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

A switch construct is faster (or at least not slower).

That's mostly because a switch construct gives static data to the compiler, while a runtime structure like a hash map doesn't.

When possible compilers should compile switch constructs into array of code pointers: each item of the array (indexed by your indexes) points to the associated code. At runtime this takes O(1), while a hash map could take more: O(log n) at average case or O(n) at worst case, usually, and anyway a bigger constant number of memory accesses.