std::ranges::size

 

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Ranges library

Range access begin cbegin   end cend rbegin crbegin   rend crend size ssize   data cdata empty

Range conversions std::from_range_t std::from_range (C++23)(C++23)     to (C++23)

Dangling iterator handling dangling borrowed_iterator_t borrowed_subrange_t

Range primitives
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)

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

Views
view_interface	subrange

Range factories
empty_viewviews::empty	single_viewviews::single	basic_istream_viewviews::istream	iota_viewviews::iota	repeat_viewviews::repeat (C++23)(C++23)

Range adaptors
views::all_tviews::all ref_view owning_view as_rvalue_viewviews::as_rvalue (C++23)(C++23)   filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while concat_viewviews::concat (C++26)(C++26)	drop_viewviews::drop drop_while_viewviews::drop_while join_viewviews::join join_with_viewviews::join_with (C++23)(C++23)   lazy_split_viewviews::lazy_split split_viewviews::split views::counted common_viewviews::common	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)

Range adaptor closure objects range_adaptor_closure (C++23)

Range adaptor objects

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

 

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.

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 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

• std::invocable<__size_fn, Args...>,
• std::invocable<const __size_fn, Args...>,
• std::invocable<__size_fn&, Args...>, and
• std::invocable<const __size_fn&, Args...>.
  

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

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_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

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 (niebloid)
sizessize (C++17)(C++20)	returns the size of a container or array (function template)