CODEWITHSUNDEEP
c++scope
SuryaRao
SuryaRao

Reputation: 51

Scope of an object created in a function

I have created an object of a class inside a function. The created object is passed as a parameter to another class. I expect that when I exit the function in which the object is created, the object must be destroyed.

Also any references to this object must be invalid, but I find that the object referenced after exiting the function is still valid. Trying to understand the scope of the object. The code snipped is given below.

class TextWidget
{
    public:
        TextWidget()
        {
            cout <<  "Constructor TextWidget" << endl;
        }

        TextWidget(int id)
        {
            _ID = id;
            cout <<  "Constructor TextWidget" << endl;
        }

        ~TextWidget()
        {
            cout <<  "Destructor TextWidget" << endl;
        }

        void printWidgetInstance()
        {
            cout << this << endl;
        }
        int _ID;
};

class AnotherWidget
{
    public:
        AnotherWidget()
        {
            cout << "Constructor AnotherWidget" << endl;
        }

        ~AnotherWidget()
        {
            cout << "Destructor AnotherWidget" << endl;
        }

        TextWidget* getTextWidget()
        {
            return _textWidget;
        }

        void setTextWidget(TextWidget* t)
        {
            _textWidget = t;
        }

        int getTextID() { return _textWidget->_ID; }

    private:
        TextWidget* _textWidget;
};


ButtonWidget b;
AnotherWidget a;

void fun()
{
    TextWidget t(7);
    b.setTextWidget(&t);
    a.setTextWidget(&t);
    a.getTextWidget()->printWidgetInstance();
    b.getTextWidget()->printWidgetInstance();
}

int main()
{
    fun();
    cout << "TextWidget in AnotherWidget is ";
    a.getTextWidget()->printWidgetInstance();
    cout << "Text ID in a is " << a.getTextID() << endl;
    getchar();
    return 0;
}

OUTPUT

Constructor AnotherWidget
Constructor TextWidget
Before deleting TextWidget in class ButtonWidget 0x73fdf0
Before deleting TextWidget in class AnotherWidget 0x73fdf0
0x73fdf0
0x73fdf0
Destructor TextWidget
TextWidget in AnotherWidget is 0x73fdf0
Text ID in a is 7

Upvotes: 2

Views: 1107

Answers (2)

cmks
cmks

Reputation: 527

You need to understand what classes, objects, pointers and references are. Classes are code in memory, this code deals with the member variables. A class does not occupy any data memory. When you create a object you instantiate the class. This step will reserve some data memory for the local data of one object instance of the class. To access this instance you get a reference to the object (this). An object variable holds a pointer to the data memory of an instance of a class.

When an object gets destroyed the occupied memory block gets listed as free but not cleared. So if you still have a reference to that memory block you may access it. And as long as the system doesnt use this memory block for other purposes you still will find your bits and bytes there.

You may declare some variables:

long      long_array[10];
myclass   myobject;

These variables are stored sequential in one memory block. When you now access the memory behind the long_array through that array:

long_array[10] = 12345; // valid range is from long_array[0] to long_array[9]

Than you will overwrite the data of the object myobject.

Upvotes: 1

JGroven
JGroven

Reputation: 613

A variable declared using automatic storage duration (not using new) has a lifetime of its scope. Accessing the variable outside the scope results in undefined behavior.

Upvotes: 4

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