c ++删除不存在的指针

Question

class C {

public:
    std::vector<C*> list;
    int value;

    C(int value,  C* parent)
    {
        this->value = value;

        if(parent)
        {
            parent->registerChild(this);
        }
    }

    void registerChild(C* child)
    {
        this->list.push_back(child);
    }

    ~C()
    {
        for(std::vector<C*>::iterator it = list.begin(); it != list.end(); ++it)
        {
            if( (*it))
            {
                delete (*it);
            }
        }
    }
};

This is for a GUI project. A child class that has a parent, must notify parent class of it so that when the parent class gets deleted, all of its children should be deleted too.

C* main = new C(100, 0);

C* child1 = new C(250, main);
C* child2 = new C(450, main);

delete main;

^ everything works as expected - main is deleted along with child1 and child2.

C* main = new C(100, 0);

C* child1 = new C(250, main);
C* child2 = new C(450, main);

delete child1;

delete main; // windows error

If I decided to get rid of child1 first and then sometime later decided to delete main, I get a windows error that is traced back to vector loop where apparently delete tries to delete a now non-existent pointer. I would have thought that if( (*it)) would return false for a pointer that doesn't exist anymore.

What can I do here?

EDIT: This appears to work flawlessly

class C {

    typedef std::vector<C*> cList;

public:
    std::vector<C*> list;
    int value;
    C* parent;

    C(int value,  C* parent)
    {
        this->value = value;
        this->parent = parent;

        if(parent)
        {
            parent->registerChild(this);
        }
    }

    void registerChild(C* child)
    {
        this->list.push_back(child);
    }

    void removeChild(C* child)
    {
        cList::iterator it = std::find(list.begin(), list.end(), child);

        if(it != list.end())
        {
            list.erase(it);
        }
    }

    ~C()
    {
        if(this->parent)
        {
            // this child is being removed - notify parent and remove this from its child_list
            this->parent->removeChild(this);
        }

        cList::iterator it = list.begin();

        while(it != list.end())
        {
            delete (*it);

            // find a new beginning
            it = list.begin();
        };
    }
};

Answer 1

When you call delete main , it will call delete on its children. child1 is one of them. So you are calling delete on the same memory address twice. And that is undefined behaviour .

I would have thought that if( (*it)) would return false for a pointer that doesn't exist anymore.

No, there is no mechanism for one pointer to know that the object it points to has been deleted via another pointer.

What can I do here?

Make sure only one entity is responsible for managing dynamically allocated objects.

Answer 2

It is a semi-widely missconception that a pointer is set to NULL when the memory it points to is deleted. It is not. Also, it is safe to delete NULL so checking whether a pointer is not NULL before deleting it is not necessary (off topic...).

Answer 3

There's nothing magic about pointers. They are just (generally speaking) integers of machine word width which can be used to refer to specific memory addresses. There's no magic that NULLs that integer when the memory area it mentions is no longer owned by your program.

It's a bad idea to deal with raw pointers like this. People are too easily confused by who is responsible for managing the memory pointed to.

Look into std::unique_ptr and std::shared_ptr ( http://en.cppreference.com/w/cpp/memory ).

Consider keeping the parent pointer as a member of the class, adding a deregister member function. Then in the destructor of the class, have the object deregister itself from the parent. You'll need to change how you manage the child window list.

Also, the names of your class member variables really should be more descriptive. list doesn't tell us much. Consider a name like children or child_list .