Question

Consider below functions

f(int a[])
{
     ///CODE
     for
      for
       if(a[i] > 0)
       //change i on some condition
     for
        //CODE
        if(a[i] > 0)
    ///CODE

}
f(int a[], int th)
{
     ///CODE
     for
      for
        if(a[i] < th)
       //change i on some condition
     for
        //CODE
        if(a[i] < th)
    ///CODE
}

So we have f function with a optimized huge body,
several lines are the same: if(a[i] > 0)
I want to add extend this function so that if a parameter is added, those lines should change to if(a[i] < th)
When I overload the function, hundreds of line is duplicated (becomes hard to maintain), so i don’t want this. Also I cannot divide the body into functions because “the line” appears in too many inner loops.

First idea:

f(int a[], int th = -1)
{
     ///CODE
        if(th == -1)
            if(a[i] > 0)...
        else
            if(a[i] < th)...
     ///CODE
}

I cannot do this because of the performance overhead of introducing the additional if to the inner loop.
Is there a way to solve it both efficiently and clearly, perhaps using templates or macros?

Answer 1

You can pass functions via template arguments:

template<bool Cmp(int,int)>
void f(int a[], int th)
{
    //..
    if (Cmp(a[i],th)) {
       //...
    }
    //...
}

You can then use f like this:

bool cmp(int a, int b) { return a < b; }
f<cmp>(...)