Objective Scope

Assigning values to object variables

You can assign one object pointer variable to another object pointer variable if the variables are of the same type or assignment compatible, just as you can assign variables of any type other than objects to variables of the same type or assignment-compatible types. For example, if the variables Form1 and Form2 have been declared as pointers to TForm, you could assign Form1 to Form2:

Form2 = Form1;

You can also assign an object pointer variable to another object pointer variable if the type of the variable you are assigning a value to is an ancestor of the type of the variable being assigned. For example, here is a TDataForm class declaration:

class TTDataForm : public TForm

{

__published:     // IDE-managed Components

TButton *Button1;

TEdit *Edit1;

TDBGrid *DBGrid1;

TDatabase *Database1;

private: // User declarations

public:              // User declarations

virtual __fastcall TTDataForm(TComponent* Owner);

};

Here are the declarations of two objects; one of type TForm, and one of type TDataForm:

TDataForm *DataForm;

TForm *AForm;

AForm is of type pointer to TForm, and DataForm is of type pointer to TDataForm. Because TDataForm is a descendant of TForm, this assignment statement is legal:

AForm = DataForm;

You might wonder why this is important. Taking a look at what happens behind the scenes when your application calls an event handler shows you why.

Suppose you write an event handler for the OnClick event of a button. When the button is clicked, the event handler for the OnClick event is called.

Each event handler has a Sender parameter of type pointer to TObject. For example, note the Sender parameter in this empty Button1Click handler:

void __fastcall TForm1::Button1Click(TObject *Sender)

{

}

TObject is at the top of the C++Builder Visual Component Library hierarchy. That means that all C++Builder objects are descendants of TObject. Because Sender is of type TObject, any object can be assigned to Sender. Although you don’t see the code that makes the assignment, the component or control to which the event happened is assigned to Sender. This means the value of Sender is always the control or component that responds to the event that occurred.

How is this information useful? You can test Sender to find the type of component or control that called the event handler. For example:

if (typeid(*Sender) == typeid(TEdit))

DoSomething;

Else  DoSomethingElse;

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Objective-C Tutorial – Lesson 11: Part 1: Scope of Variables

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