Virtual destructors

[acorn]

Can someone elaberate a little on virtual destructors. What happens when you don't use them ect. I skimmed over the tutorial section but couldnt find any information on them. I'm trying to understand what is really happening when using certain features.

[Computergeek01]

It has often been said that when learning C++ a man must also learn a martial art, I highley suggest Google-Fu:

http://en.wikipedia.org/wiki/Virtual_function

So this tells me that until something better roles along, virtuals are default behaviors for those functions.

[firedraco]

virtuals are default behaviors for those functions.

Not really...

Try looking for it here: http://www.parashift.com/c++-faq-lite/

[Computergeek01]

First of all let's remember that most people do not have the experiance doing research that we do, and that a little bit of frustration is all it takes to chase you into an inferior language:
http://www.parashift.com/c++-faq-lite/virtual-functions.html#faq-20.1

Second, how is this not the same as what I said, conceptually not analytically as we're trying not to fry brains here?

ComputerGeek01: ...until something better roles along, virtuals are default behaviors of those functions.

compared to

www.parashift.com: A virtual function allows derived classes to replace the implementation provided by the base class...

[Disch]

Virtual destructors are not the default behaviors because by default functions are not virtual. I'm not sure what you were trying to get at by saying that, but I had the same reaction as firedraco.

Virtual destructors are not quite the same as other virtual functions. A more on point FAQ Lite link is this one:

http://www.parashift.com/c++-faq-lite/virtual-functions.html#faq-20.7

To actually answer the OP's question:

virtual destructors allow you to polymorphically destroy objects. Without making the dtor virtual, attempting to destroy an object through a parent class's pointer could cause severe problems and/or memory leaks.

Here's an example:

class Parent_A {
public:
  ~Parent_A() { cout << "~Parent_A\n"; }
};

class Child_A : public Parent_A {
public:
  ~Child_A() { cout << "~Child_A\n"; }
};

class Parent_B {
public:
  virtual ~Parent_B() { cout << "~Parent_B\n"; }
};

class Child_B : public Parent_B {
public:
  ~Child_B() { cout << "~Child_B\n"; }
};

/*
  Basically, 'A' classes have virtual dtors
  'B' classes do not have virtual dtors
*/

int main()
{
  Parent_A* a = new Child_A;
  Parent_B* b = new Chlid_B;

  // polymorphically destroy the objects
  delete b; // calls ~Child_B, followed by ~Parent_B, as you'd expect
  delete a; // only calls ~Parent_A!  ~Child_A never called!  object not properly destroyed!
}

[sohguanh]

So can I say if as long as I don't have class inheritance design in my program, I don't need to fret over the keyword virtual ? :P

I like to use class composition over class inheritance as it is easier to debug hahahaa... ya I know I breach the GoF design patterns.

[Disch]

So can I say if as long as I don't have class inheritance design in my program, I don't need to fret over the keyword virtual ? :P

Yes, the virtual keyword is completely useless if you're not using inheritance.

I like to use class composition over class inheritance as it is easier to debug hahahaa

I don't see why it'd be easier to debug =P

Anyway both composition and inheritance have their uses. Inheritance forms an "is a" relationship whereas composition forms a "has a" relationship.

[sohguanh]

I don't see why it'd be easier to debug =P

Using inheritance, sometimes when you are poring over the subclass methods implementation, it make a call to it's parent. Without using an IDE help, you would think maybe it is calling another method within the subclass instead of it's parent isn't it ?

If you have lot's of such subclasses then you got problem visualizing the whole flow. I use Notepad++ but I believe IDE should help me out but I not IDE fan so go figure.

Using composition, you would see that object.method or object->method signature which is easy to identify isn't it ?

So when I say easier to debug, I am referring to tracing the flow of the code.

[Disch]

Without using an IDE help

Debugging without an IDE is masochism. How do you even use a debugger? Can they even operate without an environment to connect to?

If you have lot's of such subclasses then you got problem visualizing the whole flow.

That's more of a design problem. If the design is good, you won't have any of these problems you're describing. It should be immediately clear what a function call is doing and where it's going. If it isn't, your code is sloppy and/or you need to add comments.

Using composition, you would see that object.method or object->method signature which is easy to identify isn't it ?

I suppose so.

[moorecm]

Debugging without an IDE is masochism. How do you even use a debugger? Can they even operate without an environment to connect to?

How do you feel about gdb?

[Disch]

I've never used gdb by itself. I've always used it through another environment (like C::B).

My question above was sincere, not rhetorical. How does it work? How can you set breakpoints, watches, etc if not from an IDE? I can't visualize it.

[sohguanh]

My question above was sincere, not rhetorical. How does it work? How can you set breakpoints, watches, etc if not from an IDE? I can't visualize it.

Agree totally. gdb is way too old-school and command line based.

For Java I uses Eclipse so I presume I can make it work for my C++ programs too :)

Days without IDE to do debugging is your good old trusted printf and cout isn't it ? But for multi-threading debugging is extremely tricky. I wonder how ppl debug multi-threaded C++ program.

[firedraco]

How does it work? How can you set breakpoints, watches, etc if not from an IDE? I can't visualize it.

Basically you type "break" or whatever and a line number, function name, etc. When stepping through it generally shows the line you are on and 2 or 3 lines above/below it. I agree it's quite annoying.

[moorecm]

Yes, gdb is actually quite popular. My current company uses it extensively. Most of us there use Emacs or Vi and a number of modified gdbs. No IDE is necessary although they might be nice to have.

[sohguanh]

Yes, gdb is actually quite popular. My current company uses it extensively. Most of us there use Emacs or Vi and a number of modified gdbs. No IDE is necessary although they might be nice to have.

Let me guess. The average age of your company developers is in the range 35-50 ?

Nowadays, free IDE is rampant and as much as I try to keep away, the GUI effects sway me over bit by bit. Ok not only the GUI but the graphical tools for set breakpoints, watch variables etc etc.

I maybe old but I am willing to adapt new things especially if it is free and good to use :P