std::set find() method

since 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;

until C++14

// (1) Const version
size_type count( const Key& key ) const;

• (1-2) Finds an element with key equivalent to key.
• (3-4) Finds an element 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 - a value of any type that can be transparently compared with a key

Return value

Iterator to an element with key equivalent to key. If no such element is found, past-the-end (see end()) iterator is returned.

Complexity

Logarithmic in the size of the container - O(log size()).

Exceptions

(none)

Notes

Feature testing macro: __cpp_lib_generic_unordered_lookup (for overload (3-4)).

Example

Main.cpp

#include <iostream>
#include <set>
 
struct FatKey   { int x; int data[1000]; };
struct LightKey { int x; };
// Note: as detailed above, the container must use std::less<> (or other 
//   transparent Comparator) to access these overloads.
// This includes standard overloads, such as between std::string and std::string_view.
bool operator<(const FatKey& fk, const LightKey& lk) { return fk.x < lk.x; }
bool operator<(const LightKey& lk, const FatKey& fk) { return lk.x < fk.x; }
bool operator<(const FatKey& fk1, const FatKey& fk2) { return fk1.x < fk2.x; }
 
int main()
{  
// simple comparison demo
    std::set<int> example = {1, 2, 3, 4};
 
    auto search = example.find(2);
    if (search != example.end()) {
        std::cout << "Found " << (*search) << '\n';
    } else {
        std::cout << "Not found\n";
    }
 
// transparent comparison demo
    std::set<FatKey, std::less<>> example2 = { {1, {} }, {2, {} }, {3, {} }, {4, {} } };
 
    LightKey lk = {2};
    auto search2 = example2.find(lk);
    if (search2 != example2.end()) {
        std::cout << "Found " << search2->x << '\n';
    } else {
        std::cout << "Not found\n";
    }
 
}

Possible output

Found 2
Found 2