在非结构/联合上使用 function 指针

Question

Is there a simpler way to 'bind' a function pointer to an object in C?

int add_two(int num) {
    return num + 2;
}
int main(void)
{
    struct thing {
        int num; 
        int (*add_func)(int);
    };
    struct thing xx = {.num=4, .add_func=add_two};
    int yy = xx.add_func(xx.num);
    printf("%d", xx.add_func(xx.num));
}

6

For example, is it possible to do something like this:

int x = 4;
&x->add_two();        // using pseudo-syntax

Or doing it in a single-go, such as:

(void*) "7" . add_two(7);

Or is the only way to use a function pointer within the context of a struct or union?

Answer 1

No, there isn't. C++ was invented originally as a preprocessor for C compilers to do this kind of simple "object-oriented" programming paradigm.

Note that usually you'd want the reference to xx be passed to the method. For that you could use a macro such as

INVOKE(obj, name, ...) (obj)->name((obj), ## __VA_ARGS__)

Then you can use

int add_two(thing *this) {
    return this->num + 2;
}

...

    struct thing {
        int num; 
        int (*add_func)(thing *);
    };

    ...

    int yy = INVOKE(&xx, add_func);
    ...
}

And not need to write the xx twice; you could then make it a bit more complicated like put the methods into a separate virtual table object... but all in all that's about as far as you can get with standard C.