Constructors

A constructor is a special method that creates and initializes instance objects. The declaration of a constructor looks like a procedure declaration, but it begins with the word constructor. Examples:

constructor Create;

constructor Create(AOwner: TComponent);

Constructors must use the default register calling convention. Although the declaration specifies no return value, a constructor returns a reference to the object it creates or is called in.

A class can have more than one constructor, but most have only one. It is conventional to call the constructor Create.

To create an object, call the constructor method on a class type. For example,

MyObject := TMyClass.Create;

This allocates storage for the new object, sets the values of all ordinal fields to zero, assigns nil to all pointer and class-type fields, and makes all string fields empty. Other actions specified in the constructor implementation are performed next; typically, objects are initialized based on values passed as parameters to the constructor. Finally, the constructor returns a reference to the newly allocated and initialized object. The type of the returned value is the same as the class type specified in the constructor call.

If an exception is raised during execution of a constructor that was invoked on a class reference, the Destroy destructor is automatically called to destroy the unfinished object.

When a constructor is called using an object reference (rather than a class reference), it does not create an object. Instead, the constructor operates on the specified object, executing only the statements in the constructor's implementation, and then returns a reference to the object. A constructor is typically invoked on an object reference in conjunction with the reserved word inherited to execute an inherited constructor.

Here is an example of a class type and its constructor.

type

TShape = class(TGraphicControl) private

FPen: TPen; FBrush: TBrush;

procedure PenChanged(Sender: TObject); procedure BrushChanged(Sender: TObject);

public constructor Create(Owner: TComponent); override; destructor Destroy; override;

end;

constructor TShape.Create(Owner: TComponent); begin inherited Create(Owner); // Initialize inherited parts

Width := 65; // Change inherited properties

FPen := TPen.Create; // Initialize new fields FPen.OnChange := PenChanged; FBrush := TBrush.Create; FBrush.OnChange := BrushChanged;

end;

The first action of a constructor is usually to call an inherited constructor to initialize the object's inherited fields. The constructor then initializes the fields introduced in the descendant class. Because a constructor always clears the storage it allocates for a new object, all fields start with a value of zero (ordinal types), nil (pointer and class types), empty (string types), or Unassigned (variants). Hence there is no need to initialize fields in a constructor's implementation except to nonzero or nonempty values.

When invoked through a class-type identifier, a constructor declared as virtual is equivalent to a static constructor. When combined with class-reference types, however, virtual constructors allow polymorphic construction of objectsthat is, construction of objects whose types aren't known at compile time. (See Class references.)

Note: For more information on constructors, destructors, and memory management issues on the .NET platform, please see the topic Memory Management Issues on the . NET Platform.

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