On Mon, Aug 19, 2013 at 10:44 PM, Xiangrong Fang <xrf...@gmail.com> wrote: > Hi All, > > I am reading this document: > http://www.freepascal.org/docs-html/ref/refsu29.html and doing an > experiment with the following code: > > program project1; > {$mode objfpc}{$H+} > type > TBase = class > constructor Create; virtual; > end; > TDerived = class(TBase) > constructor Create; override; > end; > > var > c1, c2: TBase; > c3: TDerived; > > constructor TDerived.Create; > begin > WriteLn('Entering TDerived.Create'); > inherited Create; > WriteLn('Leaving TDerived.Create'); > end; > > constructor TBase.Create; > begin > WriteLn('Entering TBase.Create'); > Writeln('Leaving TBase.Create'); > end; > > begin > WriteLn('Creating a TBase and assigning to TBase variable...'); > c1 := TBase.Create; > WriteLn('Creating a TDerived and assigning to TBase variable...'); > c2 := TDerived.Create; > WriteLn('Creating a TDerived and assigning to TDerived variable...'); > c3 := TDerived.Create; > end. > > > The problem is, it makes NO DIFFERENCE at all in the following cases: > > CASE 1: > > TBase.Create; > TDerived.Create; > > CASE 2: > > TBase.Create; virtual; > TDerived.Create; virtual; > > CASE 3: > > TBase.Create; virtual; > TDerived.Create; override; > > According to the document, "inherited" cannot be used in non-virtual > methods,
At least that's the result I would expect :) I can confirm your findings; contrary to the documentation, a simple test works correctly (the compiler prints no warnings or hints, both method and constructor work correctly, calling either a non-virtual from virtual, and virtual from non-virtual). > and it is wrong to use virtual in sub-class. It doesn't say that, it simply says that the redeclaration will not override the base implementation (the different behavior for 'object' is news to me!). FWIW your test doesn't actually "exercise" inheritance, since virtual constructors only make a difference when you call from a class reference (similarly, you'll only see a difference in virtual methods when calling from a base-typed variable). Code>>> var MyClass: TComponentClass = TDataModule; begin MyClass.Create(nil); <<< will instantiate a TDataModule. Code>>> var MyControl: TControl; begin MyControl := TButton.Create; MyControl.ExecuteDefaultAction; <<< will invoke TButton's implementation of ExecuteDefaultAction. > But my test shows the contrary. BTW, I am running Linux on 64bit platform. > > Regards, > Shannon Best regards, Flávio _______________________________________________ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
