CODEWITHSUNDEEP
c++invalid-pointer
vaeVictis
vaeVictis

Reputation: 492

Invalid pointer after a call to delete

I have this simple code in a file called virtual.cpp:

#include <iostream>


class Parent
{
    public:
    virtual ~Parent(){ std::cout << "Parent Destructor" << std::endl; }

    virtual void VirtualFunction()
    {
        std::cout << "VirtualFunctionInParent" << std::endl;
    }
};

class Child: public Parent
{
    public:
    virtual ~Child() { std::cout << "Child Destructor" << std::endl; }
    virtual void VirtualFunction()
    {
        std::cout << "VirtualFunctionInChild" << std::endl;
    }    
};

int main()
{
    Child child1;
    Parent *parent1 = &child1;
    delete parent1;

}

It compiles correctly

g++ -Wall --pedantic -o virtual virtual.cpp

But when I run it, I obtain this error:

./virtual 
Child Destructor
Parent Destructor
free(): invalid pointer
Annullato (core dump creato)

I think it is due to the fact that I delete the pointer parent1, so this erases the memory to which the pointer point, namely &child1. So, when child1 goes out of scope, the Child destructor try to deallocate memory which no longer exists.

My questions:

  1. is my reasoning correct?
  2. if my reasoning is correct, which is the correct way to handle this "going out of scope" situation?

Upvotes: -1

Views: 375

Answers (4)

Blaine McMahon
Blaine McMahon

Reputation: 197

To answer your question more specifically

  1. is my reasoning correct?

You are never going out of scope because you are declaring variables to the current stack frame in main(). In order to declare the variable to the heap (out of scope), you have to use dynamic memory allocation.

Parent * parent1 = new(Child);
delete(parent1);

What this code is doing is creating a child class in heap and then creating a parent1 pointer local to the scope that effectively points to the child class.

Upvotes: 1

Robert Kwiatkowski
Robert Kwiatkowski

Reputation: 461

First thing is the fact that you don't use new or any other way to allocate the dynamic memory. So there is no point to use delete.

To answer your next questions you must first tell us why do you want to create the parent pointer on address of the child?

Upvotes: 1

Drew Dormann
Drew Dormann

Reputation: 63745

I delete the pointer parent1, so this erases the memory to which the pointer point. Is my reasoning correct?

No. You are getting a core dump from your delete.

Only addresses returned from new may be passed to delete.

delete does not erase memory, as memory has no concept of an "erased" state.

which is the correct way to handle this "going out of scope" situation?

Remove the delete line. It is not necessary.

Any variable created within a scope is automatically managed when that scope is left.

Upvotes: 3

David Schwartz
David Schwartz

Reputation: 182753

Don't call delete on an object you didn't allocate with new. This object will be deleted when it goes out of scope, so there's nothing special to do to destroy it properly.

When you want objects to start their lifetime inside a scope and end their lifetime when the scope ends, don't use new and delete. Just declare them normally.

Use new and delete for objects whose lifetimes you want to manage. Allocate them with new and call delete when you are done with them.

Upvotes: 1

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