类的非静态成员函数的函数指针

Question

I want to define a member function in class and use its pointer. I know that I can use static member function but the problem with it is that I can only access the static members of the class. Is there a way other than static member function to be able to get function pointer.

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To be more specific: There is a library which I'm using which gets a function pointer as its input. I want to write a member function and assign its function pointer to that external library. Should I create an object of class or use this pointer to do this?

Answer 1

You can get the pointer of the method, but it has to be called with an object

typedef void (T::*MethodPtr) ();
MethodPtr method = &T::MethodA;
T *obj = new T();
obj->*method();

If you need to have non-object pointer and you want to use object then you have to store instance of object somewhere, but you are restricted to use only one object (singleton).

class T {
  static T *instance;
public:
  T::T() {
    instance = this;
  }
  static void func() {
    instance->doStuff();
  }
  void doStuff() {}
};

If library supports user data for function pointers, then you may have multiple instances

class T {
public:
  static void func(void *instance) {
    ((T*)instance)->doStuff();
  }
  void doStuff() {}
};

Answer 2

• If:
• you want to get the function pointer of a nonstatic member from within the class
• And use it within the class:
• Then: It can work, because when you get the member function address, there is a "this" pointer. The syntax was not obvious to me, and it may look somewhat ugly, but not TOO bad. This may not be new to the true experts, but I have wanted to have this in my bag of tricks for a long time.

Here is a complete sample program:

#include <iostream>
class CTestFncPtr
{
public:
    CTestFncPtr(int data) : mData(data)
    {
//      Switch = &CTestFncPtr::SwitchC; // Won't compile - wrong function prototype - this is type safe
        if (data == 1)
            Switch = &CTestFncPtr::SwitchA;
        else
            Switch = &CTestFncPtr::SwitchB;
    }
    void CallSwitch(char *charData)
    {
        (this->*Switch)(charData);
    }

private:
    void SwitchA(char * charData)
    {
        std::cout << "Called Switch A " << "Class Data is " << mData<<" Parameter is " << charData<< "\n";
        Switch = &CTestFncPtr::SwitchB;
    }
    void SwitchB(char * charData)
    {
        std::cout << "Called Switch B " << "Class Data is " << mData<<" Parameter is " << charData<< "\n";
        Switch = &CTestFncPtr::SwitchA;
    }
    void SwitchC()
    {
    }
    void(CTestFncPtr::*Switch)(char * charData);
    int mData;
};

int main(int argc, char * argv[])
{
    CTestFncPtr item1(1);
    item1.CallSwitch("Item1");
    item1.CallSwitch("Switched call Item 1");
    CTestFncPtr item2(0);
    item2.CallSwitch("Item2");
    item2.CallSwitch("Switched call Item 2");

    return 0;
}