Designing A class with multiple set of method implementation [c++]

I am working on an existing project which has one class for which I need alternate member function definition. I don't want to modify existing member function or their signature, just an alternate definition which has to be selected compile time based on some xml filed (Compile time flag not preferred) I am new C++ experience so may be a stupid question. Any design Guidelines for such situation where I don't have to change and test existing code base and just plug my implementation. May be I am thinking in wrong direction, what would be the right approach in that case.