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.
key. This is either 1 or 0 since this container does not allow duplicates.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.| key | - | key value of the elements to count |
| x | - | alternative value to compare to the keys |
Number of elements with key that compares equivalent to key or x, which, for overload (1), is either 1 or 0.
Logarithmic in the size of the container plus linear in the number of elements found.
#include <set>
#include <iostream>
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.| finds element with specific key (public member function) |
|
| returns range of elements matching a specific key (public member function) |
© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/container/set/count