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

In C, I believe all variable declarations are applied as if they were at the top of the function declaration; if you declare them in a block, I think it's just a scoping thing (I don't think it's the same in C++). The compiler will perform all optimizations on the variables, and some may even effectively disappear in the machine code in higher optimizations. The compiler will then decide how much space is needed by the variables, and then later, during execution, create a space known as the stack where the variables live.

When a function is called, all of the variables that are used by your function are put on the stack, along with information about the function that is called (i.e. the return address, parameters, etc.). It doesn't matter where the variable was declared, just that it was declared - and it will be allocated onto the stack, regardless.

Declaring variables isn't "expensive," per se; if it's easy enough to be not used as a variable, the compiler will probably remove it as a variable.

Check this out:

Wikipedia on call stacks, Some other place on the stack

Of course, all of this is implementation-dependent and system-dependent.