std::ranges::ssize

inline namespace /*unspecified*/ {
    inline constexpr /*unspecified*/ ssize = /*unspecified*/;
}

template< class T >
    requires /* see below */
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.

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

#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';
}

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.

See also