function pointer - ambiguous symbol error

Question

MyClass is a singleton class (There will only ever be one of these in my whole program).

What I want to do is follows.

1. Add data to my class using AddData, get a function pointer returned that I can then pass to 'another' function in a dll.
2. Then this 'other' function calls my call back function

My class is like so.

typedef void (*DataReceivedCallback)(int, int);  

class MyClass
{
    MyClass();
    ~MyClass();

    void AddData(int sourceId, DataReceivedCallback &callback);
    static void MyCallBackFunction(int var1, int var2);
};

void MyClass::AddData(int sourceId, DataReceivedCallback &callback)
{
    callback = &MyCallBackFunction;

}
void MyClass::MyCallBackFunction(int var1, int var2 )
{
    //do something blah blah
}

I can then do:

int main()
{
     DataReceivedCallback callback;
     MyClass->GetInstance()->AddData(1, callback);
     callback(1,100);
}

When I step through this I see that I do actually step into the callback MyCallBackFunction which is what I want :)

What I then want to do now is pass this 'callback' defined in main to a dll function that will call back into my callback function.

I have the code for the dll so I want to modify one if its functions so that it accepts my callback function parameter.

I am doing this in the dll function signature:

void * someDllFunction( int var1, int var2, DataReceivedCallback& callback)
{
   callback(2, 200);
}

But I get the error: error C2872: 'DataReceivedCallback' : ambiguous symbol

How can I solve this? Does this have to do with only being allowed to use c-style parameters across dll boundaries??

Answer 1

typedef void (*DataReceivedCallback)(int, int);

should be,

typedef void (MyClass::*DataReceivedCallback)(int, int);

Because, MyCallBackFunction is a non-static member method of MyClass. So it cannot have regular function signature. Also change assignment to,

callback = &MyClass::MyCallBackFunction;

Demo.

Answer 2

You got your types wrong. DataReceivedCallback, alias void(*)(int, int), is a function pointer, but &MyClass::MyCallBackFunction is a pointer-to-member-function (PTMF). Those two are entirely unrelated and incompatible types!

You cannot treat a member function as a free function.

Since you only have one single instance of your class (Note: "one instance", not "one class"; you always only have one class), why bother with member functions at all? Just make the function global (inside a namespace) and you're done. Though perhaps I'm misunderstanding your requirements.

Another note: You don't need to pass function pointers by reference to the DLL function, just pass them by value. They're just pointers, so they're light-weight.