std::set<Key,Compare,Allocator>::count
From cppreference.com
| size_type count( const Key& key ) const; |
(1) | |
| template< class K > size_type count( const K& x ) const; |
(2) | (since C++14) |
Returns the number of elements with key that compares equivalent to the specified argument.
1) Returns the number of elements with key key. This is either 1 or 0 since this container does not allow duplicates.
2) Returns the number of elements with key that compares equivalent to the value x. This overload participates in overload resolution only if the qualified-id Compare::is_transparent is valid and denotes a type. It allows calling this function without constructing an instance of
Key.Parameters
| key | - | key value of the elements to count |
| x | - | alternative value to compare to the keys |
Return value
Number of elements with key that compares equivalent to key or x, which, for overload (1), is either 1 or 0.
Complexity
Logarithmic in the size of the container plus linear in the number of elements found.
Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_generic_associative_lookup |
201304L | (C++14) | Heterogeneous comparison lookup in associative containers; overload (2) |
Example
Run this code
#include <functional> #include <iostream> #include <set> struct S { int x; S(int i) : x{i} { std::cout << "S{" << i << "} "; } bool operator<(S const& s) const { return x < s.x; } }; struct R { int x; R(int i) : x{i} { std::cout << "R{" << i << "} "; } bool operator<(R const& r) const { return x < r.x; } }; bool operator<(R const& r, int i) { return r.x < i; } bool operator<(int i, R const& r) { return i < r.x; } int main() { std::set<int> t{3, 1, 4, 1, 5}; std::cout << t.count(1) << ", " << t.count(2) << ".\n"; std::set<S> s{3, 1, 4, 1, 5}; std::cout << ": " << s.count(1) << ", " << s.count(2) << ".\n"; // Two temporary objects S{1} and S{2} were created. // Comparison function object is defaulted std::less<S>, // which is not transparent (has no is_transparent member type). std::set<R, std::less<>> r{3, 1, 4, 1, 5}; std::cout << ": " << r.count(1) << ", " << r.count(2) << ".\n"; // C++14 heterogeneous lookup; temporary objects were not created. // Comparator std::less<void> has predefined is_transparent. }
Output:
1, 0.
S{3} S{1} S{4} S{1} S{5} : S{1} 1, S{2} 0.
R{3} R{1} R{4} R{1} R{5} : 1, 0.See also
| finds element with specific key (public member function) | |
| returns range of elements matching a specific key (public member function) |