std::ranges::ssize

 

C++

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Feature test macros (C++20)

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Concepts library (C++20)

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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 /* unspecified */ ssize = /* unspecified */; }		(since C++20) (customization point object)
Call signature
template< class T >     requires /* see below */ constexpr auto ssize( T&& t );		(since C++20)

Calculates the number of elements in t in constant time, and converts the result to a signed type.

Given the subexpression of which t denotes the (possibly materialized) result object as E:

• If ranges::size(t) is ill-formed, ranges::ssize(E) is also ill-formed.
• Otherwise, let Signed be make-signed-like-t <decltype(ranges::size(t))>:
  • If std::ptrdiff_t is wider than Signed, ranges::ssize(E) is expression-equivalent to static_cast<std::ptrdiff_t>(ranges::size(t)).
  • Otherwise, ranges::ssize(E) is expression-equivalent to static_cast<Signed>(ranges::size(t)).

Customization point objects

The name ranges::ssize 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 __ssize_fn.

All instances of __ssize_fn are equal. The effects of invoking different instances of type __ssize_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::ssize 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::ssize above, __ssize_fn models

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

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

Notes

If ranges::ssize(e) is valid for an expression e, the return type is a signed-integer-like type.

Example

Run this code

#include <array>
#include <iostream>
#include <ranges>
#include <type_traits>
 
int main()
{
    std::array arr{1, 2, 3, 4, 5};
    auto s = std::ranges::ssize(arr);
 
    std::cout << "ranges::ssize(arr) = " << s << '\n'
              << "ranges::ssize is "
              << (std::is_signed_v<decltype(s)> ? "signed" : "unsigned")
              << '\n';
 
    std::cout << "reversed arr: ";
 
    for (--s; s >= 0; --s)
        std::cout << arr[s] << ' ';
 
    std::cout << "\n" "s = " << s << '\n';
}

Output:

ranges::ssize(arr) = 5
ranges::ssize is signed
reversed arr: 5 4 3 2 1
s = -1

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3403	C++20	ranges::size worked for some non-range types but ranges::ssize did not	made work

See also

ranges::size (C++20)	returns an 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)