finding all the values with given key for multimap

I am searching for all pairs for a particular key in a multimap using the code below.

int main() {
    multimap<int,int> mp;
    mp.insert({1,2});
    mp.insert({11,22});
    mp.insert({12,42});
    mp.insert({1,2});
    mp.insert({1,2});

    for (auto itr = mp.find(1); itr != mp.end(); itr++)
        cout << itr->first<< '\t' << itr->second << '\n';
}

You're calling find only a single time in your code. It's perfectly acceptable for this call to return the same value as mp.begin() resulting in you iterating though all the entries in the map before reaching mp.end().

You can use the equal_range member function to get iterators for the start and end of the elements with key 1:

for (auto[itr, rangeEnd] = mp.equal_range(1); itr != rangeEnd; ++itr)
{
    std::cout << itr->first<< '\t' << itr->second << '\n';
}