Calculates the number of elements int in constant time.

Given thesubexpression of whicht denotes the (possiblymaterialized) result object asE, and the type ofE asT:

• IfT is an array of unknown bound,ranges::size(E) is ill-formed.
• Otherwise, ifT is an array type,ranges::size(E) isexpression-equivalent todecay-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 todecay-copy (t.size())(until C++23)auto(t.size())(since C++23):
  • ranges::disable_sized_range<std::remove_cv_t<T>> isfalse.
  • decay-copy (t.size())(until C++23)auto(t.size())(since C++23) is a valid expression ofinteger-like type.
• Otherwise, if all following conditions are satisfied,ranges::size(E) is expression-equivalent todecay-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>> isfalse.
  • decay-copy (size(t))(until C++23)auto(size(t))(since C++23) is a valid expression of integer-like type, where the meaning ofsize is established as if by performingargument-dependent lookup only.
• Otherwise, if all following conditions are satisfied,ranges::size(E) is expression-equivalent toto-unsigned-like (ranges::end(t)-ranges::begin(t)):
  • T modelsforward_range.
  • Given the type ofranges::begin(t) asI and the type ofranges::end(t) asS, bothsized_sentinel_for<S, I> andforward_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 insubstitution failure whenranges::size(E) appears in the immediate context of a template instantiation.

Customization point objects

The nameranges::size denotes acustomization point object, which is a constfunction object of aliteralsemiregular class type. SeeCustomizationPointObject for details.

[edit]Notes

Wheneverranges::size(e) is valid for an expressione, the return type isinteger-like.

The C++20 standard requires that if the underlyingsize 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 proposalP0849R8 to match the implementations.

The expressionranges::distance(e) can also be used to determine the size of a rangee. Unlikeranges::size(e),ranges::distance(e) works even ife 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_liststd::cout<<"ranges::size(il) == "<< std::ranges::size(il)<<'\n'; int array[]{4,5};// array has a known boundstd::cout<<"ranges::size(array) == "<< std::ranges::size(array)<<'\n';     static_assert(std::is_signed_v<decltype(std::ranges::size(v))>==false);}

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

[edit]Defect reports

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

[edit]See also

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