Question

Code

class cat{

        public:
           void walk(){
                printf("cat is walking \n");
           }

           void myAbc(){
               void (*pKoo)();
               pKoo = &this->walk;
           }

          void myDef(){
               void(cat::*pFoo)();
               pFoo = &this->walk;
          }
        }

    };

Result

1. void (*pKoo)(); – No problem
2. pKoo = &this->walk; – Compiling error
3. void(cat::*pFoo)(); – No problem
4. pFoo = &this->walk; – compile error;

Question

• Why no 2 impossible? If Impossible, Then what is the use of No 1?
• Why no 4 impossible? If Impossible, Then what is the use of No 3?

Please help for conceptual explanation. thank you

Answer 1

Why no 2 impossible?

My compiler gives an error message:

cannot convert ‘void (cat::*)()’ to ‘void (*)()’ in assignment

meaning that you can’t convert a pointer-to-member-function to a pointer-to-function. They are incompatible types; a pointer-to-function can be called directly, while a pointer-to-member-function needs to be applied to an object. So the type you assign to has to be pointer-to-member-function, or void (cat::*)();, as you correctly use in 3.

If Impossible, Then what is the use of No 1?

It’s used to store a pointer to a static or non-member function.

Why no 3 impossible?

(I assume you mean 4, not 3.) My compiler gives an error message:

ISO C++ forbids taking the address of a bound member function to form a 
pointer to member function.  Say ‘&cat::walk’

For some reason, you’re not allowed to take the address of a member function using an object (or pointer); you have to use the class name instead. We can only speculate on why this isn’t allowed; perhaps because it’s not entirely clear which override should be chosen if it’s virtual, or perhaps for some other reason.

Whatever the reason, use the allowed syntax: pFoo = &cat::walk;