std::ranges::ssize

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)
• 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

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...>
• 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

#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