C++: What is faster - lookup in hashmap or switch statement?

Question

I have a code pattern which translates one integer to another. Just like this:

int t(int value) {
    switch (value) {
        case 1: return const_1;
        ca         


        

Answer 1

I think it is not obvious which is going to be faster. You might need to profile both approaches.

The hash map should have complexity of O(1).

The switch (with non-contiguous keys like yours) may be optimized into a binary search (at least with GCC), which has complexity of O(log n).

On the other hand, any operation done on a hash map will be much more expensive than an operation done in a switch.