成员 function 指的是对象的成员 function

Question

I'm a c++ noob so apologies in advance if this question is not properly structured.

I have a class which instantiates an object of a different class, I'd like for the member function of the main class (excuse my naive language) to be in turn referring to the member function of the object instantiated in the main class.

I've attached a snippet for reference

foo.h

class foo {
public : 

int data;
void printFunc(){
cout<< "data is"<< data;
}

};

bar.h

class bar {

public:
void updatebarr ();
}

bar.cpp

bar::bar()
{
foo foo1;
foo1.data = 1; 
// the line below is flagged as syntax error
// I want the object of type bar's updatebarr() function to be essentially be calling 
// foo::PrintFunc(). I figured this "should" work as the signatures are identical.
this->updatebarr = &foo1.printFunc(); 
}

The error I get is "Reference to non-static member function must be called",

Answer 1

The signatures might look similar but these are not free functions but member functions . Have a look at this faq!

The type of the first one is: void (foo::*)() and the second is void (bar::*)() . See the difference?

They can only be invoked on an object of their respective type. What you can do is this:

class bar {
  public:
    void updatebarr(foo& f) {
      f.printFunc();
    }
};

Answer 2

Pointers to member methods do not work the way you are trying. Not only what you have is a syntax error, but it is also a logic error since foo1 will go out of scope and be destroyed when bar() exits, thus leaving updatebarr to call printFunc() on an invalid foo object.

For what you are attempting, you would need something more like this:

foo.h

class foo {
public : 
    int data;
    void printFunc(){
        cout << "data is " << data;
    }
};

bar.h

#include "foo.h"

class bar {
private:
    foo foo1;
public:
    bar();
    void updatebarr();
};

bar.cpp

#include "bar.h"

bar::bar()
{
    foo1.data = 1; 
}

void bar::updatebarr()
{
    foo1.printFunc();
}

main.cpp

#include "bar.h"

int main()
{
    bar b;
    b.updatebarr();
    return 0;
}

Live Demo

Alternatively, you can make updatebarr() be a std::function that calls printFunc() via a lambda, eg:

foo.h

class foo {
public : 
    int data;
    void printFunc(){
      cout << "data is " << data;
    }
};

bar.h

#include <functional>
#include "foo.h"

class bar {
private:
    foo foo1;
public:
    bar();
    std::function<void()> updatebarr;
};

bar.cpp

#include "bar.h"

bar::bar()
{
    foo1.data = 1; 
    updatebarr = [&](){ foo1.printFunc(); };
}

main.cpp

#include "bar.h"

int main()
{
    bar b;
    b.updatebarr();
    return 0;
}

Live Demo

In which case, you could then have the lambda capture a copy of the foo object instead, which will give you a result very close to your original attempt, eg:

foo.h

class foo {
public : 
    int data;
    void printFunc() const {
      cout << "data is " << data;
    }
};

bar.h

#include <functional>

class bar {
public:
    bar();
    std::function<void()> updatebarr;
};

bar.cpp

#include "bar.h"
#include "foo.h"

bar::bar()
{
    foo foo1;
    foo1.data = 1; 
    updatebarr = [foo1](){ foo1.printFunc(); };
}

main.cpp

#include "bar.h"

int main()
{
    bar b;
    b.updatebarr();
    return 0;
}

Live Demo