Difference between revisions of "cpp/ranges/empty"

Revision as of 09:31, 8 September 2023

Determines whether or not t has any elements.

A call to ranges::empty is expression-equivalent to:

1. bool(t.empty()), if that expression is valid.
2. Otherwise, (ranges::size(t) == 0), if that expression is valid.
3. Otherwise, bool(ranges::begin(t) == ranges::end(t)), if that expression is valid and decltype(ranges::begin(t)) models std::forward_iterator.

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.

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(char id, R&& r)
{
    if (std::ranges::empty(r))
    {
        std::cout << '\t' << id << ") Empty\n";
        return;
    }
 
    std::cout << '\t' << id << ") Elements:";
    for (const auto& element : r)
        std::cout << ' ' << element;
 
    std::cout << '\n';
}
 
int main()
{
    {
        auto v = std::vector<int>{1, 2, 3};
        std::cout << "(1) ranges::empty uses std::vector::empty:\n";
        print('a', v);
 
        v.clear();
        print('b', v);
    }
    {
        std::cout << "(2) ranges::empty uses ranges::size(initializer_list):\n";
        auto il = {7, 8, 9};
        print('a', il);
 
        print('b', std::initializer_list<int>{});
    }
    {
        std::cout << "(2) ranges::empty on a raw array uses ranges::size:\n";
        int array[] = {4, 5, 6}; // array has a known bound
        print('a', array);
    }
    {
        struct Scanty : private std::vector<int> {
            using std::vector<int>::begin;
            using std::vector<int>::end;
            using std::vector<int>::push_back;
            // Note: both empty() and size() are hidden
        };
 
        std::cout << "(3) calling ranges::empty on an object w/o empty() or size():\n";
        Scanty y;
        print('a', y);
        y.push_back(42);
        print('b', y);
    }
}

(1) ranges::empty uses std::vector::empty:
        a) Elements: 1 2 3
        b) Empty
(2) ranges::empty uses ranges::size(initializer_list):
        a) Elements: 7 8 9
        b) Empty
(2) ranges::empty on a raw array uses ranges::size:
        a) Elements: 4 5 6
(3) calling ranges::empty on an object w/o empty() or size():
        a) Empty
        b) Elements: 42

See also