std::ranges::empty(3) | C++ Standard Libary | std::ranges::empty(3) |
NAME¶
std::ranges::empty - std::ranges::empty
Synopsis¶
Defined in header <ranges>
Defined in header <iterator>
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.
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)
2024.06.10 | http://cppreference.com |