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 will add my 5 cents:

For the number of entries at about 50 std::unordered_map (hash based, O(1)) is typically slower then std::map (tree based O(ln(N))), and both of them are slower then boost::flat_map(sorted vector O(ln(N))) which I tend to use in such cases. It is not always the case that switch can be compiled to jump table, and when it is, you can simply put your values (or functions) in vector yourself and access by index. Otherwise switch is marginally faster than boost::flat_map.

Please note the word "typically" in the beginning, if you do care about performance of this piece of code do the profiling (and share results with us :)).