std::ranges::size

 

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Ranges library

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begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
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to (C++23)
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range_size_trange_difference_trange_value_trange_reference_t	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)	elements_of (C++23)
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dangling	borrowed_iterator_t	borrowed_subrange_t
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range borrowed_range sized_range constant_range (C++23)	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
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empty_viewviews::empty single_viewviews::single	iota_viewviews::iota repeat_viewviews::repeat (C++23)(C++23)	basic_istream_viewviews::istream
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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	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)
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range_adaptor_closure (C++23)
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copyable-box movable-box (until C++23)(C++23)	simple-view non-propagating-cache

 

Defined in header <ranges>
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 overload resolution is performed with the following candidates:
   • any declarations of size found by argument-dependent lookup
   • void size(auto&) = delete;
   • void size(const auto&) = delete;
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.

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.

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

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)