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

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

int funcName(int foo, bar* = NULL);

Also, is function overloading possible in ANSI C?

Would need

const char* foo_property(foo_t* /* this */, int /* property_number*/);

const char* foo_property(foo_t* /* this */, const开发者_运维知识库 char* /* key */, int /* iter */);

Could of course just name them differently but being used to C++ I kinda used to function overloading.

No, Standard C does not support either. Why do you feel you need to convert your C++ code to C? That could get quite tricky - I'd have thought writing wrappers would be the way to go, if your C++ must be callable from C.

Nevertheless I found a "trick" to do so if you use GCC (edit December 2020) or any compiler compatible with C++2a -yes it works with 'plain C' since it is a pre-compiler trick-.

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.

[Edit - December 2020] : So the above trick, with __VA_OPT__, becomes:

#define DEF_OR_ARG(value,...) value
#define SUM(a,...) sum( a, DEF_OR_ARG(__VA_ARGS__ __VA_OPT__(,) 5))

Unlike the 'sequence trick' that might complain for unused variables, this only involves the pre-compiler and is more readable.

When SUM is called with only one argument, ... is empty and __VA_OPT__ does not output anything, thus DEF_OR_ARG(__VA_ARGS__ __VA_OPT(,) 5) becomes DEF_OR_ARG( 5)

When SUM is called with a second argument, ... is this second argument and __VA_OPT__ expands to the value given which is a coma. In that case DEF_OR_ARG(__VA_ARGS__ __VA_OPT(,) 5) becomes DEF_OR_ARG(second_argument_of_SUM , 5)

Now the expansion of DEF_OR_ARG happens. This one is easy since it considers only the first argument and just discards the rest. So, when SUM was called with no second argument (first case above), the first argument to DEF_OR_ARG is our default value. In the case there was a second argument to SUM, it becomes the first argument to DEF_OR_ARG that will expand to that and discard the default which is now second argument.

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;
}

There is a way to support as many default parameters you need, just use a structure.

// Populate structure with var list and set any default values
struct FooVars {
  int int_Var1 = 1;  // One is the default value
  char char_Command[2] = {"+"};
  float float_Var2 = 10.5;
};
struct FooVars MainStruct;

//...
// Switch out any values needed, leave the rest alone
MainStruct.float_Var2 = 22.8;
Myfunc(MainStruct);  // Call the function which at this point will add 1 to 22.8.
//...

void Myfunc( struct FooVars *MyFoo ) {
  switch(MyFoo.char_Command) {
    case '+':
      printf("Result is %i %c %f.1 = %f\n" MyFoo.int_Var1, MyFoo.char_Command, MyFoo.float_Var2, (MyFoo.float_Var2 + MyFoo.int_Var1);
      break;
    case '*':
      // Insert multiply here, you get the point...
      break;
    case '//':
      // Insert divide here...
      break;
  }
}

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.

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

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)

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".

i think u can use a function with variable arguments here is my example

#include <stdarg.h>
#include <stdio.h>
void baz( int flag, ... )
{
    va_list ap;
    char *bar = "baz"; /* default value */
    va_start( ap, flag );
    if ( flag == 1 )
        bar = va_arg( ap, char * );
    va_end( ap );
    printf( "%s\n", bar );
}
int main( void )
{
    baz( 0 );
    baz( 1, "foo");
    baz( 2 );
    baz( 1, "bar");
    return 0;
}

the output is

baz
foo
baz
bar

if u look for example man 2 open they say

SYNOPSIS
       #include <sys/types.h>
       #include <sys/stat.h>
       #include <fcntl.h>

       int open(const char *pathname, int flags);
       int open(const char *pathname, int flags, mode_t mode);

       int creat(const char *pathname, mode_t mode);

       int openat(int dirfd, const char *pathname, int flags);
       int openat(int dirfd, const char *pathname, int flags, mode_t mode);

but mode is actually a ... argument