NAME

std::ranges::empty - std::ranges::empty

Synopsis

Defined in header <ranges>
inline namespace /*unspecified*/ {
(since C++20)
inline constexpr auto empty = /*unspecified*/; (customization point object)

}
Call signature
template< class T >

requires /* see below */ (since C++20)

constexpr bool empty( T&& t );

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.

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

// Run this code

#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);
}
}

Output:

(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

empty checks whether the container is empty
(C++17) (function template)