Preprocessor behind the scenes, how does it interpret the following lines of code C++

Question

So i'm using a preprocessing macro for this basic square function :

#define SQUARE(a) a*a

it is then passed to these functions, to perform these tasks respectively:

$f(x) = x^2, g(x) = (1-x)^2, h(x) = 1/x^2$

double f(double x) {

    return SQUARE(x);

}

double g(double x) {

    return SQUARE(1-x);

}

double h(double x) {

    return 1/SQUARE(x);

}

with such functions, for g(x) and h(x) i'm getting results such as:

 g(2) = -3, h(2) = 1

 g(3) = -5, h(3) = 1

 g(4) = -7, h(4) = 1

 etc...

I have changed the functions to get their expected results, but I'm curious as to what is happening in the above functions to give me such strange results?

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Answer 1

SQUARE(1-x) will be expanded to 1-x*1-x which is not correct at all.

It would be much better to use a function instead of a macro. A function only evaluates the parameter once.

double square(double a)
{ return a * a; }

Preprocessor behind the scenes, how does it interpret the following lines of code C++

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