std::ranges::empty - cppreference.com (original) (raw)

| Defined in header | | | | ---------------------------------------------------------------------------------------- | | ------------------------------------------ | | Defined in header | | | | inline namespace /*unspecified*/ { inline constexpr auto empty = /*unspecified*/; } | | (since C++20) (customization point object) | | Call signature | | | | template< class T > requires /* see below */ constexpr bool empty( T&& t ); | | (since C++20) |

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. See CustomizationPointObject for details.

[edit] Example

#include #include #include   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{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{}); } { 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 { using std::vector::begin; using std::vector::end; using std::vector::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

[edit] See also

| | checks whether the container is empty (function template) [edit] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |