C++ switch table performance

Question

Does the time taken by a switch statement (or jump table in compiled form) to "decide" where to jump rise by the number of casees it contains?

Answer 1

It depends on the compiler and (typically) the values you supply -- if the values are "dense" (i.e., all or almost all values in a range have cases in the switch statement) you'll typically get a jump table, which takes the same time for all values (in that range). If you have relatively sparse values, it may compile to code roughly equivalent to an if/then/else ladder, in which case adding more (sparse) case values can increase execution time.

Answer 2

Not really, there's usually a hash table or some other O(1) lookup data structure in the compiled code (unless you only have a tiny amount of switches, then the compiler may decide to use jumps instead). In general a large amount of switches should outperform a large amount of if statements, although normally you wouldn't have enough cases for it to be noticeable anyways.