Switch betweeen method versions

I need to rewrite some c++ classes and I would like to be able to switch between the new code and the old code (reliable and fast). What would be good options to do so? In general I need some kind of switch that decides what to do, like:

int foo::bar()
{
   if (yesUseNew == true)
  {
    return 1 + 1;
  }
  else
  {
    return 1 + 2;
  }
}

HOW and where could i set yesUseNew? It should be useable in Debug-Build and in Release and it should be applied directly. So reading some xml-Config would be to late.

The attempt, with two code-versions directly in the methods, is only an example. At this point I am not sure on which abstraction level I will do this. The primary question is actually HOW I can distinguish between the versions (fast).

Thanks!

>Solution :

Well, I see the following alternatives :

1. At compile time with #define

e.g:

#define V1  //Comment this line if you want V2. You can also define it on the command line of your compiler

void myfunction() {
#ifdef V1
//V1 Implementation
#else
//V2 Impl
#endif

2. At runtime with env variables for example

This means you can switch without recompiling.

if (std::getenv("V1"))
    //V1 Code
else
    //V2 Code

3. Add a configuration option to your app, either in the command line or the config file if it has one. If the dual behaviour will last this is the way to go