用void重载函数

Question

Trying to overloading a function in C programming language with _Generic . Have found examples:

• https://www.quora.com/Does-C-support-function-overloading
• How to achieve function overloading in C?

This the example:

#include <stdio.h>
#include <math.h>

void display_float(float dispid) ;
void display_int(int dispid) ;
void display_void(void) ;

#define display(x) _Generic((x), \
          float: display_float, \
          int: display_int,  \
          default: display_float  \
)(x)

void display_void(void){
    printf("display id: 0\n") ;
}

void display_float(float dispid){
   printf("display id: %f\n", dispid) ;
}

void display_int(int dispid){
    printf("display id: %d\n", dispid) ;
}

void main(void){

    display(5) ;
    display(6.5) ;
}

Now I also want to overload the function with display() . Meaning the function would intake a void , call the function display_void() , and display 0. Can't seem to do this. Any help would be appreciated.

EDIT 1:

Also, see the examples here . One of the examples (I think) is passing a pointer to a void. Can this be implemented?

Answer 1

Sure. Overload Macro on Number of Arguments .

void display_float(float dispid) ;
void display_int(int dispid) ;
void display_void(void) ;

#define _display_1(x) _Generic((x), \
          float: display_float, \
          int: display_int,  \
          default: display_float  \
)(x)
#define _display_0()  display_void()
#define _display_N(_0,_1,N,...)  _display_ ## N
#define display(...) _display_N(_0,##__VA_ARGS__,1,0)(__VA_ARGS__)

int main() {
    display();
    display(1);
    display(1.1);
}

Tested on godbolt .

Note that , ##__VA_ARGS__ is a gcc extension not described in the C standard.

Answer 2

Trying to overloading a function in C programming language

C11 does not have any function overloading and _Generic is for genericity , not for overloading, check that by reading n1570 . Consider switching to C++ (or even better, Rust ) which provides function overloading. Notice that it is illegal to have a void (not void* , just void ) argument to a C function, so there is no any "top" super-type in C, which would be a super type of other types (and this because of calling conventions and ABI considerations making C a low-level programming language, sometimes even called a " portable assembler"). Read more about abstract interpretation and type systems please.

If _Generic was for overloading, the members of the C11 standard committee - all programming language experts and good enough English writers - would have named it like _Overload , but they wisely did not.

Food for thought (assuming both stderr and stdin works like you want them to work) :
what about some hypothetical display(stderr) or display(*stdin) or display("abc") ?

^{(a wrong suggestion below, followed by a better advice)}

bad approach

( this does not work! )

Your display_void should take some argument, eg

void display_void(void*p) { printf("p@%p\n", p);

Then you might have:

#define display_anything(P) display_void((void*)(&(P)))

with

#define display(x) _Generic((x), \
      float: display_float, \
      int: display_int,  \
      default: display_anything  \
)(x)

better advice

Since that does not work (and it does not), design your code in some other way . For example, consider having some tagged union , like here . Look into GVariant (from Glib) implementation for inspiration.