Книга: Standard Template Library Programmer

value_type

value_type

Category: iterators

Component type: function

Value_type is overloaded; it is in fact five different functions.

template <class T, class Distance>
inline T* value_type(const input_iterator<T, Distance>&);
template <class T, class Distance>
inline T* value_type(const forward_iterator<T, Distance>&);
template <class T, class Distance>
inline T* value_type(const bidirectional_iterator<T, Distance>&);
template <class T, class Distance>
inline T* value_type(const random_access_iterator<T, Distance>&);
template <class T>
inline T* value_type(const T*);

Value_type is an iterator tag function: it is used to determine the value type associated with an iterator. An iterator's value type is the type of object returned when the iterator is dereferenced; Output Iterators do not have value types (Output Iterators may only be used for storing values, not for accessing values), but Input Iterators, Forward Iterators, Bidirectional Iterators, and Random Access Iterators do. [1]

In some cases, such as an algorithm that must declare a local variable that holds a value returned from dereferencing an iterator, it is necessary to find out an iterator's value type. Accordingly, value_type(Iter) returns (T*)0 , where T is Iter's value type.

Although value_type looks like a single function whose return type depends on its argument type, in reality it is a set of functions; the name value_type is overloaded. The function value_type must be overloaded for every iterator type [1].

In practice, ensuring that value_type is defined requires essentially no work at all. It is already defined for pointers, and for the base classes input_iterator, forward_iterator, bidirectional_iterator, and random_access_iterator. If you are implementing a new type of forward iterator, for example, you can simply derive it from the base class forward_iterator; this means that value_type (along with iterator_category and distance_type) will automatically be defined for your iterator. These base classes are empty: they contain no member functions or member variables, but only type information. Using them should therefore incur no overhead.

Note that, while the function value_type was present in the original STL, it is no longer present in the most recent draft C++ standard: it has been replaced by the iterator_traits class At present both mechanisms are supported [2], but eventually value_type will be removed.

Defined in the standard header iterator, and in the nonstandard backward-compatibility header iterator.h. This function is no longer part of the C++ standard, although it was present in early drafts of the standard. It is retained in this implementation for backward compatibility.

The argument of value_type must be an Input Iterator, Forward Iterator, Bidirectional Iterator, or Random Access Iterator. [1]

None. Value_type's argument is even permitted to be a singular iterator.

At most amortized constant time. In many cases, a compiler should be able to optimize away value_type entirely.

This example uses the value_type iterator tag function in order to declare a temporary variable of an iterator's value type.

template <class ForwardIterator 1, class ForwardIterator 2, class ValueType>
inline void __iter_swap(ForwardIterator1 a, ForwardIterator2 b, ValueType*) {
 T tmp = *a;
 *a = *b;
 *b = tmp;
}
template <class ForwardIterator 1, class ForwardIterator 2>
inline void iter_swap(ForwardIterator1 a, ForwardIterator2 b) {
 __iter_swap(a, b, value_type (a));
}

[1] Note that distance_type is not defined for Output Iterators or for Trivial Iterators. In the case of Output Iterators, this is because an Output Iterator does not have a value type: it is not possible to dereference an Output Iterator and obtain a value. In the case of Trivial Iterators, this is because the concept was introduced only for conceptual clarity, in order to separate the axioms related to an object that refers to another object from those related to iteration over a range. In fact, the STL does not define any types that are Trivial Iterators. Although built-in C pointers may be Trivial Iterators, the C type system does not allow a distinction between pointers that are Trivial Iterators and pointers that are Random Access Iterators into C arrays. Consequently, there is no Trivial Iterator category tag or iterator base.

[2] The iterator_traits class relies on a C++ feature known as partial specialization. Many of today's compilers don't implement the complete standard; in particular, many compilers do not support partial specialization. If your compiler does not support partial specialization, then you will not be able to use iterator_traits, and you will have to continue using the functions iterator_category, distance_type, and value_type. This is one reason that those functions have not yet been removed.

The Iterator Tags overview, iterator_traits, iterator_category, distance_type, output_iterator_tag, input_iterator_tag, forward_iterator_tag, bidirectional_iterator_tag, random_access_iterator_tag

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