Linker errors with Interfaces

[Andrej Mitrovic]

Private does not prevent overriding, final prevents overriding. Private
prevents the methods from being explicitly called from either user code or
any derived classes.

On Wed, Aug 11, 2010 at 8:08 AM, Igor Lesik <curoles at yahoo.com> wrote:

> I think, it is not linker error at all.
> "private" in Transmogrifier prevents overriding; and since those 2
> functions are
> not
> defined but still in use (vtbl) linker does not find them and complains.
> Replace "private" with protected (in CardboardBox also) and it all works.
>
> compiler should say that the functions are not overrided; I am sure a bug
> report
> already filed for this issue
>
> Igor
>
> From: Andrej Mitrovic
> Subject: Linker errors with Interfaces
>
> Excerpt from TDPL, p213-214:
>
> interface Transmogrifier
> {
>    // client interface
>    final void thereAndBack()
>    {
>        transmogrify();
>        untransmogrify();
>    }
>
>    // implementation interface
> private:
>    void transmogrify();
>    void untransmogrify();
> }
>
> class CardboardBox : Transmogrifier
> {
>    override private void transmogrify()
>    {
>        // get in the box
>    }
>
>    override private void untransmogrify()
>    {
>        // get out of the box
>    }
> }
>
> void play()
> {
>    writeln("just playing");
> }
>
> void aDayInLife(Transmogrifier device, string mood)
> {
>    if (mood == "play")
>    {
>        device.transmogrify();
>        play();
>        writeln(typeid(device));
>        device.untransmogrify();
>    }
>    else if (mood == "experiment")
>    {
>        device.thereAndBack();
>    }
> }
>
> import std.stdio;
>
> void main()
> {
>    aDayInLife(new CardboardBox, "play");
> }
>
>
> Calling device.transmogrify() and device.untransmogrify() should not even
> be
> allowed to compile. I can comment these out, but the linker will still fail
> when
>
> there's a call to device.thereAndBack(); device is an object that inherits
> from
> interface Transmogrifier, and "thereAndBack()" is a public method of the
> interface, so I'm not sure whats wrong with that call.
>
> I get these in both cases:
>
> output\interface_test.obj(interface_test)
> Error 42: Symbol Undefined
> _D14interface_test14Transmogrifier12transmogrifyMFZv
> output\interface_test.obj(interface_test)
> Error 42: Symbol Undefined
> _D14interface_test14Transmogrifier14untransmogrifyMFZv
>
>
> One more thing, here's a class that inherits from the same interface, but
> tries
> to make the overriden methods non-private by mistake:
> interface Transmogrifier
> {
>
>    // client interface
>    final void thereAndBack()
>    {
>        transmogrify();
>        untransmogrify();
>    }
>
>    // implementation interface
> private:
>    void transmogrify();
>    void untransmogrify();
> }
>
> class CardboardBox : Transmogrifier
> {
>    override void transmogrify()
>    {
>        // get in the box
>    }
>
>    override void untransmogrify()
>    {
>        // get out of the box
>    }
> }
>
> I get these errors:
> interface_test.d(23): Error: function
> interface_test.CardboardBox.transmogrify
> does not override any function
> interface_test.d(28): Error: function
> interface_test.CardboardBox.untransmogrify
>
> does not override any function
>
> The book has nice plausible error message alternatives:
> // Error: Cannot change protection of transmogrify from private to public
> // Error: Cannot change protection of untransmogrify from private to public
>
> I'd prefer if DMD put out those kinds of error messages in this case (or
> something similar).
>
>
>
>
>
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