iter2 must point to the same type - excluding <font size="2" face="Courier New">const</font>
and <font size="2" face="Courier New">volatile</font>-qualifiers.</li>
<li>The type pointed to by Iter1 must have a trivial assignment operator.</li>
</ul>
<p>By trivial assignment operator we mean that the type is either a scalar
type[3] or:</p>
<ul>
<li>The type has no user defined assignment operator.</li>
<li>The type does not have any data members that are references.</li>
<li>All base classes, and all data member objects must have trivial assignment
operators.</li>
</ul>
<p>If all these conditions are met then a type can be copied using <font size="2" face="Courier New">memcpy</font>
rather than using a compiler generated assignment operator. The type-traits
library provides a class <i>has_trivial_assign</i>, such that <code>has_trivial_assign&lt;T&gt;::value</code>
is true only if T has a trivial assignment operator. This class &quot;just
works&quot; for scalar types, but has to be explicitly specialised for
class/struct types that also happen to have a trivial assignment operator. In
other words if <i>has_trivial_assign</i> gives the wrong answer, it will give
the &quot;safe&quot; wrong answer - that trivial assignment is not allowable.</p>
<p>The code for an optimised version of copy that uses <font size="2" face="Courier New">memcpy</font>
where appropriate is given in listing 1. The code begins by defining a template
class <i>copier</i>, that takes a single Boolean template parameter, and has a
static template member function <font size="2" face="Courier New">do_copy</font>
which performs the generic version of <font size="2">copy</font> (in other words
the &quot;slow but safe version&quot;). Following that there is a specialisation
for <i>copier&lt;true&gt;</i>: again this defines a static template member
function <font size="2" face="Courier New">do_copy</font>, but this version uses
memcpy to perform an &quot;optimised&quot; copy.</p>
<p>In order to complete the implementation, what we need now is a version of
copy, that calls <code>copier&lt;true&gt;::do_copy</code> if it is safe to use <font size="2" face="Courier New">memcpy</font>,
and otherwise calls <code>copier&lt;false&gt;::do_copy</code> to do a
&quot;generic&quot; copy. This is what the version in listing 1 does. To
understand how the code works look at the code for <font size="2" face="Courier New">copy</font>
and consider first the two typedefs <i>v1_t</i> and <i>v2_t</i>. These use <code>std::iterator_traits&lt;Iter1&gt;::value_type</code>
to determine what type the two iterators point to, and then feed the result into
another type-traits class <i>remove_cv</i> that removes the top-level
const-volatile-qualifiers: this will allow copy to compare the two types without
regard to const- or volatile-qualifiers. Next, <font size="2" face="Courier New">copy</font>
declares an enumerated value <i>can_opt</i> that will become the template
parameter to copier - declaring this here as a constant is really just a
convenience - the value could be passed directly to class <font size="2" face="Courier New">copier</font>.
The value of <i>can_opt</i> is computed by verifying that all of the following
are true:</p>
<ul>
<li>first that the two iterators point to the same type by using a type-traits
class <i>is_same</i>.</li>
<li>Then that both iterators are real pointers - using the class <i>is_pointer</i>
described above.</li>
<li>Finally that the pointed-to types have a trivial assignment operator using
<i>has_trivial_assign</i>.</li>
</ul>
<p>Finally we can use the value of <i>can_opt</i> as the template argument to
copier - this version of copy will now adapt to whatever parameters are passed
to it, if its possible to use <font size="2" face="Courier New">memcpy</font>,
then it will do so, otherwise it will use a generic copy.</p>
<h4>Was it worth it?</h4>
<p>It has often been repeated in these columns that &quot;premature optimisation
is the root of all evil&quot; [4]. So the question must be asked: was our
optimisation premature? To put this in perspective the timings for our version
of copy compared a conventional generic copy[5] are shown in table 1.</p>
<p>Clearly the optimisation makes a difference in this case; but, to be fair,
the timings are loaded to exclude cache miss effects - without this accurate
comparison between algorithms becomes difficult. However, perhaps we can add a
couple of caveats to the premature optimisation rule:</p>
<ul>
<li>If you use the right algorithm for the job in the first place then
optimisation will not be required; in some cases, <font size="2" face="Courier New">memcpy</font>
is the right algorithm.</li>
<li>If a component is going to be reused in many places by many people then
optimisations may well be worthwhile where they would not be so for a single
case - in other words, the likelihood that the optimisation will be
absolutely necessary somewhere, sometime is that much higher. Just as
importantly the perceived value of the stock implementation will be higher:
there is no point standardising an algorithm if users reject it on the
grounds that there are better, more heavily optimised versions available.</li>
</ul>
<h4>Table 1: Time taken to copy 1000 elements using copy&lt;const T*, T*&gt;
(times in micro-seconds)</h4>
<table border="1" cellpadding="7" cellspacing="1" width="529">
<tr>
<td valign="top" width="33%">
<p align="center">Version</p>
</td>
<td valign="top" width="33%">
<p align="center">T</p>
</td>
<td valign="top" width="33%">
<p align="center">Time</p>
</td>
</tr>
<tr>
<td valign="top" width="33%">&quot;Optimised&quot; copy</td>
<td valign="top" width="33%">char</td>
<td valign="top" width="33%">0.99</td>
</tr>
<tr>
<td valign="top" width="33%">Conventional copy</td>
<td valign="top" width="33%">char</td>
<td valign="top" width="33%">8.07</td>
</tr>
<tr>
<td valign="top" width="33%">&quot;Optimised&quot; copy</td>
<td valign="top" width="33%">int</td>
<td valign="top" width="33%">2.52</td>
</tr>
<tr>
<td valign="top" width="33%">Conventional copy</td>
<td valign="top" width="33%">int</td>
<td valign="top" width="33%">8.02</td>
</tr>
</table>
<p>&nbsp;</p>
<h4>Pair of References</h4>
<p>The optimised copy example shows how type traits may be used to perform
optimisation decisions at compile-time. Another important usage of type traits
is to allow code to compile that otherwise would not do so unless excessive
partial specialization is used. This is possible by delegating partial
specialization to the type traits classes. Our example for this form of usage is
a pair that can hold references [6].</p>
<p>First, let us examine the definition of &quot;std::pair&quot;, omitting the
comparision operators, default constructor, and template copy constructor for
simplicity:</p>
<pre>template &lt;typename T1, typename T2&gt;
struct pair
{
typedef T1 first_type;
typedef T2 second_type;
T1 first;
T2 second;
pair(const T1 &amp; nfirst, const T2 &amp; nsecond)
:first(nfirst), second(nsecond) { }
};</pre>
<p>Now, this &quot;pair&quot; cannot hold references as it currently stands,
because the constructor would require taking a reference to a reference, which
is currently illegal [7]. Let us consider what the constructor's parameters
would have to be in order to allow &quot;pair&quot; to hold non-reference types,
references, and constant references:</p>
<table border="1" cellpadding="7" cellspacing="1" width="638">
<tr>
<td valign="top" width="50%">Type of &quot;T1&quot;</td>
<td valign="top" width="50%">Type of parameter to initializing constructor</td>
</tr>
<tr>
<td valign="top" width="50%">
<pre>T</pre>
</td>
<td valign="top" width="50%">
<pre>const T &amp;</pre>
</td>
</tr>
<tr>
<td valign="top" width="50%">
<pre>T &amp;</pre>
</td>
<td valign="top" width="50%">
<pre>T &amp;</pre>
</td>
</tr>
<tr>
<td valign="top" width="50%">
<pre>const T &amp;</pre>
</td>
<td valign="top" width="50%">
<pre>const T &amp;</pre>
</td>
</tr>
</table>
<p>A little familiarity with the type traits classes allows us to construct a
single mapping that allows us to determine the type of parameter from the type
of the contained class. The type traits classes provide a transformation &quot;add_reference&quot;,
which adds a reference to its type, unless it is already a reference.</p>
<table border="1" cellpadding="7" cellspacing="1" width="580">
<tr>
<td valign="top" width="21%">Type of &quot;T1&quot;</td>
<td valign="top" width="27%">Type of &quot;const T1&quot;</td>
<td valign="top" width="53%">Type of &quot;add_reference&lt;const
T1&gt;::type&quot;</td>
</tr>
<tr>
<td valign="top" width="21%">
<pre>T</pre>
</td>
<td valign="top" width="27%">
<pre>const T</pre>
</td>
<td valign="top" width="53%">
<pre>const T &amp;</pre>
</td>
</tr>
<tr>
<td valign="top" width="21%">
<pre>T &amp;</pre>
</td>
<td valign="top" width="27%">
<pre>T &amp; [8]</pre>
</td>
<td valign="top" width="53%">
<pre>T &amp;</pre>
</td>
</tr>
<tr>
<td valign="top" width="21%">
<pre>const T &amp;</pre>
</td>
<td valign="top" width="27%">
<pre>const T &amp;</pre>
</td>
<td valign="top" width="53%">
<pre>const T &amp;</pre>
</td>
</tr>
</table>
<p>This allows us to build a primary template definition for &quot;pair&quot;
that can contain non-reference types, reference types, and constant reference
types:</p>
<pre>template &lt;typename T1, typename T2&gt;
struct pair
{
typedef T1 first_type;
typedef T2 second_type;
T1 first;
T2 second;
pair(boost::add_reference&lt;const T1&gt;::type nfirst,
boost::add_reference&lt;const T2&gt;::type nsecond)
:first(nfirst), second(nsecond) { }
};</pre>
<p>Add back in the standard comparision operators, default constructor, and
template copy constructor (which are all the same), and you have a std::pair
that can hold reference types!</p>
<p>This same extension <i>could</i> have been done using partial template
specialization of &quot;pair&quot;, but to specialize &quot;pair&quot; in this
way would require three partial specializations, plus the primary template. Type
traits allows us to define a single primary template that adjusts itself
auto-magically to any of these partial specializations, instead of a brute-force
partial specialization approach. Using type traits in this fashion allows
programmers to delegate partial specialization to the type traits classes,
resulting in code that is easier to maintain and easier to understand.</p>
<h4>Conclusion</h4>
<p>We hope that in this article we have been able to give you some idea of what
type-traits are all about. A more complete listing of the available classes are
in the boost documentation, along with further examples using type traits.
Templates have enabled C++ uses to take the advantage of the code reuse that
generic programming brings; hopefully this article has shown that generic
programming does not have to sink to the lowest common denominator, and that
templates can be optimal as well as generic.</p>
<h4>Acknowledgements</h4>
<p>The authors would like to thank Beman Dawes and Howard Hinnant for their
helpful comments when preparing this article.</p>
<h4>References</h4>
<ol>
<li>Nathan C. Myers, C++ Report, June 1995.</li>
<li>The type traits library is based upon contributions by Steve Cleary, Beman
Dawes, Howard Hinnant and John Maddock: it can be found at www.boost.org.</li>
<li>A scalar type is an arithmetic type (i.e. a built-in integer or floating
point type), an enumeration type, a pointer, a pointer to member, or a
const- or volatile-qualified version of one of these types.</li>
<li>This quote is from Donald Knuth, ACM Computing Surveys, December 1974, pg
268.</li>
<li>The test code is available as part of the boost utility library (see
algo_opt_examples.cpp), the code was compiled with gcc 2.95 with all
optimisations turned on, tests were conducted on a 400MHz Pentium II machine
running Microsoft Windows 98.</li>
<li>John Maddock and Howard Hinnant have submitted a &quot;compressed_pair&quot;
library to Boost, which uses a technique similar to the one described here
to hold references. Their pair also uses type traits to determine if any of
the types are empty, and will derive instead of contain to conserve space --
hence the name &quot;compressed&quot;.</li>
<li>This is actually an issue with the C++ Core Language Working Group (issue
#106), submitted by Bjarne Stroustrup. The tentative resolution is to allow
a &quot;reference to a reference to T&quot; to mean the same thing as a
&quot;reference to T&quot;, but only in template instantiation, in a method
similar to multiple cv-qualifiers.</li>
<li>For those of you who are wondering why this shouldn't be const-qualified,
remember that references are always implicitly constant (for example, you
can't re-assign a reference). Remember also that &quot;const T &amp;&quot;
is something completely different. For this reason, cv-qualifiers on
template type arguments that are references are ignored.</li>
</ol>
<h2>Listing 1</h2>
<pre>namespace detail{
template &lt;bool b&gt;
struct copier
{
template&lt;typename I1, typename I2&gt;
static I2 do_copy(I1 first,
I1 last, I2 out);
};
template &lt;bool b&gt;
template&lt;typename I1, typename I2&gt;
I2 copier&lt;b&gt;::do_copy(I1 first,
I1 last,
I2 out)
{
while(first != last)
{
*out = *first;
++out;
++first;
}
return out;
}
template &lt;&gt;
struct copier&lt;true&gt;
{
template&lt;typename I1, typename I2&gt;
static I2* do_copy(I1* first, I1* last, I2* out)
{
memcpy(out, first, (last-first)*sizeof(I2));
return out+(last-first);
}
};
}
template&lt;typename I1, typename I2&gt;
inline I2 copy(I1 first, I1 last, I2 out)
{
typedef typename
boost::remove_cv&lt;
typename std::iterator_traits&lt;I1&gt;
::value_type&gt;::type v1_t;
typedef typename
boost::remove_cv&lt;
typename std::iterator_traits&lt;I2&gt;
::value_type&gt;::type v2_t;
enum{ can_opt =
boost::is_same&lt;v1_t, v2_t&gt;::value
&amp;&amp; boost::is_pointer&lt;I1&gt;::value
&amp;&amp; boost::is_pointer&lt;I2&gt;::value
&amp;&amp; boost::
has_trivial_assign&lt;v1_t&gt;::value
};
return detail::copier&lt;can_opt&gt;::
do_copy(first, last, out);
}</pre>
<hr>
<p>© Copyright John Maddock and Steve Cleary, 2000</p>
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<h1><img src="../../c++boost.gif" width="276" height="86">Header
&lt;<a href="../../boost/detail/type_traits.hpp">boost/type_traits.hpp</a>&gt;</h1>
<p>The contents of &lt;boost/type_traits.hpp&gt; are declared in
namespace boost.</p>
<p>The file &lt;<a href="../../boost/detail/type_traits.hpp">boost/type_traits.hpp</a>&gt;
contains various template classes that describe the fundamental
properties of a type; each class represents a single type
property or a single type transformation. This documentation is
divided up into the following sections:</p>
<pre><a href="#fop">Fundamental type operations</a>
<a href="#fp">Fundamental type properties</a>
<a href="#misc">Miscellaneous</a>
<code> </code><a href="#cv">cv-Qualifiers</a>
<code> </code><a href="#ft">Fundamental Types</a>
<code> </code><a href="#ct">Compound Types</a>
<code> </code><a href="#ot">Object/Scalar Types</a>
<a href="#cs">Compiler Support Information</a>
<a href="#ec">Example Code</a></pre>
<h2><a name="fop"></a>Fundamental type operations</h2>
<p>Usage: &quot;class_name&lt;T&gt;::type&quot; performs
indicated transformation on type T.</p>
<table border="1" cellpadding="7" cellspacing="1" width="100%">
<tr>
<td valign="top" width="45%"><p align="center">Expression.</p>
</td>
<td valign="top" width="45%"><p align="center">Description.</p>
</td>
<td valign="top" width="33%"><p align="center">Compiler.</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>remove_volatile&lt;T&gt;::type</code></td>
<td valign="top" width="45%">Creates a type the same as T
but with any top level volatile qualifier removed. For
example &quot;volatile int&quot; would become &quot;int&quot;.</td>
<td valign="top" width="33%"><p align="center">P</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>remove_const&lt;T&gt;::type</code></td>
<td valign="top" width="45%">Creates a type the same as T
but with any top level const qualifier removed. For
example &quot;const int&quot; would become &quot;int&quot;.</td>
<td valign="top" width="33%"><p align="center">P</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>remove_cv&lt;T&gt;::type</code></td>
<td valign="top" width="45%">Creates a type the same as T
but with any top level cv-qualifiers removed. For example
&quot;const int&quot; would become &quot;int&quot;, and
&quot;volatile double&quot; would become &quot;double&quot;.</td>
<td valign="top" width="33%"><p align="center">P</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>remove_reference&lt;T&gt;::type</code></td>
<td valign="top" width="45%">If T is a reference type
then removes the reference, otherwise leaves T unchanged.
For example &quot;int&amp;&quot; becomes &quot;int&quot;
but &quot;int*&quot; remains unchanged.</td>
<td valign="top" width="33%"><p align="center">P</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>add_reference&lt;T&gt;::type</code></td>
<td valign="top" width="45%">If T is a reference type
then leaves T unchanged, otherwise converts T to a
reference type. For example &quot;int&amp;&quot; remains
unchanged, but &quot;double&quot; becomes &quot;double&amp;&quot;.</td>
<td valign="top" width="33%"><p align="center">P</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>remove_bounds&lt;T&gt;::type</code></td>
<td valign="top" width="45%">If T is an array type then
removes the top level array qualifier from T, otherwise
leaves T unchanged. For example &quot;int[2][3]&quot;
becomes &quot;int[3]&quot;.</td>
<td valign="top" width="33%"><p align="center">P</p>
</td>
</tr>
</table>
<p>&nbsp;</p>
<h2><a name="fp"></a>Fundamental type properties</h2>
<p>Usage: &quot;class_name&lt;T&gt;::value&quot; is true if
indicated property is true, false otherwise. (Note that class_name&lt;T&gt;::value
is always defined as a compile time constant).</p>
<h3><a name="misc"></a>Miscellaneous</h3>
<table border="1" cellspacing="1" width="100%">
<tr>
<td width="37%"><p align="center">Expression</p>
</td>
<td width="36%"><p align="center">Description</p>
</td>
<td width="27%"><p align="center">Compiler</p>
</td>
</tr>
<tr>
<td width="37%"><div align="center"><center><pre><code>is_same&lt;T,U&gt;::value</code></pre>
</center></div></td>
<td width="36%"><p align="center">True if T and U are the
same type.</p>
</td>
<td width="27%">&nbsp; </td>
</tr>
<tr>
<td width="37%"><div align="center"><center><pre>is_convertible&lt;T,U&gt;::value</pre>
</center></div></td>
<td width="36%"><p align="center">True if type T is
convertible to type U.</p>
</td>
<td width="27%">&nbsp;</td>
</tr>
<tr>
<td width="37%"><div align="center"><center><pre>alignment_of&lt;T&gt;::value</pre>
</center></div></td>
<td width="36%"><p align="center">An integral value
representing the minimum alignment requirements of type T
(strictly speaking defines a multiple of the type's
alignment requirement; for all compilers tested so far
however it does return the actual alignment).</p>
</td>
<td width="27%">&nbsp;</td>
</tr>
</table>
<p>&nbsp;</p>
<h3><a name="cv"></a>cv-Qualifiers</h3>
<p>The following classes determine what cv-qualifiers are present
on a type (see 3.93).</p>
<table border="1" cellpadding="7" cellspacing="1" width="100%">
<tr>
<td valign="top" width="37%"><p align="center">Expression.</p>
</td>
<td valign="top" width="37%"><p align="center">Description.</p>
</td>
<td valign="top" width="27%"><p align="center">Compiler.</p>
</td>
</tr>
<tr>
<td valign="top" width="37%"><code>is_const&lt;T&gt;::value</code></td>
<td valign="top" width="37%">True if type T is top-level
const qualified.</td>
<td valign="top" width="27%">&nbsp; </td>
</tr>
<tr>
<td valign="top" width="37%"><code>is_volatile&lt;T&gt;::value</code></td>
<td valign="top" width="37%">True if type T is top-level
volatile qualified.</td>
<td valign="top" width="27%">&nbsp; </td>
</tr>
</table>
<p>&nbsp;</p>
<h3><a name="ft"></a>Fundamental Types</h3>
<p>The following will only ever be true for cv-unqualified types;
these are closely based on the section 3.9 of the C++ Standard.</p>
<table border="1" cellpadding="7" cellspacing="1" width="100%">
<tr>
<td valign="top" width="45%"><p align="center">Expression.</p>
</td>
<td valign="top" width="45%"><p align="center">Description.</p>
</td>
<td valign="top" width="33%"><p align="center">Compiler.</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_void&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True only if T is void.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_standard_unsigned_integral&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True only if T is one of the
standard unsigned integral types (3.9.1 p3) - unsigned
char, unsigned short, unsigned int, and unsigned long.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_standard_signed_integral&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True only if T is one of the
standard signed integral types (3.9.1 p2) - signed char,
short, int, and long.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_standard_integral&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T is a standard
integral type(3.9.1 p7) - T is either char, wchar_t, bool
or either is_standard_signed_integral&lt;T&gt;::value or
is_standard_integral&lt;T&gt;::value is true.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_standard_float&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T is one of the
standard floating point types(3.9.1 p8) - float, double
or long double.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_standard_arithmetic&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T is a standard
arithmetic type(3.9.1 p8) - implies is_standard_integral
or is_standard_float is true.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_standard_fundamental&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T is a standard
arithmetic type or if T is void.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_extension_unsigned_integral&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True for compiler specific
unsigned integral types.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_extension_signed_integral&lt;T&gt;&gt;:value</code></td>
<td valign="top" width="45%">True for compiler specific
signed integral types.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_extension_integral&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if either is_extension_unsigned_integral&lt;T&gt;::value
or is_extension_signed_integral&lt;T&gt;::value is true.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_extension_float&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True for compiler specific
floating point types.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_extension_arithmetic&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if either is_extension_integral&lt;T&gt;::value
or is_extension_float&lt;T&gt;::value are true.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
<tr>
<td valign="top" width="45%"><code>&nbsp;is_extension_fundamental&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if either is_extension_arithmetic&lt;T&gt;::value
or is_void&lt;T&gt;::value are true.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
<tr>
<td valign="top" width="45%"><code>&nbsp;is_unsigned_integral&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if either is_standard_unsigned_integral&lt;T&gt;::value
or is_extention_unsigned_integral&lt;T&gt;::value are
true.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_signed_integral&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if either is_standard_signed_integral&lt;T&gt;::value
or is_extention_signed_integral&lt;T&gt;&gt;::value are
true.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_integral&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if either is_standard_integral&lt;T&gt;::value
or is_extention_integral&lt;T&gt;::value are true.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_float&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if either is_standard_float&lt;T&gt;::value
or is_extention_float&lt;T&gt;::value are true.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_arithmetic&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if either is_integral&lt;T&gt;::value
or is_float&lt;T&gt;::value are true.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_fundamental&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if either is_arithmetic&lt;T&gt;::value
or is_void&lt;T&gt;::value are true.</td>
<td valign="top" width="33%">&nbsp;</td>
</tr>
</table>
<p>&nbsp;</p>
<h3><a name="ct"></a>Compound Types</h3>
<p>The following will only ever be true for cv-unqualified types,
as defined by the Standard.&nbsp;</p>
<table border="1" cellpadding="7" cellspacing="1" width="100%">
<tr>
<td valign="top" width="45%"><p align="center">Expression</p>
</td>
<td valign="top" width="45%"><p align="center">Description</p>
</td>
<td valign="top" width="33%"><p align="center">Compiler</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_array&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T is an array type.</td>
<td valign="top" width="33%"><p align="center">P</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_pointer&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T is a regular
pointer type - including function pointers - but
excluding pointers to member functions (3.9.2 p1 and 8.3.1).</td>
<td valign="top" width="33%">&nbsp; </td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_member_pointer&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T is a pointer to a
non-static class member (3.9.2 p1 and 8.3.1).</td>
<td valign="top" width="33%">&nbsp; </td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_reference&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T is a reference
type (3.9.2 p1 and 8.3.2).</td>
<td valign="top" width="33%">&nbsp; </td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_class&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T is a class or
struct type.</td>
<td valign="top" width="33%"><p align="center">PD</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_union&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T is a union type.</td>
<td valign="top" width="33%"><p align="center">C</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_enum&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T is an enumerator
type.</td>
<td valign="top" width="33%"><p align="center">C</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_compound&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T is any of the
above compound types.</td>
<td valign="top" width="33%"><p align="center">PD</p>
</td>
</tr>
</table>
<p>&nbsp;</p>
<h3><a name="ot"></a>Object/Scalar Types</h3>
<p>The following ignore any top level cv-qualifiers: if <code>class_name&lt;T&gt;::value</code>
is true then <code>class_name&lt;cv-qualified-T&gt;::value</code>
will also be true.</p>
<table border="1" cellpadding="7" cellspacing="1" width="100%">
<tr>
<td valign="top" width="45%"><p align="center">Expression</p>
</td>
<td valign="top" width="45%"><p align="center">Description</p>
</td>
<td valign="top" width="33%"><p align="center">Compiler</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_object&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T is not a reference
type, or a (possibly cv-qualified) void type.</td>
<td valign="top" width="33%"><p align="center">P</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_standard_scalar&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T is a standard
arithmetic type, an enumerated type, a pointer or a
member pointer.</td>
<td valign="top" width="33%"><p align="center">PD</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_extension_scalar&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T is an extentions
arithmetic type, an enumerated type, a pointer or a
member pointer.</td>
<td valign="top" width="33%"><p align="center">PD</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_scalar&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T is an arithmetic
type, an enumerated type, a pointer or a member pointer.</td>
<td valign="top" width="33%"><p align="center">PD</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_POD&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T is a &quot;Plain
Old Data&quot; type (see 3.9 p2&amp;p3). Note that
although this requires compiler support to be correct in
all cases, if T is a scalar or an array of scalars then
we can correctly define T as a POD.</td>
<td valign="top" width="33%"><p align="center">PC</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>is_empty&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T is an empty struct
or class. If the compiler implements the &quot;zero sized
empty base classes&quot; optimisation, then is_empty will
correctly guess whether T is empty. Relies upon is_class
to determine whether T is a class type. Screens out enum
types by using is_convertible&lt;T,int&gt;, this means
that empty classes that overload operator int(), will not
be classified as empty.</td>
<td valign="top" width="33%"><p align="center">PCD</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>has_trivial_constructor&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T has a trivial
default constructor - that is T() is equivalent to memset.</td>
<td valign="top" width="33%"><p align="center">PC</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>has_trivial_copy&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T has a trivial copy
constructor - that is T(const T&amp;) is equivalent to
memcpy.</td>
<td valign="top" width="33%"><p align="center">PC</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>has_trivial_assign&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T has a trivial
assignment operator - that is if T::operator=(const T&amp;)
is equivalent to memcpy.</td>
<td valign="top" width="33%"><p align="center">PC</p>
</td>
</tr>
<tr>
<td valign="top" width="45%"><code>has_trivial_destructor&lt;T&gt;::value</code></td>
<td valign="top" width="45%">True if T has a trivial
destructor - that is if T::~T() has no effect.</td>
<td valign="top" width="33%"><p align="center">PC</p>
</td>
</tr>
</table>
<p>&nbsp;</p>
<h2><a name="cs"></a>Compiler Support Information</h2>
<p>The legends used in the tables above have the following
meanings:</p>
<table border="0" cellpadding="7" cellspacing="0" width="480">
<tr>
<td valign="top" width="50%"><p align="center">P</p>
</td>
<td valign="top" width="90%">Denotes that the class
requires support for partial specialisation of class
templates to work correctly.</td>
</tr>
<tr>
<td valign="top" width="50%"><p align="center">C</p>
</td>
<td valign="top" width="90%">Denotes that direct compiler
support for that traits class is required.</td>
</tr>
<tr>
<td valign="top" width="50%"><p align="center">D</p>
</td>
<td valign="top" width="90%">Denotes that the traits
class is dependent upon a class that requires direct
compiler support.</td>
</tr>
</table>
<p>&nbsp;</p>
<p>For those classes that are marked with a D or C, if compiler
support is not provided, this type trait may return &quot;false&quot;
when the correct value is actually &quot;true&quot;. The single
exception to this rule is &quot;is_class&quot;, which attempts to
guess whether or not T is really a class, and may return &quot;true&quot;
when the correct value is actually &quot;false&quot;. This can
happen if: T is a union, T is an enum, or T is a compiler-supplied
scalar type that is not specialised for in these type traits.</p>
<p><i>If there is no compiler support</i>, to ensure that these
traits <i>always</i> return the correct values, specialise 'is_enum'
for each user-defined enumeration type, 'is_union' for each user-defined
union type, 'is_empty' for each user-defined empty composite type,
and 'is_POD' for each user-defined POD type. The 'has_*' traits
should also be specialized if the user-defined type has those
traits and is <i>not</i> a POD.</p>
<p>The following rules are automatically enforced:</p>
<p>is_enum implies is_POD</p>
<p>is_POD implies has_*</p>
<p>This means, for example, if you have an empty POD-struct, just
specialize is_empty and is_POD, which will cause all the has_* to
also return true.</p>
<h2><a name="ec"></a>Example code</h2>
<p>Type-traits comes with two sample programs: <a
href="type_traits_test.cpp">type_traits_test.cpp</a> tests the
type traits classes - mostly this is a test of your compiler's
support for the concepts used in the type traits implementation,
while <a href="algo_opt_examples.cpp">algo_opt_examples.cpp</a>
uses the type traits classes to &quot;optimise&quot; some
familiar standard library algorithms.</p>
<p>There are four algorithm examples in algo_opt_examples.cpp:</p>
<table border="0" cellpadding="7" cellspacing="0" width="638">
<tr>
<td valign="top" width="50%"><pre>opt::copy</pre>
</td>
<td valign="top" width="50%">If the copy operation can be
performed using memcpy then does so, otherwise uses a
regular element by element copy (<i>c.f.</i> std::copy).</td>
</tr>
<tr>
<td valign="top" width="50%"><pre>opt::fill</pre>
</td>
<td valign="top" width="50%">If the fill operation can be
performed by memset, then does so, otherwise uses a
regular element by element assign. Also uses call_traits
to optimise how the parameters can be passed (<i>c.f.</i>
std::fill).</td>
</tr>
<tr>
<td valign="top" width="50%"><pre>opt::destroy_array</pre>
</td>
<td valign="top" width="50%">If the type in the array has
a trivial destructor then does nothing, otherwise calls
destructors for all elements in the array - this
algorithm is the reverse of std::uninitialized_copy / std::uninitialized_fill.</td>
</tr>
<tr>
<td valign="top" width="50%"><pre>opt::iter_swap</pre>
</td>
<td valign="top" width="50%">Determines whether the
iterator is a proxy-iterator: if it is then does a &quot;slow
and safe&quot; swap, otherwise calls std::swap on the
assumption that std::swap may be specialised for the
iterated type.</td>
</tr>
</table>
<p>&nbsp;</p>
<hr>
<p>Revised 01 September 2000</p>
<p>© Copyright boost.org 2000. Permission to copy, use, modify,
sell and distribute this document is granted provided this
copyright notice appears in all copies. This document is provided
&quot;as is&quot; without express or implied warranty, and with
no claim as to its suitability for any purpose.</p>
<p>Based on contributions by Steve Cleary, Beman Dawes, Howard
Hinnant and John Maddock.</p>
<p>Maintained by <a href="mailto:John_Maddock@compuserve.com">John
Maddock</a>, the latest version of this file can be found at <a
href="http://www.boost.org/">www.boost.org</a>, and the boost
discussion list at <a href="http://www.egroups.com/list/boost">www.egroups.com/list/boost</a>.</p>
</body>
</html>

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@ -1,672 +0,0 @@
// (C) Copyright Steve Cleary, Beman Dawes, Howard Hinnant & John Maddock 2000.
// Permission to copy, use, modify, sell and
// distribute this software is granted provided this copyright notice appears
// in all copies. This software is provided "as is" without express or implied
// warranty, and with no claim as to its suitability for any purpose.
// standalone test program for <boost/type_traits.hpp>
/* Release notes:
31 Jan 2001:
Added a test for is_array using a const array and a test for
is_convertible with a user-defined implicit conversion. Changed
signature of main() so that this program will link under
MSVC. (Jeremy Siek)
20 Jan 2001:
Suppress an expected warning for MSVC
Added a test to prove that we can use void with is_same<>
Removed "press any key to exit" as it interferes with testing in large
batches.
(David Abahams)
31st July 2000:
Added extra tests for is_empty, is_convertible, alignment_of.
23rd July 2000:
Removed all call_traits tests to call_traits_test.cpp
Removed all compressed_pair tests to compressed_pair_tests.cpp
Improved tests macros
Tidied up specialistions of type_types classes for test cases. */
#include <iostream>
#include <typeinfo>
#include <boost/type_traits.hpp>
#include <boost/utility.hpp>
#include "type_traits_test.hpp"
using namespace boost;
// Since there is no compiler support, we should specialize:
// is_enum for all enumerations (is_enum implies is_POD)
// is_union for all unions
// is_empty for all empty composites
// is_POD for all PODs (except enums) (is_POD implies has_*)
// has_* for any UDT that has that trait and is not POD
enum enum_UDT{ one, two, three };
struct UDT
{
UDT();
~UDT();
UDT(const UDT&);
UDT& operator=(const UDT&);
int i;
void f1();
int f2();
int f3(int);
int f4(int, float);
};
struct POD_UDT { int x; };
struct empty_UDT{ ~empty_UDT(){}; };
struct empty_POD_UDT{};
union union_UDT
{
int x;
double y;
~union_UDT();
};
union POD_union_UDT
{
int x;
double y;
};
union empty_union_UDT
{
~empty_union_UDT();
};
union empty_POD_union_UDT{};
#ifndef BOOST_NO_INCLASS_MEMBER_INITIALIZATION
namespace boost {
template <> struct is_enum<enum_UDT>
{ static const bool value = true; };
template <> struct is_POD<POD_UDT>
{ static const bool value = true; };
// this type is not POD, so we have to specialize the has_* individually
template <> struct has_trivial_constructor<empty_UDT>
{ static const bool value = true; };
template <> struct has_trivial_copy<empty_UDT>
{ static const bool value = true; };
template <> struct has_trivial_assign<empty_UDT>
{ static const bool value = true; };
template <> struct is_POD<empty_POD_UDT>
{ static const bool value = true; };
template <> struct is_union<union_UDT>
{ static const bool value = true; };
template <> struct is_union<POD_union_UDT>
{ static const bool value = true; };
template <> struct is_POD<POD_union_UDT>
{ static const bool value = true; };
template <> struct is_union<empty_union_UDT>
{ static const bool value = true; };
// this type is not POD, so we have to specialize the has_* individually
template <> struct has_trivial_constructor<empty_union_UDT>
{ static const bool value = true; };
template <> struct has_trivial_copy<empty_union_UDT>
{ static const bool value = true; };
template <> struct has_trivial_assign<empty_union_UDT>
{ static const bool value = true; };
template <> struct is_union<empty_POD_union_UDT>
{ static const bool value = true; };
template <> struct is_POD<empty_POD_union_UDT>
{ static const bool value = true; };
}
#else
namespace boost {
template <> struct is_enum<enum_UDT>
{ enum{ value = true }; };
template <> struct is_POD<POD_UDT>
{ enum{ value = true }; };
// this type is not POD, so we have to specialize the has_* individually
template <> struct has_trivial_constructor<empty_UDT>
{ enum{ value = true }; };
template <> struct has_trivial_copy<empty_UDT>
{ enum{ value = true }; };
template <> struct has_trivial_assign<empty_UDT>
{ enum{ value = true }; };
template <> struct is_POD<empty_POD_UDT>
{ enum{ value = true }; };
template <> struct is_union<union_UDT>
{ enum{ value = true }; };
template <> struct is_union<POD_union_UDT>
{ enum{ value = true }; };
template <> struct is_POD<POD_union_UDT>
{ enum{ value = true }; };
template <> struct is_union<empty_union_UDT>
{ enum{ value = true }; };
// this type is not POD, so we have to specialize the has_* individually
template <> struct has_trivial_constructor<empty_union_UDT>
{ enum{ value = true }; };
template <> struct has_trivial_copy<empty_union_UDT>
{ enum{ value = true }; };
template <> struct has_trivial_assign<empty_union_UDT>
{ enum{ value = true }; };
template <> struct is_union<empty_POD_union_UDT>
{ enum{ value = true }; };
template <> struct is_POD<empty_POD_union_UDT>
{ enum{ value = true }; };
}
#endif
class Base { };
class Derived : public Base { };
class NonDerived { };
enum enum1
{
one_,two_
};
enum enum2
{
three_,four_
};
struct VB
{
virtual ~VB(){};
};
struct VD : VB
{
~VD(){};
};
//
// struct non_pointer:
// used to verify that is_pointer does not return
// true for class types that implement operator void*()
//
struct non_pointer
{
operator void*(){return this;}
};
//
// struct non_empty:
// used to verify that is_empty does not emit
// spurious warnings or errors.
//
struct non_empty : boost::noncopyable
{
int i;
};
struct implicitly_convertible_to_int {
operator int() { return 0; }
};
// Steve: All comments that I (Steve Cleary) have added below are prefixed with
// "Steve:" The failures that BCB4 has on the tests are due to Borland's
// not considering cv-qual's as a part of the type -- they are considered
// compiler hints only. These failures should be fixed before long.
int main(int, char*[])
{
std::cout << "Checking type operations..." << std::endl << std::endl;
// cv-qualifiers applied to reference types should have no effect
// declare these here for later use with is_reference and remove_reference:
typedef int& r_type;
#ifdef BOOST_MSVC
# pragma warning(push)
# pragma warning(disable:4181) // qualifier applied to reference type ignored
#endif
typedef const r_type cr_type;
#ifdef BOOST_MSVC
# pragma warning(pop)
#endif
type_test(int, remove_reference<int>::type)
type_test(const int, remove_reference<const int>::type)
type_test(int, remove_reference<int&>::type)
type_test(const int, remove_reference<const int&>::type)
type_test(volatile int, remove_reference<volatile int&>::type)
type_test(int, remove_reference<cr_type>::type)
type_test(int, remove_const<const int>::type)
// Steve: fails on BCB4
type_test(volatile int, remove_const<volatile int>::type)
// Steve: fails on BCB4
type_test(volatile int, remove_const<const volatile int>::type)
type_test(int, remove_const<int>::type)
type_test(int*, remove_const<int* const>::type)
type_test(int, remove_volatile<volatile int>::type)
// Steve: fails on BCB4
type_test(const int, remove_volatile<const int>::type)
// Steve: fails on BCB4
type_test(const int, remove_volatile<const volatile int>::type)
type_test(int, remove_volatile<int>::type)
type_test(int*, remove_volatile<int* volatile>::type)
type_test(int, remove_cv<volatile int>::type)
type_test(int, remove_cv<const int>::type)
type_test(int, remove_cv<const volatile int>::type)
type_test(int, remove_cv<int>::type)
type_test(int*, remove_cv<int* volatile>::type)
type_test(int*, remove_cv<int* const>::type)
type_test(int*, remove_cv<int* const volatile>::type)
type_test(const int *, remove_cv<const int * const>::type)
type_test(int, remove_bounds<int>::type)
type_test(int*, remove_bounds<int*>::type)
type_test(int, remove_bounds<int[3]>::type)
type_test(int[3], remove_bounds<int[2][3]>::type)
std::cout << std::endl << "Checking type properties..." << std::endl << std::endl;
value_test(true, (is_same<void, void>::value))
value_test(false, (is_same<int, void>::value))
value_test(false, (is_same<void, int>::value))
value_test(true, (is_same<int, int>::value))
value_test(false, (is_same<int, const int>::value))
value_test(false, (is_same<int, int&>::value))
value_test(false, (is_same<int*, const int*>::value))
value_test(false, (is_same<int*, int*const>::value))
value_test(false, (is_same<int, int[2]>::value))
value_test(false, (is_same<int*, int[2]>::value))
value_test(false, (is_same<int[4], int[2]>::value))
value_test(false, is_const<int>::value)
value_test(true, is_const<const int>::value)
value_test(false, is_const<volatile int>::value)
value_test(true, is_const<const volatile int>::value)
value_test(false, is_volatile<int>::value)
value_test(false, is_volatile<const int>::value)
value_test(true, is_volatile<volatile int>::value)
value_test(true, is_volatile<const volatile int>::value)
value_test(true, is_void<void>::value)
// Steve: fails on BCB4
// JM: but looks as though it should according to [3.9.3p1]?
//value_test(false, is_void<const void>::value)
value_test(false, is_void<int>::value)
value_test(false, is_standard_unsigned_integral<UDT>::value)
value_test(false, is_standard_unsigned_integral<void>::value)
value_test(false, is_standard_unsigned_integral<bool>::value)
value_test(false, is_standard_unsigned_integral<char>::value)
value_test(false, is_standard_unsigned_integral<signed char>::value)
value_test(true, is_standard_unsigned_integral<unsigned char>::value)
value_test(false, is_standard_unsigned_integral<wchar_t>::value)
value_test(false, is_standard_unsigned_integral<short>::value)
value_test(true, is_standard_unsigned_integral<unsigned short>::value)
value_test(false, is_standard_unsigned_integral<int>::value)
value_test(true, is_standard_unsigned_integral<unsigned int>::value)
value_test(false, is_standard_unsigned_integral<long>::value)
value_test(true, is_standard_unsigned_integral<unsigned long>::value)
value_test(false, is_standard_unsigned_integral<float>::value)
value_test(false, is_standard_unsigned_integral<double>::value)
value_test(false, is_standard_unsigned_integral<long double>::value)
#ifdef ULLONG_MAX
value_test(false, is_standard_unsigned_integral<long long>::value)
value_test(false, is_standard_unsigned_integral<unsigned long long>::value)
#endif
#if defined(__BORLANDC__) || defined(_MSC_VER)
value_test(false, is_standard_unsigned_integral<__int64>::value)
value_test(false, is_standard_unsigned_integral<unsigned __int64>::value)
#endif
value_test(false, is_standard_signed_integral<UDT>::value)
value_test(false, is_standard_signed_integral<void>::value)
value_test(false, is_standard_signed_integral<bool>::value)
value_test(false, is_standard_signed_integral<char>::value)
value_test(true, is_standard_signed_integral<signed char>::value)
value_test(false, is_standard_signed_integral<unsigned char>::value)
value_test(false, is_standard_signed_integral<wchar_t>::value)
value_test(true, is_standard_signed_integral<short>::value)
value_test(false, is_standard_signed_integral<unsigned short>::value)
value_test(true, is_standard_signed_integral<int>::value)
value_test(false, is_standard_signed_integral<unsigned int>::value)
value_test(true, is_standard_signed_integral<long>::value)
value_test(false, is_standard_signed_integral<unsigned long>::value)
value_test(false, is_standard_signed_integral<float>::value)
value_test(false, is_standard_signed_integral<double>::value)
value_test(false, is_standard_signed_integral<long double>::value)
#ifdef ULLONG_MAX
value_test(false, is_standard_signed_integral<long long>::value)
value_test(false, is_standard_signed_integral<unsigned long long>::value)
#endif
#if defined(__BORLANDC__) || defined(_MSC_VER)
value_test(false, is_standard_signed_integral<__int64>::value)
value_test(false, is_standard_signed_integral<unsigned __int64>::value)
#endif
value_test(false, is_standard_arithmetic<UDT>::value)
value_test(false, is_standard_arithmetic<void>::value)
value_test(true, is_standard_arithmetic<bool>::value)
value_test(true, is_standard_arithmetic<char>::value)
value_test(true, is_standard_arithmetic<signed char>::value)
value_test(true, is_standard_arithmetic<unsigned char>::value)
value_test(true, is_standard_arithmetic<wchar_t>::value)
value_test(true, is_standard_arithmetic<short>::value)
value_test(true, is_standard_arithmetic<unsigned short>::value)
value_test(true, is_standard_arithmetic<int>::value)
value_test(true, is_standard_arithmetic<unsigned int>::value)
value_test(true, is_standard_arithmetic<long>::value)
value_test(true, is_standard_arithmetic<unsigned long>::value)
value_test(true, is_standard_arithmetic<float>::value)
value_test(true, is_standard_arithmetic<double>::value)
value_test(true, is_standard_arithmetic<long double>::value)
#ifdef ULLONG_MAX
value_test(false, is_standard_arithmetic<long long>::value)
value_test(false, is_standard_arithmetic<unsigned long long>::value)
#endif
#if defined(__BORLANDC__) || defined(_MSC_VER)
value_test(false, is_standard_arithmetic<__int64>::value)
value_test(false, is_standard_arithmetic<unsigned __int64>::value)
#endif
value_test(false, is_standard_fundamental<UDT>::value)
value_test(true, is_standard_fundamental<void>::value)
value_test(true, is_standard_fundamental<bool>::value)
value_test(true, is_standard_fundamental<char>::value)
value_test(true, is_standard_fundamental<signed char>::value)
value_test(true, is_standard_fundamental<unsigned char>::value)
value_test(true, is_standard_fundamental<wchar_t>::value)
value_test(true, is_standard_fundamental<short>::value)
value_test(true, is_standard_fundamental<unsigned short>::value)
value_test(true, is_standard_fundamental<int>::value)
value_test(true, is_standard_fundamental<unsigned int>::value)
value_test(true, is_standard_fundamental<long>::value)
value_test(true, is_standard_fundamental<unsigned long>::value)
value_test(true, is_standard_fundamental<float>::value)
value_test(true, is_standard_fundamental<double>::value)
value_test(true, is_standard_fundamental<long double>::value)
#ifdef ULLONG_MAX
value_test(false, is_standard_fundamental<long long>::value)
value_test(false, is_standard_fundamental<unsigned long long>::value)
#endif
#if defined(__BORLANDC__) || defined(_MSC_VER)
value_test(false, is_standard_fundamental<__int64>::value)
value_test(false, is_standard_fundamental<unsigned __int64>::value)
#endif
value_test(false, is_arithmetic<UDT>::value)
value_test(true, is_arithmetic<char>::value)
value_test(true, is_arithmetic<signed char>::value)
value_test(true, is_arithmetic<unsigned char>::value)
value_test(true, is_arithmetic<wchar_t>::value)
value_test(true, is_arithmetic<short>::value)
value_test(true, is_arithmetic<unsigned short>::value)
value_test(true, is_arithmetic<int>::value)
value_test(true, is_arithmetic<unsigned int>::value)
value_test(true, is_arithmetic<long>::value)
value_test(true, is_arithmetic<unsigned long>::value)
value_test(true, is_arithmetic<float>::value)
value_test(true, is_arithmetic<double>::value)
value_test(true, is_arithmetic<long double>::value)
value_test(true, is_arithmetic<bool>::value)
#ifdef ULLONG_MAX
value_test(true, is_arithmetic<long long>::value)
value_test(true, is_arithmetic<unsigned long long>::value)
#endif
#if defined(__BORLANDC__) || defined(_MSC_VER)
value_test(true, is_arithmetic<__int64>::value)
value_test(true, is_arithmetic<unsigned __int64>::value)
#endif
value_test(false, is_array<int>::value)
value_test(false, is_array<int*>::value)
value_test(false, is_array<const int*>::value)
value_test(false, is_array<const volatile int*>::value)
value_test(true, is_array<int[2]>::value)
value_test(true, is_array<const int[2]>::value)
value_test(true, is_array<const volatile int[2]>::value)
value_test(true, is_array<int[2][3]>::value)
value_test(true, is_array<UDT[2]>::value)
value_test(false, is_array<int(&)[2]>::value)
value_test(true, is_array<const int[2]>::value)
typedef void(*f1)();
typedef int(*f2)(int);
typedef int(*f3)(int, bool);
typedef void (UDT::*mf1)();
typedef int (UDT::*mf2)();
typedef int (UDT::*mf3)(int);
typedef int (UDT::*mf4)(int, float);
value_test(false, is_const<f1>::value)
value_test(false, is_reference<f1>::value)
value_test(false, is_array<f1>::value)
value_test(false, is_pointer<int>::value)
value_test(false, is_pointer<int&>::value)
value_test(true, is_pointer<int*>::value)
value_test(true, is_pointer<const int*>::value)
value_test(true, is_pointer<volatile int*>::value)
value_test(true, is_pointer<non_pointer*>::value)
// Steve: was 'true', should be 'false', via 3.9.2p3, 3.9.3p1
value_test(false, is_pointer<int*const>::value)
// Steve: was 'true', should be 'false', via 3.9.2p3, 3.9.3p1
value_test(false, is_pointer<int*volatile>::value)
// Steve: was 'true', should be 'false', via 3.9.2p3, 3.9.3p1
value_test(false, is_pointer<int*const volatile>::value)
// JM 02 Oct 2000:
value_test(false, is_pointer<non_pointer>::value)
value_test(false, is_pointer<int*&>::value)
value_test(false, is_pointer<int(&)[2]>::value)
value_test(false, is_pointer<int[2]>::value)
value_test(false, is_pointer<char[sizeof(void*)]>::value)
value_test(true, is_pointer<f1>::value)
value_test(true, is_pointer<f2>::value)
value_test(true, is_pointer<f3>::value)
// Steve: was 'true', should be 'false', via 3.9.2p3
value_test(false, is_pointer<mf1>::value)
// Steve: was 'true', should be 'false', via 3.9.2p3
value_test(false, is_pointer<mf2>::value)
// Steve: was 'true', should be 'false', via 3.9.2p3
value_test(false, is_pointer<mf3>::value)
// Steve: was 'true', should be 'false', via 3.9.2p3
value_test(false, is_pointer<mf4>::value)
value_test(false, is_reference<bool>::value)
value_test(true, is_reference<int&>::value)
value_test(true, is_reference<const int&>::value)
value_test(true, is_reference<volatile int &>::value)
value_test(true, is_reference<r_type>::value)
value_test(true, is_reference<cr_type>::value)
value_test(true, is_reference<const UDT&>::value)
value_test(false, is_class<int>::value)
value_test(false, is_class<const int>::value)
value_test(false, is_class<volatile int>::value)
value_test(false, is_class<int*>::value)
value_test(false, is_class<int* const>::value)
value_test(false, is_class<int[2]>::value)
value_test(false, is_class<int&>::value)
value_test(false, is_class<mf4>::value)
value_test(false, is_class<f1>::value)
value_test(false, is_class<enum_UDT>::value)
value_test(true, is_class<UDT>::value)
value_test(true, is_class<UDT const>::value)
value_test(true, is_class<UDT volatile>::value)
value_test(true, is_class<empty_UDT>::value)
value_test(true, is_class<std::iostream>::value)
value_test(false, is_class<UDT*>::value)
value_test(false, is_class<UDT[2]>::value)
value_test(false, is_class<UDT&>::value)
value_test(true, is_object<int>::value)
value_test(true, is_object<UDT>::value)
value_test(false, is_object<int&>::value)
value_test(false, is_object<void>::value)
value_test(true, is_standard_scalar<int>::value)
value_test(true, is_extension_scalar<void*>::value)
value_test(false, is_enum<int>::value)
value_test(true, is_enum<enum_UDT>::value)
value_test(false, is_member_pointer<f1>::value)
value_test(false, is_member_pointer<f2>::value)
value_test(false, is_member_pointer<f3>::value)
value_test(true, is_member_pointer<mf1>::value)
value_test(true, is_member_pointer<mf2>::value)
value_test(true, is_member_pointer<mf3>::value)
value_test(true, is_member_pointer<mf4>::value)
value_test(false, is_empty<int>::value)
value_test(false, is_empty<int*>::value)
value_test(false, is_empty<int&>::value)
#if defined(__MWERKS__) || defined(BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION)
// apparent compiler bug causes this to fail to compile:
value_fail(false, is_empty<int[2]>::value)
#else
value_test(false, is_empty<int[2]>::value)
#endif
#if defined(BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION)
value_fail(false, is_empty<f1>::value)
#else
value_test(false, is_empty<f1>::value)
#endif
value_test(false, is_empty<mf1>::value)
value_test(false, is_empty<UDT>::value)
value_test(true, is_empty<empty_UDT>::value)
value_test(true, is_empty<empty_POD_UDT>::value)
#if defined(BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION)
value_fail(true, is_empty<empty_union_UDT>::value)
#else
value_test(true, is_empty<empty_union_UDT>::value)
#endif
value_test(false, is_empty<enum_UDT>::value)
value_test(true, is_empty<boost::noncopyable>::value)
value_test(false, is_empty<non_empty>::value)
value_test(true, has_trivial_constructor<int>::value)
value_test(true, has_trivial_constructor<int*>::value)
value_test(true, has_trivial_constructor<int*const>::value)
value_test(true, has_trivial_constructor<const int>::value)
value_test(true, has_trivial_constructor<volatile int>::value)
value_test(true, has_trivial_constructor<int[2]>::value)
value_test(true, has_trivial_constructor<int[3][2]>::value)
value_test(true, has_trivial_constructor<int[2][4][5][6][3]>::value)
value_test(true, has_trivial_constructor<f1>::value)
value_test(true, has_trivial_constructor<mf2>::value)
value_test(false, has_trivial_constructor<UDT>::value)
value_test(true, has_trivial_constructor<empty_UDT>::value)
value_test(true, has_trivial_constructor<enum_UDT>::value)
value_test(true, has_trivial_copy<int>::value)
value_test(true, has_trivial_copy<int*>::value)
value_test(true, has_trivial_copy<int*const>::value)
value_test(true, has_trivial_copy<const int>::value)
// Steve: was 'false' -- should be 'true' via 3.9p3, 3.9p10
value_test(true, has_trivial_copy<volatile int>::value)
value_test(true, has_trivial_copy<int[2]>::value)
value_test(true, has_trivial_copy<int[3][2]>::value)
value_test(true, has_trivial_copy<int[2][4][5][6][3]>::value)
value_test(true, has_trivial_copy<f1>::value)
value_test(true, has_trivial_copy<mf2>::value)
value_test(false, has_trivial_copy<UDT>::value)
value_test(true, has_trivial_copy<empty_UDT>::value)
value_test(true, has_trivial_copy<enum_UDT>::value)
value_test(true, has_trivial_assign<int>::value)
value_test(true, has_trivial_assign<int*>::value)
value_test(true, has_trivial_assign<int*const>::value)
value_test(true, has_trivial_assign<const int>::value)
// Steve: was 'false' -- should be 'true' via 3.9p3, 3.9p10
value_test(true, has_trivial_assign<volatile int>::value)
value_test(true, has_trivial_assign<int[2]>::value)
value_test(true, has_trivial_assign<int[3][2]>::value)
value_test(true, has_trivial_assign<int[2][4][5][6][3]>::value)
value_test(true, has_trivial_assign<f1>::value)
value_test(true, has_trivial_assign<mf2>::value)
value_test(false, has_trivial_assign<UDT>::value)
value_test(true, has_trivial_assign<empty_UDT>::value)
value_test(true, has_trivial_assign<enum_UDT>::value)
value_test(true, has_trivial_destructor<int>::value)
value_test(true, has_trivial_destructor<int*>::value)
value_test(true, has_trivial_destructor<int*const>::value)
value_test(true, has_trivial_destructor<const int>::value)
value_test(true, has_trivial_destructor<volatile int>::value)
value_test(true, has_trivial_destructor<int[2]>::value)
value_test(true, has_trivial_destructor<int[3][2]>::value)
value_test(true, has_trivial_destructor<int[2][4][5][6][3]>::value)
value_test(true, has_trivial_destructor<f1>::value)
value_test(true, has_trivial_destructor<mf2>::value)
value_test(false, has_trivial_destructor<UDT>::value)
value_test(false, has_trivial_destructor<empty_UDT>::value)
value_test(true, has_trivial_destructor<enum_UDT>::value)
value_test(true, is_POD<int>::value)
value_test(true, is_POD<int*>::value)
// Steve: was 'true', should be 'false', via 3.9p10
value_test(false, is_POD<int&>::value)
value_test(true, is_POD<int*const>::value)
value_test(true, is_POD<const int>::value)
// Steve: was 'false', should be 'true', via 3.9p10
value_test(true, is_POD<volatile int>::value)
// Steve: was 'true', should be 'false', via 3.9p10
value_test(false, is_POD<const int&>::value)
value_test(true, is_POD<int[2]>::value)
value_test(true, is_POD<int[3][2]>::value)
value_test(true, is_POD<int[2][4][5][6][3]>::value)
value_test(true, is_POD<f1>::value)
value_test(true, is_POD<mf2>::value)
value_test(false, is_POD<UDT>::value)
value_test(false, is_POD<empty_UDT>::value)
value_test(true, is_POD<enum_UDT>::value)
value_test(true, (boost::is_convertible<implicitly_convertible_to_int,
int>::value));
value_test(true, (boost::is_convertible<Derived,Base>::value));
value_test(true, (boost::is_convertible<Derived,Derived>::value));
value_test(true, (boost::is_convertible<Base,Base>::value));
value_test(false, (boost::is_convertible<Base,Derived>::value));
value_test(true, (boost::is_convertible<Derived,Derived>::value));
value_test(false, (boost::is_convertible<NonDerived,Base>::value));
value_test(false, (boost::is_convertible<boost::noncopyable, int>::value));
value_test(true, (boost::is_convertible<float,int>::value));
#if defined(BOOST_MSVC6_MEMBER_TEMPLATES) || !defined(BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION)
value_test(false, (boost::is_convertible<float,void>::value));
value_test(false, (boost::is_convertible<void,float>::value));
value_test(true, (boost::is_convertible<void,void>::value));
#endif
value_test(true, (boost::is_convertible<enum1, int>::value));
value_test(true, (boost::is_convertible<Derived*, Base*>::value));
value_test(false, (boost::is_convertible<Base*, Derived*>::value));
value_test(true, (boost::is_convertible<Derived&, Base&>::value));
value_test(false, (boost::is_convertible<Base&, Derived&>::value));
value_test(true, (boost::is_convertible<const Derived*, const Base*>::value));
value_test(false, (boost::is_convertible<const Base*, const Derived*>::value));
value_test(true, (boost::is_convertible<const Derived&, const Base&>::value));
value_test(false, (boost::is_convertible<const Base&, const Derived&>::value));
value_test(false, (boost::is_convertible<const int *, int*>::value));
value_test(false, (boost::is_convertible<const int&, int&>::value));
value_test(true, (boost::is_convertible<int*, int[2]>::value));
value_test(false, (boost::is_convertible<const int*, int[3]>::value));
value_test(true, (boost::is_convertible<const int&, int>::value));
value_test(true, (boost::is_convertible<int(&)[4], const int*>::value));
value_test(true, (boost::is_convertible<int(&)(int), int(*)(int)>::value));
value_test(true, (boost::is_convertible<int *, const int*>::value));
value_test(true, (boost::is_convertible<int&, const int&>::value));
value_test(true, (boost::is_convertible<int[2], int*>::value));
value_test(true, (boost::is_convertible<int[2], const int*>::value));
value_test(false, (boost::is_convertible<const int[2], int*>::value));
align_test(int);
align_test(char);
align_test(double);
align_test(int[4]);
align_test(int(*)(int));
#ifndef BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION
align_test(char&);
align_test(char (&)(int));
align_test(char(&)[4]);
#endif
align_test(int*);
//align_test(const int);
align_test(VB);
align_test(VD);
std::cout << std::endl << test_count << " tests completed (" << failures << " failures)";
return failures;
}

114
type_traits_test.hpp
View File

@ -1,114 +0,0 @@
// boost::compressed_pair test program
// (C) Copyright John Maddock 2000. Permission to copy, use, modify, sell and
// distribute this software is granted provided this copyright notice appears
// in all copies. This software is provided "as is" without express or implied
// warranty, and with no claim as to its suitability for any purpose.
// common test code for type_traits_test.cpp/call_traits_test.cpp/compressed_pair_test.cpp
#ifndef BOOST_TYPE_TRAITS_TEST_HPP
#define BOOST_TYPE_TRAITS_TEST_HPP
// Variable declarations must come before test_align due to two-phase lookup
unsigned failures = 0;
unsigned test_count = 0;
//
// this one is here just to suppress warnings:
//
template <class T>
bool do_compare(T i, T j)
{
return i == j;
}
//
// this one is to verify that a constant is indeed a
// constant-integral-expression:
//
template <int>
struct ct_checker
{
};
#define BOOST_DO_JOIN( X, Y ) BOOST_DO_JOIN2(X,Y)
#define BOOST_DO_JOIN2(X, Y) X##Y
#define BOOST_JOIN( X, Y ) BOOST_DO_JOIN( X, Y )
#ifdef BOOST_MSVC
#define value_test(v, x) ++test_count;\
{typedef ct_checker<(x)> this_is_a_compile_time_check_;}\
if(!do_compare((int)v,(int)x)){++failures; std::cout << "checking value of " << #x << "...failed" << std::endl;}
#else
#define value_test(v, x) ++test_count;\
typedef ct_checker<(x)> BOOST_JOIN(this_is_a_compile_time_check_, __LINE__);\
if(!do_compare((int)v,(int)x)){++failures; std::cout << "checking value of " << #x << "...failed" << std::endl;}
#endif
#define value_fail(v, x) ++test_count; ++failures; std::cout << "checking value of " << #x << "...failed" << std::endl;
#ifndef BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION
#define type_test(v, x) ++test_count;\
if(do_compare(boost::is_same<v, x>::value, false)){\
++failures; \
std::cout << "checking type of " << #x << "...failed" << std::endl; \
std::cout << " expected type was " << #v << std::endl; \
std::cout << " " << typeid(boost::is_same<v, x>).name() << "::value is false" << std::endl; }
#else
#define type_test(v, x) ++test_count;\
if(typeid(v) != typeid(x)){\
++failures; \
std::cout << "checking type of " << #x << "...failed" << std::endl; \
std::cout << " expected type was " << #v << std::endl; \
std::cout << " " << "typeid(" #v ") != typeid(" #x ")" << std::endl; }
#endif
template <class T>
struct test_align
{
struct padded
{
char c;
T t;
};
static void do_it()
{
padded p;
unsigned a = reinterpret_cast<char*>(&(p.t)) - reinterpret_cast<char*>(&p);
value_test(a, boost::alignment_of<T>::value);
}
};
#ifndef BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION
template <class T>
struct test_align<T&>
{
static void do_it()
{
//
// we can't do the usual test because we can't take the address
// of a reference, so check that the result is the same as for a
// pointer type instead:
value_test(boost::alignment_of<T*>::value, boost::alignment_of<T&>::value);
}
};
#endif
#define align_test(T) test_align<T>::do_it()
//
// define tests here
//
// turn off some warnings:
#ifdef __BORLANDC__
#pragma option -w-8004
#endif
#ifdef BOOST_MSVC
#pragma warning (disable: 4018)
#endif
#endif // BOOST_TYPE_TRAITS_TEST_HPP