Confusion with DEBUG macro

Question

test.cpp

#include <iostream>
#include "Class1.h"
#define DEBUG

int main() {
    checkAssert();
}

Class1.h

#include <cassert>

#ifndef CLASS1_H_
#define CLASS1_H_

#if defined(DEBUG)

void checkAssert(){
    int number = 10;
    assert(number == 10);
}


#else

void checkAssert(){
    std::cout << "opps" << std::endl;
}

#endif /* DEBUG */
#endif /* CLASS1_H_ */

1. I have defined DEBUG in main file.

2.In Class1.h #if defined(DEBUG) is for check if DEBUG is defined or not(according to my understanding).

I am trying this program to understand how DEBUG macro works in c++, But every time I have opps output in the screen. Can anyone please help me to understand what is going on.

Answer 1

Preprocessor does text substitution. Once it pastes class1.h into your TU's file you have (I ignored expanding standard headers for the sake of brevity)

#include <iostream>

#include <cassert>

#ifndef CLASS1_H_
#define CLASS1_H_

#if defined(DEBUG)

void checkAssert(){
    int number = 10;
    assert(number == 10);
}


#else

void checkAssert(){
    std::cout << "opps" << std::endl;
}

#endif /* DEBUG */
#endif /* CLASS1_H_ */

#define DEBUG

int main() {
    checkAssert();
}

As you can see, DEBUG is defined after the check for it. Simply move it above the relevant #include to get the behaviour you want.

Answer 2

Your test.cpp sets the macro after the header file is included already. That's too late. You have to set the macro before including the header file:

#define DEBUG
#include <Class1.h>