[RESOLVED] call to non-static member function without an object argument?

[davidthecoder]

Hi, I'm trying to learn c++. I just came across a error I couldn't figure out.

I keep getting this error: "call to non-static member function without an object argument "

For this line :

void (Part::*Func)()const = Part::Display();

its declared like this:

virtual void Display() const =0;

any idea how I could fix this??

[cilu]

What is this suppose to do? It looks like a function declaration.

void (Part::*Func)()const = Part::Display();

Do you want to take the address of the Display function? Then you should use &Part:isplay. However, the first part of that statement is incorrect.

[davidthecoder]

I believe it's supposed to be a pointer to that function declaration, I'm still learning so I could be mistaken.
The code came from an online course that I'm using to learn.
The code is kind of long so I'll show you a similar example with the same type of error :

#include <iostream.h>

enum BOOL {FALSE, TRUE};
class Mammal
{
public:
    Mammal():itsAge(1) {  }
    virtual ~Mammal() { }
    virtual void Speak() const = 0;
    virtual void Move() const = 0;
protected:
    int itsAge;
};

class Dog : public Mammal
{
public:
    void Speak()const { cout << "Woof!\n"; }
    void Move() const { cout << "Walking to heel...\n"; }
};


class Cat : public Mammal
{
public:
    void Speak()const { cout << "Meow!\n"; }
    void Move() const { cout << "slinking...\n"; }
};


class Horse : public Mammal
{
public:
    void Speak()const { cout << "Winnie!\n"; }
    void Move() const { cout << "Galloping...\n"; }
};


int main()
{
    void (Mammal::*pFunc)() const =0;
    Mammal* ptr =0;
    int Animal;
    int Method;
    BOOL fQuit = FALSE;
    
    while (fQuit == FALSE)
    {
        cout << "(0)Quit (1)dog (2)cat (3)horse: ";
        cin >> Animal;
        switch (Animal)
        {
            case 1: ptr = new Dog; break;
            case 2: ptr = new Cat; break;
            case 3: ptr = new Horse; break;
            default: fQuit = TRUE; break;
        }
        if (fQuit)
            break;
        
        cout << "(1)Speak  (2)Move: ";
        cin >> Method;
        switch (Method)
        {
            case 1: pFunc = Mammal::Speak; break;   //ERROR HERE
            default: pFunc = Mammal::Move; break;   // ERROR HERE
        }
        
        (ptr->pFunc)();
        delete ptr;
    }
}

In Main(), in switch (method) I get the same error in case 1 and in default:
"call to non-static function without an object argument "

[davidthecoder]

but any how I did as you suggested and added the "&" reference operator and it came out working thanks

[cilu]

pFunc is actually a pointer to function. However, this is wrong

(ptr->pFunc)();

because pFunc is not a member of Mammal. It has to be:

(ptr->*pFunc)();

What is that chapter about? Because I think your example can be rather put like this:

   Mammal* ptr =0;
   int Animal;
   int Method;
   BOOL fQuit = FALSE;

   while (fQuit == FALSE)
   {
      cout << "(0)Quit (1)dog (2)cat (3)horse: ";
      cin >> Animal;
      switch (Animal)
      {
      case 1: ptr = new Dog; break;
      case 2: ptr = new Cat; break;
      case 3: ptr = new Horse; break;
      default: fQuit = TRUE; break;
      }
      if (fQuit)
         break;

      cout << "(1)Speak  (2)Move: ";
      cin >> Method;
      switch (Method)
      {
      case 1: ptr->Speak(); break;
      default: ptr->Move(); break;
      }

      delete ptr;
   }

[davidthecoder]

yeah i caught that too, seems that things are missing in the example codes.
This chapter is about Special Classes and Functions : Pointers to Member Functions.

Your version is simpler

[cilu]

Aha, "Pointers to Member Functions". Than it makes sense, of course.

[D_Drmmr]

This chapter is about Special Classes and Functions : Pointers to Member Functions.

Pointers to member functions is something you hardly ever use in modern C++. You should use function objects (std::function, std::tr1::function, boost::function) instead whenever possible, because they are much more flexible and the syntax is much more readable.

If you really do want to learn about pointers to member functions, here is a good reference: http://www.parashift.com/c++-faq/poi...o-members.html