delete operator in c++

[mickey22]

Hi all,

I have a class say A and created a pointer to call this class like

A *obj=NULL;
obj=new A( );

After I perform all the operations within the code and try to delete this pointer using

delete obj;
obj=NULL;

I get an exception as
" the instruction at "0x7c910f 2b" REFERENCED MEMORY AT "0x00000004".the memory could not be "read".Click OK to terminate the program.

I am not able to figure it out.

If i dont delete it the code runs smoothly.But always have to delete after use once allocated.The whole code is within a function block.

Could anyone please suggest me.

Thanks .

[Ganon11]

Try removing the obj=NULL; portion and see if that works?

[sicarie]

delete obj;
obj=NULL;

So you're deleting obj and then trying to assign it a NULL value?

[Cucumber]

Post the code of the destructor.

[weaknessforcats]

Have you stepped through your destructor with your debugger??

[mickey22]

I removed NULL. It did not work.

I have my code in this way:

my own class

///abc.h
 
class abc
{
   int off;
   int *img;
   abc(void);
   ~abc( );
   void sum( ); 
};
 
//abc.cpp
 
abc::abc(void)
{
   off=0;
}
abc::~abc( )
{
   delete[] img;
}
 
void abc::sum( )
{
   Filter *f;   //class already created by someone and I am using all its functions
   f=new Filter ( );
   std::string filename=f->datafile();
   raw *r;   //another class same like f created by someone.
   r=new raw( );
   r->add(filename );
   img=new float[100];
   delete r;
   r=NULL;
   delete f;
   f=NULL;
}

I am getting that exception just before add function .I get the filename correctly from the class but when I pass to the add( ) that filename is corrupted and I see it as Bad Ptr when I stepped in thru the debugger step by step. when I delete r, I dont get any exception.

It is little bit confusing.Please let me now if any questions.I am still trying to figure it out.

[weaknessforcats]

Filter *f; //class already created by someone and I am using all its functions
f=new Filter ( );

This should be:

Filter *f; //class already created by someone and I am using all its functions
f=new Filter;
 

Also, your abc class:

class abc
{
int off;
int *img;
abc(void);
~abc( );
void sum( );
};

Has no constructor. Therefore no guarantee that the img pointer ever got initialized. If you delete and uninitialized pointer, you crash.

[RobRussell]

The best way to find it is to start by adding error checking. For example:

abc::abc()
{
   off=0;
   img = NULL;
}
...
abc::~abc()
{
   if (img) {
      delete(img);
      }
   img = NULL;
}
 

Adding the code will benefit you in the long run because it will catch the problem every time - even if it shouldn't happen any more.

I'm pretty sure that

std::string filename=f->datafile();
 

copies the return value of datafile() then it gets copied again on the way in to add(). If you meant to get a pointer or a reference then make sure that's what's happening. For example, a pointer allocated on the stack inside datafile() won't be valid after that function returns. I'm always fuzzy on these things in C++ though.

[weaknessforcats]

The best way to find it is to start by adding error checking. For example:

Code: ( cpp )
abc::abc()
{
off=0;
img = NULL;
}
...
abc::~abc()
{
if (img) { <<<<<<<<<<<<<<<<<<<<<<<<<<
delete(img);
}
img = NULL;
}

Pointless test. delete does nothing if the pointer is null. However, setting the pointer to null after the delete is a good idea. This is different from C. There, free() crashes on a null pointer.

For example, a pointer allocated on the stack inside datafile() won't be valid after that function returns. I'm always fuzzy on these things in C++ though.

It's not the pointer that's the problem. It's the address inside the pointer. If that address is to another local variable, then you have a problem. But if the address is to something on the heap, then that address will be valid until you delete it.

[RRick]

Rob Russell had the right idea by setting img = NULL in the constructor. This is what was missing in the original constructor. When the object was created, img had a non-zero value which caused delete to crash and burn.

A minor point, in C++, NULL has fallen out of favor. Using img = 0 is preferable.