Understanding macros used as functions

Question

In this snippet, I can't understand what the FUNCTION(func,pred) is supposed to be doing.

#define FUNCTION(func,pred)\
void func(int& a, int x){ \
if(!(a pred x)) a = x; \
}

I also can't understand this:

FUNCTION(minimum, <)
FUNCTION(maximum, >)

Can somebody explain please?

• Thomas Matthews: What part of the code confuses you?
• NathanOliver: It would be helpful if you went through and explain how you think it works and we can correct you if that is incorrect/there is missing understanding.
• ethelake: Hello Thomas, I am being confused about of "FUNCTION (func,pred)" of the preprocessor and two FUNCTIONs just above int main().
• Some programmer dude: Use the macro, and then run the file though the preprocessor and see that the macro invocation is replaced with. All good compilers have options to stop after running the preprocessor.
• ethelake: Hello Nathan, thanks for the feedback I should edit my questions :)

Answer 1

The preprocessor substitutes all tokens: FUNCTION(func,pred) with

void func(int& a, int x){
if(!(a pred x)) a = x;
}

Thus, defining a function func with a binary operator as pred. So, when you write this:

FUNCTION(minimum,<)

The preprocessor replaces that with:

void minimum(int& a, int x){
 if(!(a < x))
 a = x;
}

---

Same thing happens with:

FUNCTION(maximum,>)

produces:

void maximum(int& a, int x){
 if(!(a > x))
 a = x;
}

...and so on..

• ethelake: oh I see it now, I didn't know that I can even replace the binary operators as words in preprocessor. Thank you so much for your kind explanation.

Answer 2

The macro FUNCTION is defining a function with the name func. For example:

FUNCTION(minimum, <)

Would define a function like so:

void minimum(int& a, int x) {
 if(!(a < x)) {
 a = x;
 }
}

Macros are a method of code generation. The author of this code obviously felt it was too arduous to write out this 2 line function and instead confuse everyone with macros.

This is definitely a case where macros harm (readability) rather than help. I don't recommend doing this for such basic functions.

• Lightness Races in Orbit: He's also confused everyone with terrible function naming & semantics. I would never expect this function to mutate an arbitrary selection of its arguments.
• ethelake: Hello Colin, thank you for your help. I can certainly see it better with your rephrase :)
• ethelake: Hello again Lightness, it was actually part of the questions in Hackerrank so wanted you to know that I didn't want to confuse everybody :)