Calling member function of one class from another

Question

I have this problem with a big piece of code which have some event handling objects. In an event handling class I am trying to call a function of another class through pointer but it is having same error as this code have which I have implemented to just check the logic that I am using to call there. What is wrong I am doing here?

#include <iostream>
using namespace std;
class FunctionPointer;
class UseFP
{
public:
    UseFP(void){}
    ~UseFP(void){}

    void Modified(int m,int n);
    void (FunctionPointer::*update)(int,int);

};

void UseFP::Modified(int m,int n)
    {
            //(this->*update)(m,n);// call by fp if I uncomment it it gives error.
    }

class FunctionPointer
{
    int a,b;
    UseFP * obj;
public:
    FunctionPointer(void);

    ~FunctionPointer(void);

    void updateData(int m, int n)
    {
        a = m;
        b = n;
        cout<<"\n\nUpdated: a "<<a<<", b "<<b<<endl;
    }

    void Input()
    {
        int m, n;
        cout<<"\nEnter new data: ";
        cin>>m>>n;
        obj->Modified(m,n);
    }
    };

void main()
{
    FunctionPointer obj;
    obj.Input();
}

errors after uncommenting function call

1>------ Build started: Project: FunctionPointer, Configuration: Debug Win32 ------
1>Compiling...
1>main.cpp
1>c:\users\volmo\desktop\functionpointer\functionpointer\functionpointer.h(14) : error C2440: 'newline' : cannot convert from 'UseFP *const ' to 'FunctionPointer *const '
1>        Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
1>c:\users\volmo\desktop\functionpointer\functionpointer\functionpointer.h(14) : error C2647: '->*' : cannot dereference a 'void (__thiscall FunctionPointer::* )(int,int)' on a 'UseFP *const '
1>Generating Code...
1>Compiling...
1>FunctionPointer.cpp
1>c:\users\volmo\desktop\functionpointer\functionpointer\functionpointer.h(14) : error C2440: 'newline' : cannot convert from 'UseFP *const ' to 'FunctionPointer *const '
1>        Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
1>c:\users\volmo\desktop\functionpointer\functionpointer\functionpointer.h(14) : error C2647: '->*' : cannot dereference a 'void (__thiscall FunctionPointer::* )(int,int)' on a 'UseFP *const '
1>Generating Code...
1>Build log was saved at "file://c:\Users\volmo\Desktop\FunctionPointer\FunctionPointer\Debug\BuildLog.htm"
1>FunctionPointer - 4 error(s), 0 warning(s)
    ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Answer 1

update is typed as a "pointer to member function of class FunctionPointer . This means it requires a FunctionPointer instance on the left of ->* . But you're trying to dereference it with this which is of type UseFP . Hence the error.

You need an instance of FunctionPointer to invoke update on it. I don't know what your intended semantics is, but one way to obtain one would be to add a parameter to Modified :

void UseFP::Modified(FunctionPointer &fp, int m,int n)
    {
            (fp.*update)(m,n);
    }

Answer 2

You cannot do such things. You should pass FunctionPointer object to function Modified , or store as class variable.

void UseFP::Modified(FunctionPointer* p, int m,int n)
    {
       (p->*update)(m,n);// call by fp if I uncomment it it gives error.
    }

And call it like

obj->Modified(this,m,n);