预处理器常量评估

Question

I have a small question about the preprocessor constants in C. I understand what is the goal of this kind of "variable" and how it works. However, I have a small question about their evaluation. Let's consider a small example :

#define MY_VARIABLE 5

int main() {
    int test1 = MY_VARIABLE*5;
    int test2 = 5;
    int test3 = MY_VARIABLE*test2;
}

During the preprocessor step, MY_VARIABLE will be replaced by 5 in the code. My question is: will test1 be computed during the compilation or during the execution ? Is that correct that test3 will be computed during the execution ? This question may seem a bit weird and useless but I'm studying a program where this kind of things is done a lot of times and I would like to know if this kind of operation can slow the execution.

Answer 1

As you have already understood, the C preprocessor merely replaces the macros before the actual compilation takes place, therefore I stripped out all preprocessor related stuff.

In this code no evaluation takes place during execution because the compiler calculates 5*5 during compile time and by inference is can also evaluate all other constants during compilation:

int main() {
    int test1 = 5 * 5;
    int test2 = 5;
    int test3 = 5 * test2;
}

The exact equivalent of the preceeding code fragment is:

int main() {
    int test1 = 25;
    int test2 = 5;
    int test3 = 25;
}

But in following code test3 is evaluated during run time, because the value of test2 cannot be determined during compile time, because it depends on the return value of SomeFunction that can only be known at run time.

int main() {
    int test1 = 5 * 5;
    int test2 = SomeFunction();        
    int test3 = 5 * test2;
}