Question

Say I have more than one key with the same value in a map. Then in that case how do I retrieve all keys that matches a query.

Or, Is there any possibility to tell find operation to search after a specific value.
I am using an std::map, C++.

Answer 1

The associative containers probably won’t help you too much because for std::map<K, V> the key happens to be unique and chances that your chosen query matches the ordering relation you used may not be too high. If the order matches, you can use the std::map<K, V> members lower_bound() and upper_bound(). For std::multimap<K, V> you can also use equal_range().

In general, i.e., if you query isn’t really related to the order, you can use std::copy_if() to get a sequence of objects matching a predicate:

Other other;
// ...
std::vector<Other::value_type> matches;
std::copy_if(other.begin(), other.end(), 
             std::back_inserter(matches), predicate);

When copying the elements is too expensive, you should probably consider using std:find_if() instead:

for (auto it(other.begin());
    other.end() != (it = std::find_if(it, other.end(), predicate));
    ++it) {
   // do something with it
}