Replace function-pointer implementation with std::function for using lambdas in PlayFab SDK

I'm currently trying to include the PlayFab C++ SDK into my app. This SDK is mainly for the game engine Cocos2d-x, but can be used for any C++ app basically.

It's just plain REST, hence you send requests to their servers and wait for the response. This would be perfect for using lambdas.

They declare this callback, which is called when a request is successful:

template<typename ResType> using ProcessApiCallback = void(*)(const ResType& result, void* userData);

Unfortunately, they're not using std::function, but a function pointer. This way one can not use lambdas that capture variables.

Hence, I thought I could simply replace this function pointer callback with an std::function callback like so:

template<typename ResType> using ProcessApiCallback = std::function<void(const ResType& result, void* userData)>;

Unfortunately, things are not that simply, as they stored the function pointers using ugly reinterpret_casts, here's an example (remove unnecessary parts to keep it short):

void PlayFabClientAPI::LoginWithAndroidDeviceID(
    LoginWithAndroidDeviceIDRequest& request,
    ProcessApiCallback<LoginResult> callback,
    ErrorCallback errorCallback,
    void* userData
    )
{
    // here they assign the callback to the httpRequest
    httpRequest->SetResultCallback(reinterpret_cast<void*>(callback));
    httpRequest->SetErrorCallback(errorCallback);
    httpRequest->SetUserData(userData);

    PlayFabSettings::httpRequester->AddRequest(httpRequest, OnLoginWithAndroidDeviceIDResult, userData);
}

Later on, when the request was successful, they do this:

if (request->GetResultCallback() != nullptr)
{
    ProcessApiCallback<LoginResult> successCallback = reinterpret_cast<ProcessApiCallback<LoginResult>>(request->GetResultCallback());
    successCallback(outResult, request->GetUserData());
}

The HttpRequest class has this field:

void* mResultCallback;

The problem is that I don't know how to store arbitrary std::function pointers in the HttpRequest class and then later cast them back. I tried many things, including also really ugly reinterpret_casting, but nothing did work.

I'm open to do any changes to their SDK. I also reported this as bad design and they agreed, but they don't have the time to improve it, but they will accept pull request if a good solution can be found.