Searching values in set and multiset

There are several ways to search a given value in std::set or in std::multiset:

To get the iterator of the first occurrence of a key, the find() function can be used. It returns end() if the key does not exist.

std::set<int> sut;
sut.insert(10);
sut.insert(15);
sut.insert(22);
sut.insert(3); // contains 3, 10, 15, 22    

auto itS = sut.find(10); // the value is found, so *itS == 10
itS = sut.find(555); // the value is not found, so itS == sut.end()

std::multiset<int> msut;
sut.insert(10);
sut.insert(15);
sut.insert(22);
sut.insert(15);
sut.insert(3); // contains 3, 10, 15, 15, 22

auto itMS = msut.find(10);

Another way is using the count() function, which counts how many corresponding values have been found in the set/multiset (in case of a set, the return value can be only 0 or 1). Using the same values as above, we will have:

int result = sut.count(10); // result == 1
result = sut.count(555); // result == 0

result = msut.count(10); // result == 1
result = msut.count(15); // result == 2

In the case of std::multiset, there could be several elements having the same value. To get this range, the equal_range() function can be used. It returns std::pair having iterator lower bound (inclusive) and upper bound (exclusive) respectively. If the key does not exist, both iterators would point to the nearest superior value (based on compare method used to sort the given multiset).

auto eqr = msut.equal_range(15);
auto st = eqr.first; // point to first element '15'
auto en = eqr.second; // point to element '22'

eqr = msut.equal_range(9); // both eqr.first and eqr.second point to element '10'