How to Safely Pass a `boost::function` to a Plain Function Pointer?

Demoting Boost::function to a Plain Function Pointer

Problem:

Passing a boost::function as a plain function pointer with the same signature to a method that requires one can result in errors, such as C2664.

Resolution:

While some answers suggest using target() to retrieve a pointer from a boost::function, this only works for trivial cases where the initially bound object was eligible for binding to a C callback.

For non-trivial cases, the underlying mechanism of a boost::function necessitates multiple pointers, not just one. Therefore, a more robust solution is to create a shim function that conforms to the callback signature and delegates the call to the boost::function stored as user data.

Example:

// Shim function for callback
typedef void (*CallbackType)(int x, void* user_data);
void MyCallback(int x, void* userData) {
    boost::function<void(int)>& pfn = static_cast<boost::function<void(int)> &>(userData);
    pfn(x);
}

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

This approach enables the use of boost::function with C callbacks that include user data. However, if the callback signature lacks a user data pointer, it may require a complete redesign to become usable under most circumstances.

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