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Removed the polymorphic casts and implicit_cast

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Antony Polukhin 2014-06-09 12:44:31 +04:00
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146
cast.htm
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<h1><img src="../../boost.png" alt="boost.png (6897 bytes)" align=
"middle" width="277" height="86">Header <a href=
"../../boost/cast.hpp">boost/cast.hpp</a></h1>
<h2><a name="Cast Functions">Cast Functions</a></h2>
<p>The header <a href="../../boost/cast.hpp">boost/cast.hpp</a> provides <code>
<a href="#Polymorphic_cast">polymorphic_cast</a> and</code> <a href=
"#Polymorphic_cast"><code>polymorphic_downcast</code></a> function templates designed to
complement the C++ built-in casts.</p>
<p>The program <a href="cast_test.cpp">cast_test.cpp</a> can be used to
verify these function templates work as expected.</p>
<h3><a name="Polymorphic_cast">Polymorphic casts</a></h3>
<p>Pointers to polymorphic objects (objects of classes which define at
least one virtual function) are sometimes downcast or crosscast.
Downcasting means casting from a base class to a derived class.
Crosscasting means casting across an inheritance hierarchy diagram, such
as from one base to the other in a <code>Y</code> diagram hierarchy.</p>
<p>Such casts can be done with old-style casts, but this approach is
never to be recommended. Old-style casts are sorely lacking in type
safety, suffer poor readability, and are difficult to locate with search
tools.</p>
<p>The C++ built-in <code>static_cast</code> can be used for efficiently
downcasting pointers to polymorphic objects, but provides no error
detection for the case where the pointer being cast actually points to
the wrong derived class. The <code>polymorphic_downcast</code> template retains
the efficiency of <code>static_cast</code> for non-debug compilations, but for
debug compilations adds safety via an assert() that a <code>dynamic_cast</code>
succeeds.</p>
<p>The C++ built-in <code>dynamic_cast</code> can be used for downcasts and
crosscasts of pointers to polymorphic objects, but error notification in
the form of a returned value of 0 is inconvenient to test, or worse yet,
easy to forget to test. The throwing form of <code>dynamic_cast</code>, which
works on references, can be used on pointers through the ugly expression
&amp;<code>dynamic_cast&lt;T&amp;&gt;(*p)</code>, which causes undefined
behavior if <code>p</code> is <code>0</code>. The <code>polymorphic_cast</code>
template performs a <code>dynamic_cast</code> on a pointer, and throws an
exception if the <code>dynamic_cast</code> returns 0.</p>
<p>A <code>polymorphic_downcast</code> should be used for
downcasts that you are certain should succeed. Error checking is
only performed in translation units where <code>NDEBUG</code> is
not defined, via
<pre> assert( dynamic_cast&lt;Derived&gt;(x) == x )
</pre> where <code>x</code> is the source pointer. This approach
ensures that not only is a non-zero pointer returned, but also
that it is correct in the presence of multiple inheritance.
Attempts to crosscast using <code>polymorphic_downcast</code> will
fail to compile.
<b>Warning:</b> Because <code>polymorphic_downcast</code> uses assert(), it
violates the One Definition Rule (ODR) if NDEBUG is inconsistently
defined across translation units. [See ISO Std 3.2]
</p><p>
For crosscasts, or when the success of a cast can only be known at
runtime, or when efficiency is not important,
<code>polymorphic_cast</code> is preferred. </p>
<p>The C++ built-in <code>dynamic_cast</code> must be used to cast references
rather than pointers. It is also the only cast that can be used to check
whether a given interface is supported; in that case a return of 0 isn't
an error condition.</p>
<h3>polymorphic_cast and polymorphic_downcast synopsis</h3>
<blockquote>
<pre>namespace boost {
template &lt;class Derived, class Base&gt;
inline Derived polymorphic_cast(Base* x);
// Throws: std::bad_cast if ( dynamic_cast&lt;Derived&gt;(x) == 0 )
// Returns: dynamic_cast&lt;Derived&gt;(x)
template &lt;class Derived, class Base&gt;
inline Derived polymorphic_downcast(Base* x);
// Effects: assert( dynamic_cast&lt;Derived&gt;(x) == x );
// Returns: static_cast&lt;Derived&gt;(x)
}
</pre>
</blockquote>
<h3>polymorphic_downcast example</h3>
<blockquote>
<pre>#include &lt;boost/cast.hpp&gt;
...
class Fruit { public: virtual ~Fruit(){}; ... };
class Banana : public Fruit { ... };
...
void f( Fruit * fruit ) {
// ... logic which leads us to believe it is a Banana
Banana * banana = boost::polymorphic_downcast&lt;Banana*&gt;(fruit);
...
</pre>
</blockquote>
<h3>History</h3>
<p><code>polymorphic_cast</code> was suggested by Bjarne Stroustrup in "The C++
Programming Language".<br>
<code>polymorphic_downcast</code> was contributed by <a href=
"http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a>.<code><br>
An old
numeric_cast</code> that was contributed by <a href=
"http://www.boost.org/people/kevlin_henney.htm">Kevlin Henney</a> is now superseeded by the <a href="../numeric/conversion/doc/html/index.html">Boost Numeric Conversion Library</a></p>
<hr>
<p>Revised
<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan
-->June 23, 2005<!--webbot bot="Timestamp" endspan i-checksum="30348"
--></p>
<p class="copyright">&copy; Copyright boost.org 1999.
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
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include/boost/cast.hpp
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// boost cast.hpp header file ----------------------------------------------//
// (C) Copyright Kevlin Henney and Dave Abrahams 1999.
// Distributed under the Boost
// Software License, Version 1.0. (See accompanying file
// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
// See http://www.boost.org/libs/conversion for Documentation.
// Revision History
// 23 JUn 05 numeric_cast removed and redirected to the new verion (Fernando Cacciola)
// 02 Apr 01 Removed BOOST_NO_LIMITS workarounds and included
// <boost/limits.hpp> instead (the workaround did not
// actually compile when BOOST_NO_LIMITS was defined in
// any case, so we loose nothing). (John Maddock)
// 21 Jan 01 Undid a bug I introduced yesterday. numeric_cast<> never
// worked with stock GCC; trying to get it to do that broke
// vc-stlport.
// 20 Jan 01 Moved BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS to config.hpp.
// Removed unused BOOST_EXPLICIT_TARGET macro. Moved
// boost::detail::type to boost/type.hpp. Made it compile with
// stock gcc again (Dave Abrahams)
// 29 Nov 00 Remove nested namespace cast, cleanup spacing before Formal
// Review (Beman Dawes)
// 19 Oct 00 Fix numeric_cast for floating-point types (Dave Abrahams)
// 15 Jul 00 Suppress numeric_cast warnings for GCC, Borland and MSVC
// (Dave Abrahams)
// 30 Jun 00 More MSVC6 wordarounds. See comments below. (Dave Abrahams)
// 28 Jun 00 Removed implicit_cast<>. See comment below. (Beman Dawes)
// 27 Jun 00 More MSVC6 workarounds
// 15 Jun 00 Add workarounds for MSVC6
// 2 Feb 00 Remove bad_numeric_cast ";" syntax error (Doncho Angelov)
// 26 Jan 00 Add missing throw() to bad_numeric_cast::what(0 (Adam Levar)
// 29 Dec 99 Change using declarations so usages in other namespaces work
// correctly (Dave Abrahams)
// 23 Sep 99 Change polymorphic_downcast assert to also detect M.I. errors
// as suggested Darin Adler and improved by Valentin Bonnard.
// 2 Sep 99 Remove controversial asserts, simplify, rename.
// 30 Aug 99 Move to cast.hpp, replace value_cast with numeric_cast,
// place in nested namespace.
// 3 Aug 99 Initial version
#ifndef BOOST_CAST_HPP
#define BOOST_CAST_HPP
# include <boost/config.hpp>
# include <boost/assert.hpp>
# include <typeinfo>
# include <boost/type.hpp>
# include <boost/limits.hpp>
# include <boost/detail/select_type.hpp>
namespace boost
{
// See the documentation for descriptions of how to choose between
// static_cast<>, dynamic_cast<>, polymorphic_cast<> and polymorphic_downcast<>
// polymorphic_cast --------------------------------------------------------//
// Runtime checked polymorphic downcasts and crosscasts.
// Suggested in The C++ Programming Language, 3rd Ed, Bjarne Stroustrup,
// section 15.8 exercise 1, page 425.
template <class Target, class Source>
inline Target polymorphic_cast(Source* x)
{
Target tmp = dynamic_cast<Target>(x);
if ( tmp == 0 ) throw std::bad_cast();
return tmp;
}
// polymorphic_downcast ----------------------------------------------------//
// BOOST_ASSERT() checked polymorphic downcast. Crosscasts prohibited.
// WARNING: Because this cast uses BOOST_ASSERT(), it violates
// the One Definition Rule if used in multiple translation units
// where BOOST_DISABLE_ASSERTS, BOOST_ENABLE_ASSERT_HANDLER
// NDEBUG are defined inconsistently.
// Contributed by Dave Abrahams
template <class Target, class Source>
inline Target polymorphic_downcast(Source* x)
{
BOOST_ASSERT( dynamic_cast<Target>(x) == x ); // detect logic error
return static_cast<Target>(x);
}
} // namespace boost
# include <boost/numeric/conversion/cast.hpp>
#endif // BOOST_CAST_HPP

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// Copyright David Abrahams 2003.
// Distributed under the Boost Software License, Version 1.0. (See
// accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)
#ifndef IMPLICIT_CAST_DWA200356_HPP
# define IMPLICIT_CAST_DWA200356_HPP
# include <boost/mpl/identity.hpp>
namespace boost {
// implementation originally suggested by C. Green in
// http://lists.boost.org/MailArchives/boost/msg00886.php
// The use of identity creates a non-deduced form, so that the
// explicit template argument must be supplied
template <typename T>
inline T implicit_cast (typename mpl::identity<T>::type x) {
return x;
}
// incomplete return type now is here
//template <typename T>
//void implicit_cast (...);
} // namespace boost
#endif // IMPLICIT_CAST_DWA200356_HPP

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index.html
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<h1><img border="0" src="../../boost.png" align="center" width="277" height="86">Boost
Conversion Library</h1>
<p>The Conversion Library improves program safety and clarity by performing
otherwise messy conversions.&nbsp; It includes cast-style function templates designed to complement the C++
Standard's built-in casts.</p>
<p>To reduce coupling, particularly to standard library IOStreams, the Boost
Conversion Library is
supplied by several headers:</p>
<ul>
<li>The <a href="cast.htm">boost/cast</a> header provides <b>polymorphic_cast&lt;&gt;</b>
and <b>polymorphic_downcast&lt;&gt;</b> to perform safe casting between
polymorphic types.<br>
</li>
<li>The <a href="../../doc/html/boost_lexical_cast.html">boost/lexical_cast</a> header provides <b>lexical_cast&lt;&gt;</b>
general literal text conversions, such as an <code>int</code> represented as
a <code>string</code>, or vice-versa.</li>
</ul>
<hr>
<p>Revised <!--webbot bot="Timestamp" S-Type="EDITED"
S-Format="%d %B, %Y" startspan -->June 23, 2005<!--webbot bot="Timestamp" endspan i-checksum="30348" -->
</p>
<p class="copyright">
Copyright 2001 Beman Dawes.
<!--
Copyright 2005-2007 Daniel James.
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)
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test/Jamfile.v2
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# Copyright (C) 2001-2003 Douglas Gregor
# Copyright (C) 2011-2013 Antony Polukhin
# Copyright (C) 2011-2014 Antony Polukhin
#
# Distributed under the Boost Software License, Version 1.0. (See accompanying
# file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
@ -28,11 +28,7 @@ feature.feature nowchar : on :
feature.compose <nowchar>on : <cxxflags>/Zc:wchar_t- ;
test-suite conversion
: [ run implicit_cast.cpp ]
[ compile-fail implicit_cast_fail.cpp ]
[ run cast_test.cpp ]
[ run numeric_cast_test.cpp ]
[ run lexical_cast_test.cpp ]
: [ run lexical_cast_test.cpp ]
[ run lexical_cast_loopback_test.cpp ]
[ run lexical_cast_abstract_test.cpp ]
[ run lexical_cast_noncopyable_test.cpp ]

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test/cast_test.cpp
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// boost utility cast test program -----------------------------------------//
// (C) Copyright Beman Dawes, Dave Abrahams 1999. Distributed under the Boost
// Software License, Version 1.0. (See accompanying file
// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
// See http://www.boost.org for most recent version including documentation.
// Revision History
// 28 Set 04 factored out numeric_cast<> test (Fernando Cacciola)
// 20 Jan 01 removed use of <limits> for portability to raw GCC (David Abrahams)
// 28 Jun 00 implicit_cast removed (Beman Dawes)
// 30 Aug 99 value_cast replaced by numeric_cast
// 3 Aug 99 Initial Version
#include <iostream>
#include <climits>
#include <cfloat> // for DBL_MAX (Peter Schmid)
#include <boost/cast.hpp>
# if SCHAR_MAX == LONG_MAX
# error "This test program doesn't work if SCHAR_MAX == LONG_MAX"
# endif
using namespace boost;
using std::cout;
namespace
{
struct Base
{
virtual char kind() { return 'B'; }
};
struct Base2
{
virtual char kind2() { return '2'; }
};
struct Derived : public Base, Base2
{
virtual char kind() { return 'D'; }
};
}
int main( int argc, char * argv[] )
{
# ifdef NDEBUG
cout << "NDEBUG is defined\n";
# else
cout << "NDEBUG is not defined\n";
# endif
cout << "\nBeginning tests...\n";
// test polymorphic_cast ---------------------------------------------------//
// tests which should succeed
Base * base = new Derived;
Base2 * base2 = 0;
Derived * derived = 0;
derived = polymorphic_downcast<Derived*>( base ); // downcast
assert( derived->kind() == 'D' );
derived = 0;
derived = polymorphic_cast<Derived*>( base ); // downcast, throw on error
assert( derived->kind() == 'D' );
base2 = polymorphic_cast<Base2*>( base ); // crosscast
assert( base2->kind2() == '2' );
// tests which should result in errors being detected
int err_count = 0;
base = new Base;
if ( argc > 1 && *argv[1] == '1' )
{ derived = polymorphic_downcast<Derived*>( base ); } // #1 assert failure
bool caught_exception = false;
try { derived = polymorphic_cast<Derived*>( base ); }
catch (std::bad_cast)
{ cout<<"caught bad_cast\n"; caught_exception = true; }
if ( !caught_exception ) ++err_count;
// the following is just so generated code can be inspected
if ( derived->kind() == 'B' ) ++err_count;
cout << err_count << " errors detected\nTest "
<< (err_count==0 ? "passed\n" : "failed\n");
return err_count;
} // main

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test/implicit_cast.cpp
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// Copyright David Abrahams 2003.
// Distributed under the Boost Software License, Version 1.0. (See
// accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)
#include <boost/implicit_cast.hpp>
#include <boost/detail/lightweight_test.hpp>
#include <boost/type.hpp>
using boost::implicit_cast;
using boost::type;
template <class T>
type<T> check_return(T) { return type<T>(); }
struct foo
{
foo(char const*) {}
operator long() const { return 0; }
};
typedef type<long> long_type;
typedef type<foo> foo_type;
int main()
{
type<long> x = check_return(boost::implicit_cast<long>(1));
BOOST_TEST(boost::implicit_cast<long>(1) == 1L);
type<foo> f = check_return(boost::implicit_cast<foo>("hello"));
type<long> z = check_return(boost::implicit_cast<long>(foo("hello")));
// warning supression:
(void)x;
(void)f;
(void)z;
return boost::report_errors();
}

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test/implicit_cast_fail.cpp
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// Copyright David Abrahams 2003.
// Distributed under the Boost Software License, Version 1.0. (See
// accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)
#include <boost/implicit_cast.hpp>
#include <boost/type.hpp>
#define BOOST_INCLUDE_MAIN
#include <boost/test/test_tools.hpp>
using boost::implicit_cast;
struct foo
{
explicit foo(char const*) {}
};
int test_main(int, char*[])
{
foo x = implicit_cast<foo>("foobar");
(void)x; // warning suppression.
BOOST_CHECK(false); // suppressing warning about 'boost::unit_test::{anonymous}::unit_test_log' defined but not used
return 0;
}

101
test/numeric_cast_test.cpp
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// boost utility cast test program -----------------------------------------//
// (C) Copyright Beman Dawes, Dave Abrahams 1999. Distributed under the Boost
// Software License, Version 1.0. (See accompanying file
// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
// See http://www.boost.org for most recent version including documentation.
// Revision History
// 28 Set 04 factored out numeric_cast<> test (Fernando Cacciola)
// 20 Jan 01 removed use of <limits> for portability to raw GCC (David Abrahams)
// 28 Jun 00 implicit_cast removed (Beman Dawes)
// 30 Aug 99 value_cast replaced by numeric_cast
// 3 Aug 99 Initial Version
#include <iostream>
#include <climits>
#include <cfloat> // for DBL_MAX (Peter Schmid)
#include <boost/cast.hpp>
#include "boost/test/minimal.hpp"
# if SCHAR_MAX == LONG_MAX
# error "This test program doesn't work if SCHAR_MAX == LONG_MAX"
# endif
using namespace boost;
using std::cout;
int test_main( int , char * [] )
{
# ifdef NDEBUG
cout << "NDEBUG is defined\n";
# else
cout << "NDEBUG is not defined\n";
# endif
cout << "\nBeginning tests...\n";
// test implicit_cast and numeric_cast -------------------------------------//
// tests which should succeed
long small_value = 1;
long small_negative_value = -1;
long large_value = LONG_MAX;
long large_negative_value = LONG_MIN;
signed char c = 0;
c = static_cast<signed char>(large_value);
c = numeric_cast<signed char>( small_value );
BOOST_CHECK( c == 1 );
c = 0;
c = numeric_cast<signed char>( small_value );
BOOST_CHECK( c == 1 );
c = 0;
c = numeric_cast<signed char>( small_negative_value );
BOOST_CHECK( c == -1 );
// These tests courtesy of Joe R NWP Swatosh<joe.r.swatosh@usace.army.mil>
BOOST_CHECK( 0.0f == numeric_cast<float>( 0.0 ) );
BOOST_CHECK( 0.0 == numeric_cast<double>( 0.0 ) );
// tests which should result in errors being detected
bool caught_exception = false;
try { c = numeric_cast<signed char>( large_value ); }
catch (bad_numeric_cast)
{ cout<<"caught bad_numeric_cast #1\n"; caught_exception = true; }
BOOST_CHECK ( caught_exception );
caught_exception = false;
try { c = numeric_cast<signed char>( large_negative_value ); }
catch (bad_numeric_cast)
{ cout<<"caught bad_numeric_cast #2\n"; caught_exception = true; }
BOOST_CHECK ( caught_exception );
unsigned long ul;
caught_exception = false;
try { ul = numeric_cast<unsigned long>( large_negative_value ); }
catch (bad_numeric_cast)
{ cout<<"caught bad_numeric_cast #3\n"; caught_exception = true; }
BOOST_CHECK ( caught_exception );
caught_exception = false;
try { ul = numeric_cast<unsigned long>( small_negative_value ); }
catch (bad_numeric_cast)
{ cout<<"caught bad_numeric_cast #4\n"; caught_exception = true; }
BOOST_CHECK ( caught_exception );
caught_exception = false;
try { numeric_cast<int>( DBL_MAX ); }
catch (bad_numeric_cast)
{ cout<<"caught bad_numeric_cast #5\n"; caught_exception = true; }
BOOST_CHECK ( caught_exception );
(void)ul; // Supressing GCC warning about set but unused wariable
return 0 ;
}