by John Maddock and Steve Cleary
+This is a draft of an article that will appear in a future +issue of Dr Dobb's Journal
+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 "generic" +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.
+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 strlen +and wcslen 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 char +and wchar_t into the class char_traits. +Generic code that works with character strings can simply use char_traits<>::length +to determine the length of a null terminated string, safe in the knowledge that +specialisations of char_traits will use +the most appropriate method available to them.
+Class char_traits is a classic +example of a collection of type specific properties wrapped up in a single class +- what Nathan Myers termed a baggage class[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 value, 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.
+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 type that is the result of +the transformation. All of the type-traits classes are defined inside namespace boost; +for brevity, namespace-qualification is omitted in most of the code samples +given.
+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<T> +has a member value that is true only if T is void.
+template <typename T>
+struct is_void
+{ static const bool value = false; };
+
+template <>
+struct is_void<void>
+{ static const bool value = true; };
+Here we define a primary version of the template class is_void, +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<T>: 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:
+template <typename T>
+struct is_pointer
+{ static const bool value = false; };
+
+template <typename T>
+struct is_pointer<T*>
+{ static const bool value = true; };
+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 <…> 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:
+void foo(T); +void foo(U);+
Then you can also write a partial specialisation of the form:
+template <typename T>
+class c{ /*details*/ };
+
+template <typename T>
+
+class c<U>{ /*details*/ };
+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.
+As a more complex example of partial specialisation consider the class +remove_bounds<T>. This class defines a single typedef-member type +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:
+template <typename T>
+struct remove_bounds
+{ typedef T type; };
+
+template <typename T, std::size_t N>
+struct remove_bounds<T[N]>
+{ typedef T type; };
+The aim of remove_bounds is this: imagine a generic algorithm that is passed
+an array type as a template parameter, remove_bounds
+provides a means of determining the underlying type of the array. For example remove_bounds<int[4][5]>::type
+would evaluate to the type int[5]. 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.
As an example of how the type traits classes can be used, consider the +standard library algorithm copy:
+template<typename Iter1, typename Iter2> +Iter2 copy(Iter1 first, Iter1 last, Iter2 out);+
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 memcpy. +In order to implement copy in terms of memcpy +all of the following conditions need to be met:
+By trivial assignment operator we mean that the type is either a scalar +type[3] or:
+If all these conditions are met then a type can be copied using memcpy
+rather than using a compiler generated assignment operator. The type-traits
+library provides a class has_trivial_assign, such that has_trivial_assign<T>::value
+is true only if T has a trivial assignment operator. This class "just
+works" 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 has_trivial_assign gives the wrong answer, it will give
+the "safe" wrong answer - that trivial assignment is not allowable.
The code for an optimised version of copy that uses memcpy +where appropriate is given in listing 1. The code begins by defining a template +class copier, that takes a single Boolean template parameter, and has a +static template member function do_copy +which performs the generic version of copy (in other words +the "slow but safe version"). Following that there is a specialisation +for copier<true>: again this defines a static template member +function do_copy, but this version uses +memcpy to perform an "optimised" copy.
+In order to complete the implementation, what we need now is a version of
+copy, that calls copier<true>::do_copy if it is safe to use memcpy,
+and otherwise calls copier<false>::do_copy to do a
+"generic" copy. This is what the version in listing 1 does. To
+understand how the code works look at the code for copy
+and consider first the two typedefs v1_t and v2_t. These use std::iterator_traits<Iter1>::value_type
+to determine what type the two iterators point to, and then feed the result into
+another type-traits class remove_cv 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, copy
+declares an enumerated value can_opt 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 copier.
+The value of can_opt is computed by verifying that all of the following
+are true:
Finally we can use the value of can_opt 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 memcpy, +then it will do so, otherwise it will use a generic copy.
+It has often been repeated in these columns that "premature optimisation +is the root of all evil" [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.
+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:
+|
+ Version + |
+
+ T + |
+
+ Time + |
+
| "Optimised" copy | +char | +0.99 | +
| Conventional copy | +char | +8.07 | +
| "Optimised" copy | +int | +2.52 | +
| Conventional copy | +int | +8.02 | +
+
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].
+First, let us examine the definition of "std::pair", omitting the +comparision operators, default constructor, and template copy constructor for +simplicity:
+template <typename T1, typename T2>
+struct pair
+{
+ typedef T1 first_type;
+ typedef T2 second_type;
+
+ T1 first;
+ T2 second;
+
+ pair(const T1 & nfirst, const T2 & nsecond)
+ :first(nfirst), second(nsecond) { }
+};
+Now, this "pair" 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 "pair" to hold non-reference types, +references, and constant references:
+| Type of "T1" | +Type of parameter to initializing constructor | +
+ T+ |
+
+ const T &+ |
+
+ T &+ |
+
+ T &+ |
+
+ const T &+ |
+
+ const T &+ |
+
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 "add_reference", +which adds a reference to its type, unless it is already a reference.
+| Type of "T1" | +Type of "const T1" | +Type of "add_reference<const + T1>::type" | +
+ T+ |
+
+ const T+ |
+
+ const T &+ |
+
+ T &+ |
+
+ T & [8]+ |
+
+ T &+ |
+
+ const T &+ |
+
+ const T &+ |
+
+ const T &+ |
+
This allows us to build a primary template definition for "pair" +that can contain non-reference types, reference types, and constant reference +types:
+template <typename T1, typename T2>
+struct pair
+{
+ typedef T1 first_type;
+ typedef T2 second_type;
+
+ T1 first;
+ T2 second;
+
+ pair(boost::add_reference<const T1>::type nfirst,
+ boost::add_reference<const T2>::type nsecond)
+ :first(nfirst), second(nsecond) { }
+};
+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!
+This same extension could have been done using partial template +specialization of "pair", but to specialize "pair" 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.
+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.
+The authors would like to thank Beman Dawes and Howard Hinnant for their +helpful comments when preparing this article.
+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);
+}
+© Copyright John Maddock and Steve Cleary, 2000
+ + + +