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std::ranges::ssize

From cppreference.com
< cpp‎ | ranges
 
 
 
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-integral-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, let MadeSigned denote

a call to ranges::ssize is expression-equivalent to static_cast<MadeSigned>(ranges::size(std::forward<T>(t))).

Otherwise, a call to ranges::ssize is ill-formed, which can result in substitution failure when ranges::ssize(t) appears in the immediate context of a template instantiation.

Contents

[edit] Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

[edit] 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.

[edit] 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.

[edit] 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';
}

Output:

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

[edit] 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

[edit] See also

returns an unsigned integer equal to the size of a range
(customization point object) [edit]
(C++17)(C++20)
returns the size of a container or array
(function template) [edit]