Delphi in a Nutshell

Name

Constructor Keyword

Syntax

type Class declaration
...
  constructor Name;
  constructor Name(Arguments...);
end;

Description

A constructor is a special kind of method. If you call a constructor using a class reference, Delphi creates a new instance of that class, initializes the instance, and then calls the constructor proper. If you call a constructor using an object reference, Delphi calls the constructor as an ordinary method.

A natural consequence of Delphi’s rules is that a constructor can call another constructor of the same class or call an inherited constructor. The call uses an object reference (namely, Self) so the constructor is called as an ordinary method.

When called using a class reference, Delphi calls the NewInstance method to create the instance. TObject.NewInstance allocates memory for the new object and fills that memory with all zeros, but you can override NewInstance to create the object in a different manner.

Tips and Tricks

• A common error for new Delphi programmers is to call a constructor using an object-type variable. Rather than creating the object, such a call invariably results in access violations because the object reference is most likely invalid. The compiler can warn you about such errors if you enable compiler warnings in the project options. For example:

var
  ObjRef: TSomething;
begin
  ObjRef.Create;                 // wrong
  ObjRef := TSomething.Create;   // right

• If you are writing an abstract base class, and the constructor is virtual so you can write a class factory, ...