A class constructor or destructor can also be created. They serve to instantiate some class variables or class properties which must be initialized before a class can be used. These constructors are called automatically at program startup: The constructor is called before the initialization section of the unit it is declared in, the destructor is called after the finalisation section of the unit it is declared in.
There are some caveats when using class destructors/constructors:
The following program:
{$mode objfpc} {$h+} Type TA = Class(TObject) Private Function GetA : Integer; Procedure SetA(AValue : integer); public Class Constructor create; Class Destructor destroy; Property A : Integer Read GetA Write SetA; end; {Class} Function TA.GetA : Integer; begin Result:=-1; end; {Class} Procedure TA.SetA(AValue : integer); begin // end; Class Constructor TA.Create; begin Writeln('Class constructor TA'); end; Class Destructor TA.Destroy; begin Writeln('Class destructor TA'); end; Var A : TA; begin end.
Will, when run, output the following:
Class constructor TA Class destructor TA