Example: default value for optional macro argument

main.c

/* Copyright (c) 2019 Michael Mogenson
 * MIT license https://opensource.org/licenses/MIT
 */

/* This contrived example demonstrates the use of the comma operator and
 * variadic arguments to create a function-like macro that accepts an optional
 * argument and provides a default value when no argument is supplied.
 *
 * C's comma operator will evaluate the statement to the left of the comma and
 * return the value of the statement to the right. The line `if (i++, true)` is
 * equivalent to `i++; if (true)`.
 *
 * The C preprocessor macro __VA_ARGS__ will expand to the arguments (if any)
 * passed to a function-like macro. The __VA_OPT__() macro will insert the
 * content between the parenthesis only if __VA_ARGS__ is not empty.
 *
 * This BREAK() macro will break out of a loop if a supplied conditional
 * evaluates to true. If no argument is provided, a default true value is used.
 */

#include <stdio.h>

#define BREAK(...) if (1 __VA_OPT__(,) __VA_ARGS__) break

int main() {
  
  while (1) {
    printf("break immediately\n");
    BREAK(); // => if (1) break;
  }

  for (int i = 0; i < 5; i++) {
    printf("i = %d\n", i);
    BREAK(i > 3); // => if (1, i > 3) break;
  }

  return 0;
}