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std::ranges::size

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< cpp‎ | ranges
 
 
Ranges library
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(C++23)

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(until C++23)(C++23)


 
Defined in header <ranges>
Defined in header <iterator>
inline namespace /* unspecified */ {

    inline constexpr auto size = /* unspecified */;

}
(since C++20)
(customization point object)
Call signature
template< class T >

    requires /* see below */

constexpr auto size( T&& t );
(since C++20)

Calculates the number of elements in t in constant time.

A call to ranges::size is expression-equivalent to:

  1. decay-copy(std::extent_v<T>)(until C++23)auto(std::extent_v<T>)(since C++23), if T is an array type with a known bound.
  2. Otherwise, decay-copy(t.size())(until C++23)auto(t.size())(since C++23), if ranges::disable_sized_range<std::remove_cv_t<T>> is false, and that expression is valid and has an integer-like type.
  3. Otherwise, decay-copy(size(t))(until C++23)auto(size(t))(since C++23), if ranges::disable_sized_range<std::remove_cv_t<T>> is false, and the converted expression is valid and has an integer-like type, where the meaning of size is established as if by performing argument-dependent lookup only.
  4. Otherwise, /* to-unsigned-like */(ranges::end(t) - ranges::begin(t)), if T models ranges::forward_range and ranges::sentinel_t<T> models std::sized_sentinel_for<ranges::iterator_t<T>>,
    where /* to-unsigned-like */ denotes an explicit conversion to an unsigned-integer-like type.

In all other cases, a call to ranges::size is ill-formed, which can result in substitution failure when ranges::size(t) appears in the immediate context of a template instantiation.

Contents

Customization point objects

The name ranges::size denotes a customization point object, which is a const function object of a literal semiregular class type. For exposition purposes, the cv-unqualified version of its type is denoted as __size_fn.

All instances of __size_fn are equal. The effects of invoking different instances of type __size_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, ranges::size can be copied freely and its copies can be used interchangeably.

Given a set of types Args..., if std::declval<Args>()... meet the requirements for arguments to ranges::size above, __size_fn models

Otherwise, no function call operator of __size_fn participates in overload resolution.

[edit] Notes

Whenever ranges::size(e) is valid for an expression e, the return type is integer-like.

The C++20 standard requires that if the underlying size function call returns a prvalue, the return value is move-constructed from the materialized temporary object. All implementations directly return the prvalue instead. The requirement is corrected by the post-C++20 proposal P0849R8 to match the implementations.

The expression ranges::distance(e) can also be used to determine the size of a range e. Unlike ranges::size(e), ranges::distance(e) works even if e is an unsized range, at the cost of having linear complexity in that case.

[edit] Example

#include <iostream>
#include <ranges>
#include <type_traits>
#include <vector>
 
int main()
{
    auto v = std::vector<int>{};
    std::cout << "ranges::size(v) == " << std::ranges::size(v) << '\n';
 
    auto il = {7};     // std::initializer_list
    std::cout << "ranges::size(il) == " << std::ranges::size(il) << '\n';
 
    int array[]{4, 5}; // array has a known bound
    std::cout << "ranges::size(array) == " << std::ranges::size(array) << '\n';
 
    static_assert(std::is_signed_v<decltype(std::ranges::size(v))> == false);
}

Output:

ranges::size(v) == 0
ranges::size(il) == 1
ranges::size(array) == 2

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
P2602R2 C++20 there's machinery to prohibit certain non-member size found by ADL removed such machinery

[edit] See also

returns a signed integer equal to the size of a range
(customization point object)[edit]
specifies that a range knows its size in constant time
(concept) [edit]
returns the distance between an iterator and a sentinel, or between the beginning and end of a range
(niebloid)[edit]
(C++17)(C++20)
returns the size of a container or array
(function template) [edit]