如何使用宏作为函数指针？

Question

How can I use macros as function pointers? I have no idea to solve this. I created a sketch (doesn't work, full of syntax errors) to show what I try to accomplish. Please help!

#define D0_OUT(x) (x/1024*100) //I want to use this for Pin0 calculation
#define D1_OUT(x) (x*1024) //I want to use this for Pin1 calculation

struct Pin {
  CalcMethod *calcMethod; //int methodName(int x) { return MACRO(x); }

  Pin(CalcMethod *calcMethodParam) {
    calcMethod = calcMethodParam;
  }

  int calc(int x) {
    return calcMethod(x);
  }
};

#define PIN_COUNT 2
Pin *pins[PIN_COUNT];

void start() {
    pins[0] = new Pin(D0_OUT); //use the D0_OUT macro to calculate
    pins[1] = new Pin(D1_OUT); //use the D1_OUT macro to calculate
    int pin0CalcResult=pins[0]->calc(5); // =5/1024*100
    int pin1CalcResult=pins[1]->calc(6); // =6*1024
}

Answer 1

Macros are handled by the preprocessor. They don't exist in the compiled code, therefore there is no pointer.

There is one rule you should follow in modern code and that rule is "don't use macros for furnctions". Macros for functions are a relict that still has some good uses but they are very rare.

Just declare a normal function

int do_out(int x) {
    return x / 1024 * 100;
}

Also see "static const" vs "#define" vs "enum"

Answer 2

You can, but not advisable , use macros as named lambdas. Thus

#define D0_OUT [](int x) { return x / 1024 * 100; }
#define D1_OUT [](auto x) { return x * 1024; }

and it should work.

D0_OUT example usable in C++11 and D1_OUT usable with C++14.

Answer 3

I know this is an old thread..

Assuming that you cannot just change the macro to be a function. Maybe it is part of a driver of library somewhere and you need to pass it into another function for some reason like unit testing. You can just wrap the macro within your .c file where you want to use it.

So this:

#define D0_OUT(x) (x/1024*100) //I want to use this for Pin0 calculation

becomes:

static int D0_OUT_wrapper(int x)
{
    return D0_OUT(x);
}

So wrapper goes in like normal:

pins[0] = new Pin(D0_OUT_wrapper);

If you have full control of the code you are writing then just don't use macros.