Difference between revisions of "cpp/ranges/empty"

Revision as of 11:16, 3 July 2020

Determines whether or not t has any elements.

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

1. bool(std::forward<T>(t).empty()), if that expression is valid.
2. Otherwise, (ranges::size(std::forward<T>(t)) == 0), if that expression is valid.
3. Otherwise, bool(ranges::begin(t) == ranges::end(t))

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

All instances of __empty_fn are equal. The effects of invoking different instances of type __empty_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::empty 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::empty above, __empty_fn models

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

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

Example

#include <iostream>
#include <ranges>
#include <vector>
 
template <std::ranges::input_range R>
void print(R&& r)
{
    if (std::ranges::empty(r)) {
        std::cout << "\tEmpty\n";
        return;
    }
 
    std::cout << "\tElements:";
    for (const auto& element : r) {
        std::cout << ' ' << element;
    }
 
    std::cout << '\n';
}
 
int main()
{
    {
        auto v = std::vector<int>{1, 2, 3};
        std::cout << "1. calling ranges::empty on std::vector:\n";
        print(v);
 
        v.clear();
        print(v);
    }
    {
        std::cout << "2. calling ranges::empty on std::initializer_list:\n";
        auto il = {7, 8, 9};
        print(il);
 
        print(std::initializer_list<int>{});
    }
    {
        std::cout << "2. calling ranges::empty on a raw array:\n";
        int array[] = {4, 5, 6}; // array has a known bound
        print(array);
    }
}

1. calling ranges::empty on std::vector:
	Elements: 1 2 3
	Empty
2. calling ranges::empty on std::initializer_list:
	Elements: 7 8 9
	Empty
2. calling ranges::empty on a raw array:
	Elements: 4 5 6

See also