How to elegantly implement a series of functions in different type versions using pure C?

Question

I want to write several functions that are only different in the types of arguments. I know C++ has template to handle this problem well (not very well yet thou

Answer 1

You can use union:

#include 
#include 

typedef enum {Int, Double} NumberType;

typedef struct {
  NumberType type;
  union {
    int i;
    double d;
  };
} Number;

Number addNumber(Number a, Number b) {
  Number ret;
  Number *numbers[] = {&a, &b};
  if (a.type == Int && b.type == Int ){
    ret.type = Int;
    ret.i = a.i + b.i;
  }
  else {
    ret.type = Double;
    char i;
    for (i = 0; i < 2 && numbers[i]->type == Int; i++) {
      numbers[0]->d = (double) numbers[i]->i;
    }
    ret.d = a.d + b.d;
  }
  return ret;
}

Number newNumber(NumberType type, ...) {
  va_start(ap, type);
  Number num;
  num.type = type;
  switch (type) {
    case Int: {
      num.i = va_arg(ap, int);
      break;
    }
    case Double: {
      num.d = va_arg(ap, double);
      break;
    }
    default: { /* error */
      num.type = -1;
    }
  }
  va_end(ap);
  return num;
}


int main(void) {
  Number a = newNumber(Int, 1);
  Number b = newNumber(Double, 3.0);
  Number ret = addNumber(a, b);
  switch (ret.type) {
    case Int: {
      printf("%d\n", ret.i);
    }
    case Double: {
      printf("%f\n", ret.d);
    }
  }
  return 0;
}