Example of Macro and Pre -Processor Directives

#include <iostream>
using namespace std;

int main()
{
    #define sum a+b

    int a=10, b=20,c;

    #ifdef sum
    c=sum*2;
    cout<<c<<endl;
    #endif

    #ifndef sum
    cout<<"Sum is not defined"<<endl;
    #endif

    return 0;
}