Default values on arguments in C functions and function overloading in C

Question

Converting a C++ lib to ANSI C and it seems like though ANSI C doesn\'t support default values for function variables or am I mistaken? What I want is something like

Answer 1

Nevertheless I found a "trick" to do so if you use GCC.

GCC has a handy ## extension on variadic macro that allows you to simulate a default argument.

The trick has limitations: it works only for 1 default value, and the argument must be the last of you function parameters.

Here is a working example.

#include <stdio.h>


#define SUM(a,...) sum( a, (5, ##__VA_ARGS__) )

int sum (a, b)
  int a;
  int b;
{
  return a + b;
}


main()
{

  printf("%d\n", SUM( 3, 7 ) );

  printf("%d\n", SUM( 3 ) );

}

In this case, I define SUM as a call to sum with the default second argument being 5.

If you call with 2 arguments (first call in main), it would be prepocessed as: sum( 3, (5, 7) );

This means:

• 1st argument is 3
• second argument is the result of the sequence (5, 7)... which is obviously 7!

As gcc is clever, this has no effect on runtime as the first member of the sequence is a constant and it is not needed, it will simply be discarded at compile time.

If you call with only one argument, the gcc extension will remove the VA_ARGS AND the leading coma. So it is preprocessed as:

sum( 3, (5 ) );

Thus the program gives the expected output:

10
8

So, this does perfectly simulate (with the usual macro limitations) a function with 2 arguments, the last one being optional with a default value applied if not provided.

Edit

-a) It does also work with CLANG (and possibly other compilers)

-b) A version that does NOT complain about unused arguments:

#define DEF_OR_ARG(z,a,arg,...) arg
#define SUM(a,...) sum( a, DEF_OR_ARG(,##__VA_ARGS__,__VA_ARGS__,5))

[Edit - October 2020] : You could also try the new __VA_OPT__ that was standardised with c++2a (and should work in plain C too) instead of ## which is a gcc extension. Typical usage is __VA_OPT__(,) that would add the coma when the argument list is non-empty and otherwise outputs nothing.

Answer 2

As far as I know ANSI C doesn't directly support function overloading or default arguments. The standard substitute for overloading is adding suffixes to the function name indicating the argument types. For example, in OpenGL, a "3fv" suffix to a function name means the function takes a vector of three floats.

Default arguments can be viewed as a special case of function overloading.

Answer 3

Try this.

#include <stdio.h>
#include <stdarg.h>

/* print all non-negative args one at a time;
   all args are assumed to be of int type */
void printargs(int arg1, ...)
{
  va_list ap;
  int i;

  va_start(ap, arg1); 
  for (i = arg1; i >= 0; i = va_arg(ap, int))
    printf("%d ", i);
  va_end(ap);
  putchar('\n');
}

int main(void)
{
   printargs(5, 2, 14, 84, 97, 15, 24, 48, -1);
   printargs(84, 51, -1);
   printargs(-1);
   printargs(1, -1);
   return

 0;
}

Answer 4

You can't so easily since C does not support them. The simpler way to get "fake overloading" is using suffixes as already said... default values could be simulated using variable arguments function, specifying the number of args passed in, and programmatically giving default to missing one, e.g.:

 aType aFunction(int nargs, ...)
 {
   // "initialization" code and vars
   switch(nargs)
   {
     case 0:
         // all to default values... e.g.
         aVar1 = 5; // ...
         break;
     case 1:
         aVar1 = va_arg(arglist, int); //...
         // initialize aVar2, 3, ... to defaults...
         break;
     // ...
   }
 }

Also overloading can be simulated using var args with extra informations to be added and passed and extracode... basically reproducing a minimalist object oriented runtime ... Another solution (or indeed the same but with different approach) could be using tags: each argument is a pair argument type + argument (an union on the whole set of possible argument type), there's a special terminator tag (no need to specify how many args you're passing), and of course you always need "collaboration" from the function you're calling, i.e. it must contain extra code to parse the tags and choose the actual function to be done (it behaves like a sort of dispatcher)

Answer 5

You'll have to declare each C++ overloaded function differently in C because C doesn't do name mangling. In your case "foo_property1" "foo_property2".

Answer 6

Neither of default values or function overloading exists in ANSI C, so you'll have to solve it in a different way.