Question

Let’s say you have a function in C/C++, that behaves a certain way the first time it runs. And then, all other times it behaves another way (see below for example). After it runs the first time, the if statement becomes redundant and could be optimized away if speed is important. Is there any way to make this optimization?

bool val = true; 

void function1() {

   if (val == true) {
      // do something
      val = false; 
   }
   else {
      // do other stuff, val is never set to true again 
   }

}

Answer 1

You should only make the change if you’re certain that it truly is a bottleneck. With branch-prediction, the if statement is probably instant, since it’s a very predictable pattern.

That said, you can use callbacks:

#include <iostream>
using namespace std;
typedef void (*FunPtr) (void);
FunPtr method;
void subsequentRun()
{
    std::cout << "subsequent call" << std::endl;
}
void firstRun()
{
    std::cout << "first run" << std::endl;
    method = subsequentRun;  
}
int main()
{
    method = firstRun;
    method();
    method();
    method();
}

produces the output:

first run
subsequent call
subsequent call