delete causing an error and app crashing

[paulb567]

Hello,

I'm fairly new to visual c++ and am currently working my way thourgh a few examples of windows programming.

Anyway, the curent example I'm putting together is a very simple drawing application (the kind of thing you find in all the learn visual c++ books).

The problem I'm facing is everytime I try and delete an object the app gives me one of those error messages where I can send a report to microsoft and the app dies.

Basically

I'm creating a new object like so

CElement element;
element = new CLine(start, end, color);
m_pTempElement = &element;

This part works fine, and the element is drawn ok.

The problem comes when I try and..

delete m_pTempElement
m_pTempElement = 0;

the program crashes on the delete line.

Any ideas? Am I doing something really stupid that should be obvious?

Thanks.

ps. Its being compiled by vc++ 6 running on windows xp if this makes any difference.

 

[PerFnurt]

Youre not doing delete on what you did a new on.

You did new CLine but delete on a CElement (that you've put on the stack).

CElement element = new CLine(start, end, color);

looks fishy. The CElement = pointer to a CLine assignemnt is semantically funny. What does this assignemnt mean really?



/Per
[sub]
"It was a work of art, flawless, sublime. A triumph equaled only by its monumental failure."[/sub]

 

[Zyrenthian]

The following lines dont look right unless CElement is really a pointer.

CElement element; // not a pointer
element = new CLine(start,end,color); // should see an error
m_pTempElement = &element;

If element was declared on the stack, it will call its destructor when it goes out of scope. You should not need to delete it.

I would change the code to either:

m_pTempElement = new CLine(start,end,color);
... // do stuff

You can leave the function here and have m_pTempElement still existing. Upon destruction of your class you could execute the following code. (Dont forget to initialize m_pTempElement to NULL in the constructor.)

if(m_pTempElement)
{
delete m_pTempElement;
}

You could also delete m_pTemp element within the function but if that is the case, a pointer is not required.

OR...
Judging from the code, you may not need the pointer.

CLine myLine(start,end,color);
...// do stuff
// NO DELETION REQUIRED

If myLine needs to remain around, then the pointers come into play or member variables which would require initialization in your list.

class YourClass
{
...
protected:
CLine m_line;
}

YourClass::YourClass():m_line(defaultStart,defaultEnd,defaultColor)
{}

// With pointers

class YourClass
{
...
protected:
CLine* m_pLine;
}

YourClass::YourClass():m_pLine(NULL)
{}

YourClass::~YourClass()
{
if(m_pLine)
{
delete m_pLine;
}
}

Hope these offered some help
Matt