Question

Inside a large loop, I currently have a statement similar to

if (ptr == NULL || ptr->calculate() > 5) 
  {do something} 

where ptr is an object pointer set before the loop and never changed.

I would like to avoid comparing ptr to NULL in every iteration of the loop. (The current final program does that, right?) A simple solution would be to write the loop code once for (ptr == NULL) and once for (ptr != NULL). But this would increase the amount of code making it more difficult to maintain, plus it looks silly if the same large loop appears twice with only one or two lines changed.

What can I do? Use dynamically-valued constants maybe and hope the compiler is smart? How?

Many thanks!

EDIT by Luther Blissett. The OP wants to know if there is a better way to remove the pointer check here:

loop {
 A; 
 if (ptr==0 || ptr->calculate()>5) B;
 C;
}

than duplicating the loop as shown here:

if (ptr==0) 
loop {
 A; 
 B;
 C;
} 
else loop {
 A;
 if (ptr->calculate()>5) B;
 C;
}

Answer 1

In C++, although completely overkill you can put the loop in a function and use a template. This will generate twice the body of the function, but eliminate the extra check which will be optimized out. While I certainly don’t recommend it, here is the code:

template<bool ptr_is_null>
void loop() {
    for(int i = x; i != y; ++i) {
       /**/
       if(ptr_is_null || ptr->calculate() > 5) {
          /**/
       }
       /**/
    }
}

You call it with:

if (ptr==NULL) loop<true>(); else loop<false>();

You are better off without this “optimization”, the compiler will probably do the RightThing(TM) for you.