[C++] Pointer to function giving error

[System_Error]

I have a very simple function pointer that I even got out of a book, but my compiler (dev cpp) is giving me the following errors:

35 C:\Dev-Cpp\main.cpp invalid use of non-static member function `virtual void Mammal::speak()'

35 C:\Dev-Cpp\main.cpp cannot convert `void (Mammal:()' to `void (Mammal::*)()' in assignment


The code is as follows:

#include <iostream>
using namespace std;

class Mammal
{
  public:
         Mammal() { }
         virtual ~Mammal() { }
         Mammal(const Mammal&) { }
         
         virtual void speak()
         {
            cout << "Mammal speak\n";
         }
};

class Cat: public Mammal
{
      public:
             Cat() { }
             ~Cat() { }
             Cat(const Cat&) { }
             
             void speak()
             {
               cout << "Meow\n";
             }
};


int main(int argc, char* argv[])
{
    Mammal m;
    void (Mammal::*pFunc)() = 0;
    pFunc = Mammal::speak;
    
    Mammal* ptr = new Cat;
    (ptr->*pFunc)();
    
    delete ptr;   
}

What exactly am I doing wrong to recieve this error? Everything looks correct?

[sunburnt]

I think you just need to add the 'address of' operator in front of Mammal:

    pFunc = &Mammal::speak;

HTH!

[bilm_ks]

You cant use function pointers for member functions (non static) as you do with common functions. It needs a special declaration and you must involve an instance of the object to call them.

[System_Error]

You cant use function pointers for member functions (non static) as you do with common functions. It needs a special declaration and you must involve an instance of the object to call them.

My book says you can

I think you just need to add the 'address of' operator in front of Mammal:

That's it sunburt! I'm thinking certain compilers actually accept the form without the address of operator.

Thanks.

[sunburnt]

You cant use function pointers for member functions (non static) as you do with common functions. It needs a special declaration and you must involve an instance of the object to call them.

The code System_Error posted has both the special declaration and an instance which invokes the function -- so you're right, but uneccesary


I think that when taking the address of a non-static member function, that the & is necessary. I know that with regular (non-class) functions, the & is optional.

[CornedBee]

Correct. Basically, the form without the & was kept for C compatibility, but disliked, so when it came to member functions, where C compatibility was not an issue, so the & was made mandatory.