What does preprocessing exactly mean in compiler

Question

I am trying to understand the difference between a typedef and define. There are a lot of good posts specially at this previous question on SO, however I can't understand the post that states

#define is a preprocessor token: the compiler itself will never see it.
typedef is a compiler token: the preprocessor does not care about it.

Could anyone please explain this with a little more detail. I am confused with the term preprocessor here.

Answer 1

The pre-processor is a program that runs before the compiler and essentially performs text substitution. When you write:

#define X 10

int main() 
{
    int x = X;
}

The pre-processor take that file as input, does it's thing, and outputs:

int main() 
{
    int x = 10;
}

And then the compiler does its thing with the pre-processed output.

typedef on the other hand is a construct that the compiler understands. When you write:

typedef unsigned int uint_32;

The compiler knows that uint32 is actually an alias for unsigned int. This alias is handled by the compiler itself and involves a bit more logic than simple text substitution. This becomes obvious with a simple example:

typedef int my_int;

int main()
{
    unsigned my_int x;  // oops, error
}

If a typedef were a simple text substitution mechanism (like the pre-processor is) then that would work, but it is not allowed and will fail to compile.