Question

I want to know if we can have recursive macros in C/C++? If yes, please provide a sample example.

Second thing: why am I not able to execute the below code? What is the mistake I am doing? Is it because of recursive macros?

# define pr(n) ((n==1)? 1 : pr(n-1))
void main ()
{
    int a=5;
    cout<<"result: "<< pr(5) <<endl;
    getch();
}

Answer 1

Your compiler probably provides an option to only pre-process, not actually compile. This is useful if you are trying to find a problem in a macro. For example using g++ -E:

> g++ -E recursiveMacro.c

# 1 "recursiveMacro.c"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "recursiveMacro.c"

void main ()
{
    int a=5;
    cout<<"result: "<< ((5==1)? 1 : pr(5 -1)) <<endl;
    getch();
}

As you can see, it is not recursive. pr(x) is only replaced once during pre-processing. The important thing to remember is that all the pre-processor does is blindly replace one text string with another, it doesn’t actually evaluate expressions like (x == 1).

The reason your code will not compile is that pr(5 -1) was not replaced by the pre-processor, so it ends up in the source as a call to an undefined function.