std::ranges::size

 

C++

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Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

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Technical Specifications

 

Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range primitives
iterator_tsentinel_t	range_size_trange_difference_t	range_value_t	range_reference_trange_rvalue_reference_t
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
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
Views
view_interface	subrange
Semiregular wrapper
Factories
empty_viewviews::empty	single_viewviews::single	iota_viewviews::iota	views::counted
Adaptors
views::all_tviews::all join_viewviews::join take_viewviews::take basic_istream_viewistream_view	ref_view                                split_viewviews::split take_while_viewviews::take_while elements_viewviews::elements	common_viewviews::common transform_viewviews::transform drop_viewviews::drop keys_viewviews::keys	reverse_viewviews::reverse filter_viewviews::filter drop_while_viewviews::drop_while values_viewviews::values

 

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);

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, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

Customization point objects

The name ranges::size denotes a customization point object, which is a const function object of a literal semiregular class type (denoted, for exposition purposes, as size_ftor). All instances of size_ftor are equal. 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_ftor will satisfy std::invocable<const size_ftor&, Args...>. Otherwise, no function call operator of size_ftor participates in overload resolution.

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::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';
 
    std::cout << std::boolalpha << "is_signed: "
              << std::is_signed_v<decltype(std::ranges::size(v))> << '\n';
}

Output:

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

See also

ranges::ssize (C++20)	obtains the size of a range whose size can be calculated in constant time and converts it to signed integral (customization point object)
sizessize (C++17)(C++20)	returns the size of a container or array (function template)