Correct syntax to store function pointer

Question

Surprisingly, the following code compiles well both in gcc and clang no matter what symbol before function name is used: *, & or nothing. Does standard allow any of them? What is preferred way to store function pointers?

#include <stdio.h>

typedef int foo(int a);

template <typename X>
int g(int y) {
    return y * sizeof(X);
}

int main() {

    foo* xxx;

    // 1. what is correct according to standard?
    // 2. why they all work?
    xxx = *g<float>;
    xxx = &g<float>;
    xxx = g<float>;

    printf("ok %d\n", xxx(5));
}

Piave

function decays to pointer. – Proclitic

Similar question. – Tiertza

Worth mentioning that for a pointer-to-member-function, only the syntax &ClassName::functionname will work. – Clevelandclevenger

Guess what: xxx = &&&&g<float> also works. And when you call through that pointer you can write (*xxx)(5) or (**xxx)(5) or even (******xxx)(5). – Saurel

Answer 1

All should work fine and have the same effect here. Which one is preferred is a style-issue, IMO the 1st one in your code is confusing, the other two are quite common usages.

I'll explain in the reverse order of your code for convenience,

1. For xxx = g<float>;, function-to-pointer implicit conversion is performed from g<float>, the converted pointer is assigned to xxx.

2. For xxx = &g<float>;, operator& is used explicitly to take the address of the function, the returned pointer is assigned to xxx.

3. For xxx = *g<float>;, function-to-pointer implicit conversion is performed from g<float>, then the pointer is dereferenced by operator*, which returns a function reference, on which function-to-pointer implicit conversion is performed (again), the converted pointer is assigned to xxx at last.

Heraclitean