Why putting a macros argument in parentheses leads to an error?

Question

I have one very interesting question about preprocessor directives in c++.

Consider the following macros and his usage:

    #define FUNCTION(a, b) void (a)(int &current, int candidate)\
    {\
        if ((current b candidate) == false){\      // Marked Line    
            current = candidate;\
        }\
    }

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

My question is why changing the "Marked Line" with the following line of code won't even compile:

     ... if ((current (b) candidate) == false) ...

Your error has nothing to do with macros. [See for yourself](https://ideone.com/zWCajh). — Drew Dormann, Jul 24 '18 at 16:30
Instead of showing code that works, show the code that doesn't work. — Pete Becker, Jul 24 '18 at 16:41
Why not just take a functor, like std::greater, or std::less? — Paul Belanger, Jul 24 '18 at 16:45
This question is not interesting at all. Using preprocessor directives this way is abominable and disgusting. — , Jul 24 '18 at 17:07

score 2 · Accepted Answer · answered Jul 24 '18 at 16:31

2

Because '<' is a binary operator and it cannot be evaluated without an operand on either side. You can verify this without macros simply by attempting to compile the following code:

bool LessThan( int a, int b )
{
    return( a (<) b );
}

At the very least you should see "expected an expression" or a similar error.

answered Jul 24 '18 at 16:31

Tim Randall

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Ok. I understood it but what is the idea of putting the macros argument in parentheses so? – Tony Troeff Jul 24 '18 at 16:36
1

See [this question/answer](https://stackoverflow.com/questions/7186504/c-macros-and-use-of-arguments-in-parentheses) – Kevin Jul 24 '18 at 16:47
2

Normally people advise you to put arguments in parentheses because the macro preprocessor does very simple, literal text substitutions that can lead to incorrect code. The advice breaks down when a macro argument is an operator rather than an expression. – Tim Randall Jul 24 '18 at 16:54
Thank you, Tim. I appreciate your help! – Tony Troeff Jul 24 '18 at 18:01

Why putting a macros argument in parentheses leads to an error?

1 Answers1