How to achieve function overloading in C?

Question

Is there any way to achieve function overloading in C? I am looking at simple functions to be overloaded like

foo (int a)  
foo (char b)  
foo (float c , int d)

I think there is no straight forward way; I'm looking for workarounds if any exist.

Answer 1

Yes!

In the time since this question was asked, standard C (no extensions) has effectively gained support for function overloading (not operators), thanks to the addition of the _Generic keyword in C11. (supported in GCC since version 4.9)

(Overloading isn't truly "built-in" in the fashion shown in the question, but it's dead easy to implement something that works like that.)

_Generic is a compile-time operator in the same family as sizeof and _Alignof. It is described in standard section 6.5.1.1. It accepts two main parameters: an expression (which will not be evaluated at runtime), and a type/expression association list that looks a bit like a switch block. _Generic gets the overall type of the expression and then "switches" on it to select the end result expression in the list for its type:

_Generic(1, float: 2.0,
            char *: "2",
            int: 2,
            default: get_two_object());

The above expression evaluates to 2 - the type of the controlling expression is int, so it chooses the expression associated with int as the value. Nothing of this remains at runtime. (The default clause is optional: if you leave it off and the type doesn't match, it will cause a compilation error.)

The way this is useful for function overloading is that it can be inserted by the C preprocessor and choose a result expression based on the type of the arguments passed to the controlling macro. So (example from the C standard):

#define cbrt(X) _Generic((X),                \
                         long double: cbrtl, \
                         default: cbrt,      \
                         float: cbrtf        \
                         )(X)

This macro implements an overloaded cbrt operation, by dispatching on the type of the argument to the macro, choosing an appropriate implementation function, and then passing the original macro argument to that function.

So to implement your original example, we could do this:

foo_int (int a)  
foo_char (char b)  
foo_float_int (float c , int d)

#define foo(_1, ...) _Generic((_1),                                  \
                              int: foo_int,                          \
                              char: foo_char,                        \
                              float: _Generic((FIRST(__VA_ARGS__,)), \
                                     int: foo_float_int))(_1, __VA_ARGS__)
#define FIRST(A, ...) A

In this case we could have used a default: association for the third case, but that doesn't demonstrate how to extend the principle to multiple arguments. The end result is that you can use foo(...) in your code without worrying (much[1]) about the type of its arguments.

---

For more complicated situations, e.g. functions overloading larger numbers of arguments, or varying numbers, you can use utility macros to automatically generate static dispatch structures:

void print_ii(int a, int b) { printf("int, int\n"); }
void print_di(double a, int b) { printf("double, int\n"); }
void print_iii(int a, int b, int c) { printf("int, int, int\n"); }
void print_default(void) { printf("unknown arguments\n"); }

#define print(...) OVERLOAD(print, (__VA_ARGS__), \
    (print_ii, (int, int)), \
    (print_di, (double, int)), \
    (print_iii, (int, int, int)) \
)

#define OVERLOAD_ARG_TYPES (int, double)
#define OVERLOAD_FUNCTIONS (print)
#include "activate-overloads.h"

int main(void) {
    print(44, 47);   // prints "int, int"
    print(4.4, 47);  // prints "double, int"
    print(1, 2, 3);  // prints "int, int, int"
    print("");       // prints "unknown arguments"
}

(implementation here) So with some effort, you can reduce the amount of boilerplate to looking pretty much like a language with native support for overloading.

As an aside, it was already possible to overload on the number of arguments (not the type) in C99.

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[1] note that the way C evaluates types might trip you up though. This will choose foo_int if you try to pass it a character literal, for instance, and you need to mess about a bit if you want your overloads to support string literals. Still overall pretty cool though.