demote boost::function to a plain function pointer

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Has anyone noticed that the accepted answer only works with trivial cases? The only way that function<>::target() will return an object that can be bound to a C callback, is if it was constructed with an object that can be bound to a C callback. If that’s the case, then you could have bound it directly and skipped all of the function<> nonsense to begin with.

If you think about it, there isn’t any magic solution to this. A C-style callback is stored as a single pointer which points to executable code. Any nontrivial boost::function<> is going to need at least two pointers: one to the executable code, the other to the data that’s needed to set up the call (e.g. the ‘this’ pointer, in the case of a bound member function).

The right way to use boost::function and boost::bind with C callbacks is to create a shim function that satisfies the callback signature, figures out which function<> to call, and calls it. Usually C callbacks will have some kind of a void* for ‘user data’; that’s where you stash your function pointer:

typedef void (*CallbackType)(int x, void* user_data);
void RegisterCallback(CallbackType cb, void* user_data);

void MyCallback(int x, void* userData) {
  boost::function<void(int)> pfn = static_cast<boost::function<void(int)> >(userData);
  pfn(x);
}

boost::function<void(int)> fn = boost::bind(myFunction(5));
RegisterCallback(MyCallback, &fn);

Of course, if your callback signature doesn’t include some kind of user data pointer, you’re out of luck. But any callback that doesn’t include a user data pointer is already unusable in most real-world scenarios, and needs to be rewritten.

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