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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../coroutine/../ranges/size.html below:

std::ranges::size - cppreference.com

Call signature

Calculates the number of elements in t in constant time.

Given the subexpression of which t denotes the (possibly materialized) result object as E, and the type of E as T:

• If T is an array of unknown bound, ranges::size(E) is ill-formed.
• Otherwise, if T is an array type, ranges::size(E) is expression-equivalent to decay-copy (std::extent_v<T>)(until C++23)auto(std::extent_v<T>)(since C++23).
• Otherwise, if all following conditions are satisfied, ranges::size(E) is expression-equivalent to decay-copy (t.size())(until C++23)auto(t.size())(since C++23):
  • ranges::disable_sized_range<std::remove_cv_t<T>> is false.
  • decay-copy (t.size())(until C++23)auto(t.size())(since C++23) is a valid expression of integer-like type.
• Otherwise, if all following conditions are satisfied, ranges::size(E) is expression-equivalent to decay-copy (size(t))(until C++23)auto(size(t))(since C++23):
  • T is a class or enumeration type.
  • ranges::disable_sized_range<std::remove_cv_t<T>> is false.
  • decay-copy (size(t))(until C++23)auto(size(t))(since C++23) is a valid expression of integer-like type, where the meaning of size is established as if by performing argument-dependent lookup only.
• Otherwise, if all following conditions are satisfied, ranges::size(E) is expression-equivalent to to-unsigned-like (ranges::end(t) - ranges::begin(t)):
  • T models forward_range.
  • Given the type of ranges::begin(t) as I and the type of ranges::end(t) as S, both sized_sentinel_for<S, I> and forward_iterator<I> are modeled.
  • to-unsigned-like (ranges::end(t) - ranges::begin(t)) is a valid expression.
• Otherwise, ranges::size(E) is ill-formed.

Diagnosable ill-formed cases above result in substitution failure when ranges::size(E) appears in the immediate context of a template instantiation.

The name ranges::size denotes a customization point object, which is a const function object of a literal semiregular class type. See CustomizationPointObject for details.

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.

#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

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

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