Containership in Inheritance

Question

Hi, I have learned inheritance ,but i want to know what is the containership in inheritance?

Pavi S · Answer

The Concept of Containership Visual FoxPro has a containership model built into it. This model is the implementation of the concept of one object containing other objects. Containership is defined as the ability of one object or class to contain other objects including other containers. This containership model dictates the way we address various objects with our code. Just like addressing a letter to someone, where we must supply the postal code, state, city, street and name, in VFP we must tell fox where the object exists that we want to address. Below is a diagram showing a containership. In this diagram there is a textbox in a Page of a PageFrame that is in a Form. To address the textbox it is necessary to tell VFP where the textbox is as in; Thisform.PageFrame.Page.Textbox. Notice that the syntax lists the containership for the textbox as being in the form, in the PageFrame, and in the page. Example: The syntax of containership can be made simpler through the use of certain FoxPro keywords that provide for relative addressing, that is addressing starting from the location of the object that contains the code. ThisForm, ThisFormSet, and This are three examples of this relative addressing. ThisForm refers to the form that the code is contained in regardless of which control or object in that form has the code. ThisFormSet does the same thing as ThisForm except that it refers to the formset containing the code. This refers to the immediate object that contains the code. By using these keywords we can write code that is independent of the names of the objects. There is one more keyword that can be helpful here, Parent. Every object in Visual FoxPro has a parent property. This property holds an object reference to the immediate container of the object. In the example above, the Textbox's parent is the Page, the page's parent is the PageFrame, the PageFrame's parent is the form, and the form has no parent. This concept of containership is instrumental in understanding how to best use the features of Visual Foxpro. You can construct complex classes with multiple controls and provide all the behavior for those controls in the class definition. Then using them is as simple as dropping them on a form, setting some properties and that's it.

Prabhu Raja · Answer

Hi, In containership, A class contains another object as member data. The class which contains the object cannot access any protected or private members of the contained class (unless the container it was made a friend in the definition of the contained class). Assuming the contained object is not public, the contained class is encapsulated by the container and so programs constructing a container object cannot access the contained class methods unless the container class provides "ribbon-cable" access methods to the contained class. The relationship between the container and the contained object in containership is "has-a" instead of "is-a" . The relationship between the Base and the derived class object in nheritance is "is-a" instead of "has-a" .

Javeed M Shaikh · Answer

Hi Deepak, Containership is the ability of a class to contain objects of different classes as member data. For example, class A could contain an object of class B as a member. Please do not forget to mark "Accepted Answer".

Containership in Inheritance

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