std::ranges::size

 

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Ranges library (C++20)

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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);

Calculates the number of elements in t in constant time.

Let t be an object of type T. A call to ranges::size is expression-equivalent to:

1. std::extent_v<T>, if T is an array type with a known bound.
2. Otherwise, std::forward<T>(t).size(), if not ranges::disable_sized_range<std::remove_cv_t<T>>, and std::forward<T>(t).size() is valid and returns a type that is /*integer-like*/.

   where /*integer-like*/ is a type that models std::integral or is a class that behaves like an integer type, including all operators, implicit conversions, and std::numeric_limits specializations.

3. Otherwise, size(std::forward<T>(t)), if not ranges::disable_sized_range<std::remove_cv_t<T>>, and size(std::forward<T>(t)) is valid and returns a type that is /*integer-like*/, where the overload resolution is performed with the following candidates:
   • void begin(auto&) = delete;
   • void begin(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 /*integer-like*/ type that behaves like an unsigned integer type, including all operators, implicit conversions, and std::numeric_limits specializations.

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.

Expression-equivalent

Expression e is expression-equivalent to expression f, if

• e and f have the same effects, and
• either both are constant subexpressions or else neither is a constant subexpression, and
• either both are potentially-throwing or else neither is potentially-throwing (i.e. noexcept(e) == noexcept(f)).

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.

Example

Run this code

#include <iostream>
#include <ranges>
#include <vector>
 
int main()
{
    {
        auto v = std::vector<int>{};
        std::cout << "ranges::size(v) == " << std::ranges::size(v) << '\n';
    }
    {
        auto il = {7};
        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';
    }
}

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