C++ - boost get question

Question

Does someone know if the boost::get for the boost::variant is a performance-consuming operation or not.

Right now I am refactoring some old code in a performance-critical part, where "varianting" was implementing by containers for each possible types and corresponding enum.

Obviously, this is fast, but ugly and right now when I have to refactor the code so that it would work with one more type, I want to get rid of that old part of code and replace it with boost::variant.

Also, I can't simple "profile both variants and compare" because this refactoring is a pain in the ass and would be pretty time consuming.

So, if someone knows how does boost::get<x> performs comparing to generic enum-based type dispatching, I would appreciate if you share this knowledge.

There is another variant of using boost::variant<types> with custom visitor (as described in boost::variant documentation) - may this be faster than boost::get in my case?

Thank you.

Answer 1

You could still write a simple test-application to compare the two, it doesn't have to be the production environment.

A colleague of mine had a similar problem to this one recently. In his scenario there where objects of different types, but he always knew beforehand which type he expected. Also his data-structure was huge, so memory was an issue. He solved the problem by using void * and reinterpret_cast. This prevents the memory-overhead of polymorphism and is very fast. You have to be absolutely sure of what you are doing though, otherwise things will explode.

Answer 2

Looking at the code, get<> is implemented using the internal visitor mechanism of the boost::variant. In turn, the visitor mechanism relies on mpl sequences, and it is executed step by step. That means at most n steps on an n type variant, but the loop (recursive call) is there. Again, as Space Cowboy suggests, a small performance test would be helpful.