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I have a function void f (void*, int);, that is used as a callback function. The caller expects void (*)(int). Can I use std::bind1st to convert one to another? Is there any way to do this without using C++ 11 std::bind, just std::bind1st?

Violet Giraffe
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1 Answers1

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No. Although std::bind1st() creates a function object with a call operator taking and int, it is not a void(*)(int). The only way to turn a void(*)(void*, int) into a void(*)(int) is to have a forwarding function which obtains the void* from global resources, e.g.,

static void* data = 0; // probably needs to be set to a more suitable value
void f_forward(int value) {
    f(data, value);
}

Anybody providing a callback which doesn't take a user-defined context, e.g., in the C-like interface a void* which is just passed through, didn't think too hard about the interface.

Dietmar Kühl
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  • Indeed. I was hoping to avoid a global / static variable. Thanks for the answer. – Violet Giraffe Dec 16 '13 at 10:01
  • @violetgiraffe The other way is to write self modifying code, which is both non portable and lower level than C++. Basically get a writable page from the OS, blit a known function with a constant embedded in the machine code, modify that constant, mark the page as read only and executable, then take a pointer to the function with the 'hard-coded' `void*`. And yes, this is doable on many platforms, it is just rarely wise. Use a global, and if you need more than one, use a set of `N` and a global array, and assert that you use no more than `N`. – Yakk - Adam Nevraumont Dec 16 '13 at 10:15