std::ranges::size

 

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Ranges library

Range access begin cbegin   end cend rbegin crbegin   rend crend size ssize   data cdata reserve_hint (C++26)   empty

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range_size_trange_difference_trange_value_t elements_of (C++23)	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_reference_trange_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)

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range borrowed_range    common_range	sized_range viewable_range    view	input_range output_range forward_range	bidirectional_range    random_access_range contiguous_range	approximately_sized_range (C++26) constant_range (C++23)

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views::all_tviews::all as_rvalue_viewviews::as_rvalue (C++23)(C++23)   filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while common_viewviews::common views::counted to_input_viewviews::to_input (C++26)(C++26)	ref_view drop_viewviews::drop drop_while_viewviews::drop_while lazy_split_viewviews::lazy_split split_viewviews::split join_viewviews::join join_with_viewviews::join_with (C++23)(C++23)   concat_viewviews::concat (C++26)(C++26) cache_latest_viewviews::cache_latest (C++26)(C++26)	owning_view reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	adjacent_viewviews::adjacent (C++23)(C++23) views::pairwise (C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23) views::pairwise_transform (C++23) chunk_viewviews::chunk (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) stride_viewviews::stride (C++23)(C++23) cartesian_product_viewviews::cartesian_product (C++23)(C++23)

Range generators std::generator (C++23)

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copyable-box movable-box (until C++23)  (C++23)	simple-view non-propagating-cache

 

Defined in header <ranges>
Defined in header <iterator>
inlinenamespace/* unspecified */{     inlineconstexprauto size =/* unspecified */; }		(since C++20) (customization point object)
Call signature
template<class T >     requires /* see below */ constexprauto size( T&& t );		(since C++20)

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.

Customization point objects

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

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.

Example

Run this code

#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);}

Output:

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

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

See also

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