How to use a function defined using a macro

Question

I have defined the macro as follows


#define COMPARE(OPERATOR, FUNCTION)                                         \ 
    template <typename T>                                                   \
    void FUNCTION(T *compArrayA, T *compArrayB, bool *resArray, int size) { \
        for (int idx = 0; idx <= size; ++idx ) {                                  \
            resArray[idx] = (compArrayA[idx] OPERATOR compArrayB[idx]);  \
        }                                                                   \
    }                                                                       \

COMPARE(==, eq);
COMPARE(!=, nq);
COMPARE(>=, greater_eq);

So, I tried to call the function defined in the macro as follows.

float *ArrayA, *ArrayB;
bool *ArrayRes;

ArrayA = (float *) malloc(sizeof(float)* 100);
ArrayB = (float *) malloc(sizeof(float)* 100);
ArrayRes = (bool *) malloc(sizeof(bool) * 100);

eq(ArrayA, ArrayB, ArrayRes, 100);

I received the following error.

eq.h(11): error: expected an expression

1 error detected in the compilation of "eq.cpp".

eq.h(11) represents the following line.

COMPARE(==, eq);

How do I define a function in a macro?

I just want to know how to implement functions using macros.

which line is eq.h(11)? did you remember to put your code that does things inside a function? — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented Jul 19, 2021 at 10:40
Please try to create a proper minimal reproducible example to show us. — Some programmer dude
– Some programmer dude, Commented Jul 19, 2021 at 10:41
On another couple of notes, you have some other issues in the code you show... Like a one-off error in the loop which will cause it to go out of bounds; Using malloc instead of new[] (or better yet, std::vector or std::array); And using uninitialized values. — Some programmer dude
– Some programmer dude, Commented Jul 19, 2021 at 10:42

Victor Gubin · Accepted Answer · 2021-07-19 13:10:06Z

2

Short answer, don't do that. Since you've templates you can use standard library approach from functional header

without any macro definitions at all.

For example:

template<typename E, class Pred>                                                   
void vcompare(E *compArrayA, E *compArrayB, bool *resArray, int size) {
    for (int idx = 0; idx <= size; ++idx ) {                            
        resArray[idx] = Pred(compArrayA[idx], compArrayB[idx]);
    }                                                               
}

vcompare<int,std::less>(lhs,rhs,res,len);
vcompare<int,std::greater>(lhs,rhs,res,len);

P.S. Check std::vector class and general STL part inside standard C++ library.

edited Jul 19, 2021 at 13:10

answered Jul 19, 2021 at 11:05

Victor Gubin

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1 Comment

Some programmer dude Over a year ago

Using std::vector and std::transform (as suggested in one comment) would make this even easier:

std::vector<float> v1(100), v2(100); std::vector<bool> res(100); std::transform(begin(v1), end(v1), begin(v2), begin(res), std::less<float>());

Jarod42 · Accepted Answer · 2021-07-19 10:48:40Z

1

As you define your MACRO, you shouldn't use ; after it, so

COMPARE(==, eq)
COMPARE(!=, nq)
COMPARE(>=, greater_eq)

instead of

COMPARE(==, eq);
COMPARE(!=, nq);
COMPARE(>=, greater_eq);

answered Jul 19, 2021 at 10:48

Jarod42

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2 Comments

musako Over a year ago

Thanks for answering. I removed the \ in the last line of the macro and removed the ; in the macro declaration, but I still get the same error

Eljay Over a year ago

@BoKuToTsuZenU • The code you've posted here is not the code that is producing the error you are experiencing. Please provide a minimal reproducible example. Help us help you.

kingpin · Accepted Answer · 2021-07-19 11:42:09Z

0

What you are trying to do is to pass operator as an argument.

Passing an operator as an argument to a function in C

As Victor Gubin already answered it is best to avoid macro and embrace modern alternatives to do the same you are trying to achieve.

answered Jul 19, 2021 at 11:42

kingpin

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