virtual distructor

[spicy_kid2000]

class abc
{
public:
virtual ~abc(){}
};


class bca : public abc
{
public:
~bca(){}
};

main()
{
abc *obj1 = new abc;
bca obj2;
abc = &obj2;
delete obj1
}


Now how the compiler is calling the distructor of bca????

[Graham]

class abc
{
public:
    virtual ~abc(){}
};


class bca : public abc
{
public:
    ~bca(){}
};

main()
{
    abc *obj1 = new abc;
    bca obj2;
    abc = &obj2;    // <<<---------
    delete obj1
}

The marked line is not legal. What is your question?

[Paul McKenzie]

main()

Illegal. The main() function returns an int.

   abc = &obj2;

abc is a type. Illegal.

  delete obj1

No semicolon. Illegal.

Now how the compiler is calling the distructor of bca????

First, provide compilable, legal code.

Regards,

Paul McKenzie

[spicy_kid2000]

Sorry for giving a not compiled code.

class abc
{
public:
virtual ~abc(){}
};


class bca : public abc
{
public:
~bca(){}
};

main()
{
abc *obj1 = new abc;
bca obj2;
obj1 = &obj2;
delete obj1;
}


Now how the compiler is calling the distructor of bca???

deletes calls the distr of bca also, how compiler is doing this??How compiler know till how many derive class he has to go???

Thanks

[Paul McKenzie]

Your code is ill-formed.

First, obj2 was not dynamically allocated, but you are attempting to delete an address that does not point to a dynamically created area.

When you assigned &obj2 to obj1, you can't call delete on a value that wasn't dynamically allocated. This is undefined behavior. Undefined behavior means anything can happen, including crash.

Also, your code will still not compile --

int main()

not

main()

Regards,

Paul McKenzie

[jlou]

It appears this is what you are trying to do - delete a base class pointer that points to an instance of the derived class.

class abc
{
public:
  virtual ~abc(){}
};


class bca : public abc
{
public:
  ~bca(){}
};

int main()
{
  abc *obj1 = new bca;
  delete obj1;
}

The reason the derived class destructor is called is because the base class destructor is virtual, meaning the actual type of the object pointed at should be used to find the appropriate function. When the virtual keyword is used, then the compiler knows to look up the appropriate function based on the dynamic type of the object at runtime. Even though the pointer is an abc*, there is information stored that knows that the thing it points to is actually of type bca. This is the point of the virtual keyword. If that is what you meant to ask then I hope this helped.

[Paul McKenzie]

Originally posted by jlou
It appears this is what you are trying to do - delete a base class pointer that points to an instance of the derived class.

The OP is deleting a pointer that wasn't dynamically allocated, therefore anything can happen, everything from "working" to crashing. It is equivalent to the following:

int main()
{
  abc *obj1 = new abc;
  abc obj2;
  obj1 = &obj2
  delete obj1;
}

This is clearly illegal -- the address that obj1 is pointing to at the time the delete is called was not dynamically allocated (&obj2). Therefore there are two problems, an illegal delete, and if that happened to "work", a memory leak since the original obj1 value that was dynamically allocated is now gone.

In the code you posted, it is correct -- you are deleting a pointer (regardless of whether it points to a base or derived) that was dynamically allocated.

Regards,

Paul McKenzie

[jlou]

It appears to me that the OP was having trouble coming up with an example to illustrate his or her question. Hopefully I was able to cut to the chase and answer the question that was trying to be asked. If the OP was really asking about deleting a pointer to memory that wasn't dynamically allocated, then hopefully your answer has helped him or her.

[Paul McKenzie]

OK, but if a question deals specifically with the behavior of some aspect of C++, the OP should post the exact code that they have questions about.

One variable out of place, one missing indirection, a missing semicolon, etc. may play a major part in how a piece of code would behave.

Regards,

Paul McKenzie