Function pointer casting parameter to void

Question

I wrote a simple demo(test.cpp) for my question:

#include <stdio.h>
typedef void* (*SEL)(void);

int foo(int a, int b, int c) {
    return a + b + c;
}

SEL _ptr_to_foo() {
    return (SEL)foo;
}
int main() {
    SEL sel = _ptr_to_foo();

    return 0;
}

and I compiled it with g++ test.cpp -o test in my osx and compilers complained nothing.

But I'm confused about what happened here. As my opinion, SEL defines function pointer which parameter is void and returns void*. However, function foo should be a function which accepts three int parameters and returns an int as result. I think the function pointer of foo should be declared as something like int (*ptr)(int, int, int) = foo;. Why the casting in _ptr_to_foo works? What happened here?

@Swordfish I tried to call my foo via SEL but failed because of the wrong parameter. It seems that SEL throws away my parameter information in foo. Does this mean your not compatible in your comment? — Jian Guo
– Jian Guo, Commented Nov 10, 2018 at 7:44
It works because the C++ language allows it. It allows it because you may want to store a function pointer under a different type, and cast back later based on other information stored with it it. — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Nov 10, 2018 at 7:54

Yakk - Adam Nevraumont · Accepted Answer · 2018-11-10 07:54:44Z

3

You can store function pointers as the wrong function pointer type.

If you call them as the wrong function pointer type the behaviour of your program becomes undefined by the C++ standard.

Your use of a C-style cast becomes a reinterpret_cast<SEL>, which can do very unsafe things.

answered Nov 10, 2018 at 7:54

Yakk - Adam Nevraumont

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Comments

Alan Birtles · Accepted Answer · 2018-11-10 07:50:21Z

2

c-style casts are fairly lax in their type checking. You should use static_cast instead. This fails to compile:

return static_cast<SEL>(foo);

answered Nov 10, 2018 at 7:50

Alan Birtles

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2 Comments

Jian Guo Over a year ago

Ok, this one fails. Should I reckon this c-style cast as a bug here?

Swordfish Over a year ago

No, it is not a bug, it is by design. Using a reinterpret_cast<> in C++ will allow you to do the same ... stupid thing.

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