How to Pass a `boost::function` to a C Callback Expecting a Plain Function Pointer?

Demoting Boost::function to a Plain Function Pointer

Boost::function is a versatile tool for managing function pointers, but it can be challenging to use with C callbacks that expect plain function pointers. This article explores the complexities of this issue and provides a robust solution using a shim function.

The problem arises when attempting to pass a bound boost::function directly to a method expecting a plain function pointer with the same signature. This results in compiler errors due to the difference in function pointer representations.

The accepted answer suggests using boost::function::target() to retrieve the underlying function pointer, but this approach has limitations. It only works for trivial cases where the boost::function was constructed with a function pointer that can be bound to a C callback.

To address this issue, a more reliable approach is to create a shim function that meets the callback signature and dispatches calls to the appropriate boost::function. This can be achieved by storing the boost::function pointer as user data in the callback function.

Here is an example of how this approach can be used:

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);

This solution provides a clean and reliable way to use boost::function and boost::bind with C callbacks, even when the callback signatures do not include user data pointers.

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