Vector of pointer to member functions

Question

I'm trying to write a program which creates a class that contains vector of pointers to member functions, with add() and remove() member functions. The code I wrote is -

1    #include <iostream>
2    #include <vector>
3   using namespace std;
4    
5    typedef void(*classFuncPtr)();
6    
7    class FunctionVectors
8    {
9    private:
10      vector<classFuncPtr> FunctionPointerVector;
11    public:
12      FunctionVectors(){}
13      void add(classFuncPtr funcPtr);
14      void remove(int index);
15      void run();
16      void a(){cout<<"a: Why are you calling me?"<<endl;}
17    };
18    
19    void FunctionVectors::add(classFuncPtr funcPtr)
20    {
21      FunctionPointerVector.push_back(funcPtr);
22    }
23    
24    void FunctionVectors::remove(int index)
25    {
26      FunctionPointerVector.erase(FunctionPointerVector.begin() + index);
27    }
28    
29    int main()
30    {
31      FunctionVectors f;
32      classFuncPtr fv = &(classFuncPtr)FunctionVectors::a;
33     
34      f.add(fv);
35      f.run();
36     
37      return 0;
38  }

But, it is showing error in line# 32 -

error C2440: 'type cast' : cannot convert from 'void (__thiscall FunctionVectors::* )(void)' to 'classFuncPtr'  

Please, tell me how should I modify it to work properly.

Thanks in advance.

Answer 1

typedef void(*classFuncPtr)();

This is not a pointer to method, but a pointer to function. Method differs from function, because it's being called in a context: requires this to work correctly.

Keep in mind, that in C++ you are only able to create vector of pointers to a method of specific class. So you won't be able to keep pointers to two methods of different classes in that vector.

The solution - as suggested in comments - is to use std::function or boost::function and possibly C++11 lambdas, because they provide a lot more flexibility than simple pointer-to-members.

If you want to implement an event mechanism, consider also using functors instead of methods:

1. Create base class for event handler:

   class MyEventHandler
   {
   public:
       virtual void operator()(void * sender, int data) = 0;
   }
   

2. Create simple vector of these:

   std::vector<MyEventHandler *> MyEvent;
   

3. Create specific handlers in your classes:

   class MyClass
   {
   private:
       class SpecificEventHandler : MyEventHandler
       {
       public:
           void operator()(void * sender, int data)
           {
               std::cout << "Event handled!";
           }
       }
   
   public:
       SpecificEventHandler Handler;
   
       MyClass()
       {
       }
   }
   

4. Hook the handler to your event:

   MyEvent.push_back(&(myClassInstance.Handler));
   

Code written from memory, may not compile, but you should get the idea.

Answer 2

std::function< void() > 

looks like the signature you are looking for. If it isn't available in your version of C++ but you can use boost, then you fill find it in boost. Look up documentation for appropriate header, for std, for function.

To create one for a member function, you need to bind it, and to bind it to FunctionVectors::a() you will need an instance of a FunctionVectors to call it on.

In your example, I will make the typedef for you

typedef std::function< void() > classFuncPtr; // in reality a badly named typedef

int main() { FunctionVectors f; classFuncPtr fv = std::bind( &FunctionVectors::a, f ); }

alternatively if you really have C++11 with lambdas you can do

   classFuncPtr = [ f ]() { f.a() );

In your case I reckon you don't really want a free function, you always want a member function of your class you want.

   typedef void (*FunctionVectors::classFuncPtr )();

and you would use

    (this->*func)(); 

to invoke it