std::ranges::ssize

 

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Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range conversions
to (C++23)
Range primitives
range_size_trange_difference_t range_value_t	range_reference_trange_const_reference_trange_rvalue_reference_t   (C++23)	iterator_tconst_iterator_tsentinel_t (C++23)
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
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
Views
view_interface	subrange
Factories
empty_viewviews::empty iota_viewviews::iota	single_viewviews::single basic_istream_viewviews::istream	repeat_viewviews::repeat (C++23)(C++23)
Adaptors
ref_view views::all_tviews::all common_viewviews::common owning_view views::counted reverse_viewviews::reverse zip_viewviews::zip (C++23)(C++23) as_const_viewviews::as_const (C++23)(C++23) as_rvalue_viewviews::as_rvalue (C++23)(C++23)	take_viewviews::take take_while_viewviews::take_while drop_viewviews::drop drop_while_viewviews::drop_while zip_transform_viewviews::zip_transform (C++23)(C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23)   cartesian_product_viewviews::cartesian_product (C++23)(C++23)	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform adjacent_viewviews::adjacent (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) stride_viewviews::stride (C++23)(C++23)	filter_viewviews::filter join_viewviews::join split_viewviews::split lazy_split_viewviews::lazy_split join_with_viewviews::join_with (C++23)(C++23) chunk_viewviews::chunk (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23)
Range adaptor objects
Range adaptor closure objects
Helper items
copyable-box	simple-view

 

Defined in header <ranges>
inline namespace /*unspecified*/ {     inline constexpr /*unspecified*/ ssize = /*unspecified*/; }		(since C++20) (customization point object)
Call signature
template< class T >     requires /* see below */ constexpr /*signed-integer-like*/ ssize( T&& t );		(since C++20)

Returns the size of a range converted to a signed type.

If ranges::size(std::forward<T>(t)) is well-formed, a call to ranges::ssize is expression-equivalent to static_cast<MadeSigned>(ranges::size(std::forward<T>(t))), where MadeSigned denotes

• the corresponding signed version of decltype(ranges::size(std::forward<T>(t))), if it is wider than std::ptrdiff_t, or
• std::ptrdiff_t otherwise.

If ranges::size(std::forward<T>(t)) is ill-formed, a call to ranges::ssize is also ill-formed, which can result in substitution failure when ranges::ssize(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::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, i.e. an integer type for which std::is_signed_v is true, or a signed-integer-class type.

The width of integer-like types can be detected by std::numeric_limits::digits.

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 didn't	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)
sizessize (C++17)(C++20)	returns the size of a container or array (function template)