Difference between revisions of "cpp/ranges/size"

Revision as of 20:32, 3 July 2020

Calculates the number of elements in t in constant time.

Let t be an object of type T. A call to ranges::size is expression-equivalent to:

1. std::extent_v<T>, if T is an array type with a known bound.
2. Otherwise, std::forward<T>(t).size(), if not ranges::disable_sized_range<std::remove_cv_t<T>>, and std::forward<T>(t).size() is valid and returns a type that is /*integer-like*/.

   where /*integer-like*/ is a type that models std::integral or is a class that behaves like an integer type, including all operators, implicit conversions, and std::numeric_limits specializations.

3. Otherwise, size(std::forward<T>(t)), if not ranges::disable_sized_range<std::remove_cv_t<T>>, and size(std::forward<T>(t)) is valid and returns a type that is /*integer-like*/, where the overload resolution is performed with the following candidates:
   • void begin(auto&) = delete;
   • void begin(const auto&) = delete;
4. Otherwise, /*to-unsigned-like*/(ranges::end(t) - ranges::begin(t)), if T models ranges::forward_range and ranges::sentinel_t<T> models std::sized_sentinel_for<ranges::iterator_t<T>>.

   where /*to-unsigned-like*/ denotes an explicit conversion to an /*integer-like*/ type that behaves like an unsigned integer type, including all operators, implicit conversions, and std::numeric_limits specializations.

In all other cases, a call to ranges::size is ill-formed, which can result in substitution failure when ranges::size(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::size 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 __size_fn.

All instances of __size_fn are equal. The effects of invoking different instances of type __size_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::size 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::size above, __size_fn models

• std::invocable<__size_fn, Args...>,
• std::invocable<const __size_fn, Args...>,
• std::invocable<__size_fn&, Args...>, and
• std::invocable<const __size_fn&, Args...>.
  

Otherwise, no function call operator of __size_fn participates in overload resolution.

Example

#include <iostream>
#include <ranges>
#include <vector>
 
int main()
{
    {
        auto v = std::vector<int>{};
        std::cout << "ranges::size(v) == " << std::ranges::size(v) << '\n';
    }
    {
        auto il = {7};
        std::cout << "ranges::size(il) == " << std::ranges::size(il) << '\n';
    }
    {
        int array[] = {4, 5}; // array has a known bound
        std::cout << "ranges::size(array) == " << std::ranges::size(array) << '\n';
    }
}

ranges::size(v) == 0
ranges::size(il) == 1
ranges::size(array) == 2

See also