Difference between declaring variables before or in loop?

Question

I have always wondered if, in general, declaring a throw-away variable before a loop, as opposed to repeatedly inside the loop, makes any (performance) difference? A (q

Answer 1

My practice is following:

• if type of variable is simple (int, double, ...) I prefer variant b (inside).
  Reason: reducing scope of variable.

• if type of variable is not simple (some kind of class or struct) I prefer variant a (outside).
  Reason: reducing number of ctor-dtor calls.