std::set count() method
- since C++14
- until C++14
// (1) Const version
size_type count( const Key& key ) const;
// (2) Const version
template< class K >
size_type count( const K& x ) const;
// (1) Const version
size_type count( const Key& key ) const;
Returns the number of elements with key that compares equivalent to the specified argument, which is either 1 or 0 since this container does not allow duplicates.
- (1) Returns the number of elements with key
key
. - (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-idCompare::is_transparent
is valid and denotes a type. They allow calling this function without constructing an instance ofKey
.
Parameters
key
- key value of the elements to countx
- a value of any type that can be transparently compared with a key
Return value
Number of elements with key that compares equivalent to key or x
, which is either 1 or 0 for (1).
Complexity
Logarithmic in the size of the container - O(log size()).
Exceptions
(none)
Example
Main.cpp
#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.
}
Possible 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.
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