NAME

std::ranges::ssize - std::ranges::ssize

Synopsis

Defined in header <ranges>
inline namespace /*unspecified*/ {

inline constexpr /*unspecified*/ ssize = (since C++20)
/*unspecified*/; (customization point object)

}
Call signature
template< class T >

requires /* see below */ (since C++20)

constexpr /*signed-integer-like*/ ssize( T&& t );

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 made work
but ranges::ssize didn't

See also

ranges::size returns an integer equal to the size of a range
(C++20) (customization point object)
ranges::sized_range specifies that a range knows its size in constant time
(C++20) (concept)
size
ssize returns the size of a container or array
(C++17) (function template)
(C++20)