Alternative to c++ static virtual methods

Question

In C++ is not possible to declare a static virtual function, neither cast a non-static function to a C style function pointer.

Now, I have a plain ol' C SDK that uses function pointers heavily.

I have to fill a structure with several function pointers. I was planning to use an abstract class with a bunch of static pure virtual methods, and redefine them in derived classes and fill the structure with them. It wasn't until then that I realized that static virtual are not allowed in C++.

Also this C SDKs function signature doesn't have a userData param.

Is there any good alternative? The best I can think of is defining some pure virtual methods GetFuncA(), GetFuncB(),... and some static members FuncA()/FuncB() in each derived class, which would be returned by the GetFuncX(). Then a function in the abstract class would call those functions to get the pointers and fill the structure.

Edit Answering to John Dibling, it would be great to be able to do this:

class Base
{
    FillPointers() { myStruct.funA = myFunA; myStruct.funB = myFunB; ...}
private:
    CStruct myStruct;
    static virtual myFunA(...) = 0;
    static virtual myFunB(...) = 0;
};

class Derived1 : public Base
{
    Derived1() {  FillPointers();  }
    static virtual myFunA(...) {...};
    static virtual myFunB(...) {...};
};

class Derived2 : public Base
{
    Derived2() {  FillPointers();  }
    static virtual myFunA(...) {...};
    static virtual myFunB(...) {...};
};

int main()
{
    Derived1 d1;
    Derived2 d2;
    // Now I have two objects with different functionality
}

Answer 1

I think you just need to use a plain virtual function. A static virtual function does not make sense, because a virtual function is resolved at runtime. What's there to resolve when the compiler knows exactly what the static function is?

In any case, I would suggest leaving the existing function pointer solution in place if possible. Baring that, consider using a normal virtual function.

Answer 2

Virtual functions are essentially function pointers under-the-hood. They just point to different functions for different classes. To simulate virtual-function behavior, have a function pointer stored somewhere, then to 'override' it just reassign it to some different function.

Alternatively, you might want to test this, but I think interfaces have pretty good binary compatibility. You might get away with exposing a C++ interface composed entirely of pure virtual functions, so long as all the parameters and return types have a consistent binary format (eg. C types). It's not a standard, but it might be portable enough.

Answer 3

A common pattern when passing a function pointer (a callback) to a C SDK uses the fact that many such functions allow a void * parameter that is "user data". You can define your callbacks to be simple global functions, or static class member functions. Then each callback can cast the "user data" parameter to a base class pointer so you can call a member function that does the work of the callback.

Answer 4

Assuming that the C SDK allows you to pass it a void * to your data (and you should pass it your this pointer for the derived class:)

class Base {

  public:

    void Initialize() { /* Pass /this/ and a pointer to myFuncAGate to your C SDK */ }

    virtual myFuncA()=0;

    // This is the method you pass to the C SDK:
    static myFuncAGate(void *user_data) {
        ((Base*)user_data)->myFuncA();
    }
};


class Derived1: public Base {
  public:
    virtual myFuncA() { ... } // This gets called by myFuncAGate()
};

If the C SDK doesn't allow you to pass a pointer to your data which is then passed back to you through the callbacks, then you'll have a really hard time doing this. Since you indicated in one of your comments that this is indeed the case, you're pretty much out of luck. I would suggest using simple functions as callbacks, or overloading the constructor and defining multiple static methods. You'll still have a hard time determining what's the proper object your methods are supposed to work with when your callbacks are invoked by the C code.

If you post more details about the SDK it might be possible to give you more relevant suggestions, but in the general case, even with static methods, you need some way of obtaining a this pointer to work with.

Answer 5

You could just pass the functions directly into the base class constructor:

class Base
{
    Base()(int (*myFunA)(...), int (*myFunB)(...)) 
    { myStruct.funA = funA; myStruct.funB = myFunB; ...}
private:
    CStruct myStruct;
};

class Derived1 : public Base
{
    Derived1() : Base (myFunA, myFunB) {}
    static myFunA(...) {...};
    static myFunB(...) {...};
};

class Derived2 : public Base
{
    Derived2() : Base (myFunA, myFunB) {}
    static myFunA(...) {...};
    static myFunB(...) {...};
};

int main()
{
    Derived1 d1;
    Derived2 d2;
    // Now I have two objects with different functionality
}

Answer 6

The obvious way is like this, with FillPointers implemented in each derived class.

class Base
{
private:
    CStruct myStruct;
};

class Derived1 : public Base
{
 private:
    static FillPointers() { myStruct.funA = myFunA; myStruct.funB = myFunB; ...}
    Derived1() {  FillPointers();  }
    static myFunA(...) {...};
    static myFunB(...) {...};
};

However you can probably avoid that using some template magic...

Answer 7

You can make Base be a class template that takes its function pointers from its template argument:

extern "C" {
struct CStruct
{
  void (*funA)(int, char const*);
  int (*funB)(void);
};
}

template <typename T>
class Base
{
public:
  CStruct myStruct;
  void FillPointers() {
    myStruct.funA = &T::myFunA;
    myStruct.funB = &T::myFunB;
  }
  Base() {
    FillPointers();
  }
};

Then, define your derived classes to descend from an instantiation of Base using each derived class as the template argument:

class Derived1: public Base<Derived1>
{
public:
  static void myFunA(int, char const*) { }
  static int myFunB() { return 0; }
};

class Derived2: public Base<Derived2>
{
public:
  static void myFunA(int, char const*) { }
  static int myFunB() { return 1; }
};

int main() {
  Derived1 d1;
  d1.myStruct.funA(0, 0);
  d1.myStruct.funB();
  Derived2 d2;
  d2.myStruct.funA(0, 0);
  d2.myStruct.funB();
}

That technique is known as the curiously recurring template pattern. If you neglect to implement one of the functions in a derived class, or if you change the function signature, you'll get a compilation error, which is exactly what you'd expect to get if you neglected to implement one of the pure virtual functions from your original plan.

The consequence of this technique, however, is that Derived1 and Derived2 do not have a common base class. The two instantiations of Base<> are not related in any way, as far as the type system is concerned. If you need them to be related, then you can introduce another class to serve as the base for the template, and then put the common things there:

class RealBase
{
public:
  CStruct myStruct;
};

template <typename T>
class Base: public RealBase
{
  // ...
};

int main()
  RealBase* b;
  Derived1 d1;
  b = &d1;
  b->myStruct.funA(0, 0);
  b->myStruct.funB();
  Derived2 d2;
  b = &d2;
  b->myStruct.funA(0, 0);
  b->myStruct.funB();
}

Beware: Static member functions are not necessarily compatible with ordinary function pointers. In my experience, if the compiler accepts the assignment statements shown above, then you can at least be confident that they're compatible for that compiler. This code isn't portable, but if it works on all the platforms you need to support, then you might consider it "portable enough."

Answer 8

If the C SDK wants you to perform operations without providing a userdata, then object-orientation is likely unnecessary and you should just write some functions. Else, time to find a new SDK.

Answer 9

class Base
{
    template<class T>
    FillPointers(T* dummy) { myStruct.funA = T::myFunA; myStruct.funB = T::myFunB; ...}
private:
    CStruct myStruct;
};

class Derived1 : public Base
{
    Derived1() {  FillPointers(this);  }
    static myFunA(...) {...};
    static myFunB(...) {...};
};

class Derived2 : public Base
{
    Derived2() {  FillPointers(this);  }
    static myFunA(...) {...};
    static myFunB(...) {...};
};

int main()
{
    Derived1 d1;
    Derived2 d2;
    // Now I have two objects with different functionality
}

see also http://stackoverflow.com/questions/1820477/c-static-virtual-members/2809789#2809789