Is declaration of variables expensive?

Question

While coding in C, I came across the below situation.

int function ()
{
  if (!somecondition) return false;

  internalStructure  *str1;
  internalStructure *str         


        

Answer 1

If it actually mattered the only way to avoid allocating the variables is likely to be:

int function_unchecked();

int function ()
{
  if (!someGlobalValue) return false;
  return function_unchecked();
}

int function_unchecked() {
  internalStructure  *str1;
  internalStructure *str2;
  char *dataPointer;
  float xyz;

  /* do something here with the above local variables */    
}

But in practice I think you'll find no performance benefit. If anything a minuscule overhead.

Of course if you were coding C++ and some of those local variables had non-trivial constructors you would probably need to place them after the check. But even then I don't think it would help to split the function.