Memory Management

**black_spot1984** · 10-08-2008

I am really new to C++. All my programming experience is in Java. I'm still learning about memory management. I'm trying to put together a BST. Everything works except that my clear method doesn't work yet. It is supposed to delete all the nodes of the tree. I also want it to free up the memory that the nodes used too. So the code I used was this:

Code:

void BST::deleteAll(BSTNode* current)
	{
		if(current->left!=NULL)
			deleteAll(current->left);
		if(current->right!=NULL)
			deleteAll(current->right);
		
		delete current;		
	}

When I call clear I pass the root of my tree into this deleteAll function. The problem is that my root node seems to still be there. It's not null or anything like I'd expect. Any thoughts?

Thanks for the help you guys!

**tabstop** · 10-08-2008

delete doesn't set anything to NULL. The pointer isn't valid, true, but so what. You should set head = NULL after you finish. (Note well that you can't set current = NULL in this function, as that change won't be seen in the calling function.)

**black_spot1984** · 10-08-2008

Now on that code I am getting a seg fault. My debug comes back with:

Code:

Program received signal SIGSEGV, Segmentation fault.
0x0806779d in BST::deleteAll (this=0xbff1e6a8, current=0x568b000f) at BST.cpp:175
175			if(current->left!=NULL)

What am I doing wrong here?

**Elysia** · 10-08-2008

Very difficult to say without code to test.
If I had to guess, I'd say you're using a pointer that's freed.
You should realize that you can also use smart pointers, such as std::tr1::shared_ptr (alternatively boost::shared_ptr). They will take care of freeing the memory automatically.

**black_spot1984** · 10-08-2008

Wow, now I feel really dumb. Someone wanna give me a crash course in smart pointers? That's the first time I've even heard of them.

**Elysia** · 10-08-2008

You aren't alone in that.
http://msdn.microsoft.com/en-us/library/bb982026.aspx
http://msdn.microsoft.com/en-us/libr...83(VS.80).aspx
http://msdn.microsoft.com/en-us/library/bb982126.aspx
They're a recent addition to the C++ standard library and a pretty recent design implementation I think, but future C++ will heavily build upon these smart pointers.

**black_spot1984** · 10-08-2008

Thanks for the help everyone!

**laserlight** · 10-08-2008

They're a recent addition to the C++ standard library and a pretty recent design implementation I think, but future C++ will heavily build upon these smart pointers.

In fact, they (aside from std::auto_ptr) are not even part of the C++ standard library, yet.

**Daved** · 10-08-2008

I would assume you're creating this BST for learning purposes, right? In that case, maybe a smart pointer for this particular exercise doesn't make sense. However, I'd certainly recommend you learn them for general C++ programming.

If you pass the current pointer by reference and set current = 0 after the delete current call then the pointers will all be set to null and your code might work better.

**Elysia** · 10-08-2008

Originally Posted by laserlight

In fact, they (aside from std::auto_ptr) are not even part of the C++ standard library, yet.

Well, I suppose that technically they aren't, but they're in the TR1 library which has been finalized and is moving into the standard library soon enough.

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