Creating Objects

Assume Emp is a class

Code:

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Emp o;

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here, is o itself the object? , if so how is memory allocated for it, implicity via call to new or malloc() ?

Code:

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Emp *o = new Emp();

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in C++ when objects go out of scope their destructors are automatically called

so, what if i dont call

Code:

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delete o;

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and let the pointer go out of scope, for it to call the destructor for the object automatically?

Quote:

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here, is o itself the object? , if so how is memory allocated for it, implicity via call to new or malloc() ?

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Implicitly. If it's an ordinary automatic local variable like this:

Code:

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void function() {
    Emp o;
}

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then whenever function() is executed, enough memory is reserved on the stack to hold o. This enables each call to function to have its own copy of Emp, allowing recursion.

[edit] You only create objects with new when you're storing those objects in pointers. [/edit]

malloc() and new allocate memory on the heap rather than on the stack.

Quote:

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Code:

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Emp *o = new Emp();

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in C++ when objects go out of scope their destructors are automatically called

so, what if i dont call

Code:

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delete o;

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and let the pointer go out of scope, for it to call the destructor for the object automatically?

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Destructors are only called for objects allocated on the stack. Objects allocated with new must be deleted for the destructor to be called and the memory to be freed. This allows one function to allocate some memory, and return a pointer to that memory. The calling function will be able to use that memory because it wasn't freed when the allocating function returned.

>here, is o itself the object?
Yes.
>if so how is memory allocated for it, implicity via call to new or malloc() ?
Space is set aside for the object in the area used by the compiler to store automatic (local) variables, usually the stack.

>in C++ when objects go out of scope their destructors are automatically called
True for local (automatic) variables like the first o above. Not true for objects created by operator new like the second o above.

>so, what if i dont call

Code:

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>delete o;

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>and let the pointer go out of scope, for it to call the destructor for the object automatically?
For every call to new you need a corresponding call to delete. Otherwise the memory for that object is never deallocated, until possibly when your program ends, and most operating systems will reclaim it.

If the object is created using malloc()

do i need to call the destructor for the object explicitly and then deallocate memory using free()?

Well, objects created with malloc() don't get their constructor called, so why should they get destructed? ;) Don't allocate classes with malloc(). There's no reason to.

Yeah, rite would be inappropriate to use malloc() for object creation

However i m letting my mind flow and thought of this

Code:

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int main()
{
  demo *q = create();    // assume demo class
  cout<< q->show();    // assume show() in demo
  destroy(q);   
}

demo* create()
{
  demo *p = (demo*)malloc(sizeof(demo));            // allocate space
  p -> demo();              // call constructor
  return(p);
}

void destroy(demo*q)
{
  q -> ~demo();              // call destructor
  free(q);                        // deallocate space
}

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any suggestions?

Quote:

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Originally Posted by vb.bajpai

any suggestions?

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yes. don't do that.

Code:

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int main()
{
  demo *q = new demo;
  cout<< q->show();   
  delete q;   
}

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Quote:

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Originally Posted by vart

Code:

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int main()
{
  demo *q = new demo;
  cout<< q->show();   
  delete q;   
}

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or even more betterer...

Code:

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int main()
{
  auto_ptr<demo> q(new demo);
  cout<< q->show();   
}

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:D