Override function in C ++ not working

Question

#include <cstdio> using namespace std; class A { public: virtual void func() { printf("A::func()"); } }; class B : public A { public: virtual void func() { printf("B::func()"); } }; int main() { A a = *(A *)new B(); a.func(); } 

The question is simple: why a->func() calls a function in class A , although A contains an object of class B?

Answer 1

 A a = *(A *)new B(); a.func(); 

Here, what happens in this code, step by step:

• new B() : a new object of type B is allocated in free storage, resulting in its address
• (A*) : the address of the object is added to A* , so we have a pointer of type A* , which actually points to an object of type B that is valid. All OK.
• A a : here the problems begin. A new local object of type A is created on the stack and created using the copy constructor A::A(const A&) , the first parameter being the object created earlier.
• The pointer to the original object of type B is lost after this statement, which leads to a memory leak, since it was allocated in free storage using new .
• a.func() - the method is called in the (local) object of class A.

If you change the code to:

 A& a = *( A*) new B(); a.func(); 

then only one object will be created, its pointer will be converted to a pointer of type A* , then dereferenced, and a new link will be initialized using this address . Then the call to the virtual function will be dynamically resolved to B::func() .

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But remember that you still need to free the object since it was allocated with new :

 delete &a; 

Which, incidentally, will only be correct if A has a virtual destructor for which B :: ~ B () is required (which, fortunately, is empty here, but in the general case it is not necessary) will also be called. If A does not have a virtual destructor, then you need to free it:

 delete (B*)&a; 

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If you want to use a pointer, then this is the same as with a link. The code:

 A* a = new B(); // actually you don't need an explicit cast here. a->func(); delete (B*)a; // or just delete a; if A has a virtual destructor. 

Answer 2

The problem you are facing is classic object splitting :

 A a = *(A *)new B(); 

Make a link or pointer to a , and the virtual submission will work as you expect. See this other question for further explanation.

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You commented on another answer: "The compiler should at least give a warning or what." This is why it is considered good practice to make base classes either abstract but not copyable: your source code would not be compiled in the first place.

Answer 3

Now that you have changed the code snippet, the problem is obvious. Polymorphism (i.e. virtual functions) is called only by pointers and links. You have neither one nor the other. A a = XXX does not contain an object of type B , it contains an object of type A You "chopped" the B th property of an object by following this decree and dereferencing the pointer.

If you do A *a = new B(); , you will get the expected behavior.

Answer 4

This can do the trick.

 A &a = *(A *)new B(); a.func(); 

or

 A *a = new B(); a->func(); 

Answer 5

Virtual dispatch only works with pointers or reference types:

 #include <cstdio> using namespace std; class A { public: virtual void func() { printf("A::func()"); } }; class B : public A { public: virtual void func() { printf("B::func()"); } }; int main() { A* a = new B(); a->func(); } 

Answer 6

The problem is respect and casting from B to A with new A (*) (A *) new B ();

You can fix this by simply deleting * (A *), changing it to (A * a = new B ();), but I would do it even further, since your variable name is not suitable for instance B.

It should be

 B *b = new B(); b->func(); 

Answer 7

Since you performed a slice when copying a dynamically allocated object to an object a type a (which also gave you a memory leak).

a should be a reference ( A& ) instead or just keep a pointer.