Re: default constructor and class

[Michael Goffioul]

On Mon, Jul 8, 2013 at 3:47 PM, Marco Vassallo <address@hidden> wrote:

________________________________
> Date: Mon, 8 Jul 2013 15:42:41 -0400
> Subject: Re: default constructor and class
> From: address@hidden
> To: address@hidden
> CC: address@hidden
>
>
> On Mon, Jul 8, 2013 at 3:34 PM, Marco Vassallo
> <address@hidden<mailto:address@hidden>> wrote:
> ________________________________
> > Date: Mon, 8 Jul 2013 15:27:45 -0400
> > Subject: Re: default constructor and class
> > From: address@hidden<mailto:address@hidden>
> > To: address@hidden<mailto:address@hidden>
> > CC: address@hidden<mailto:address@hidden>
> >
> > On Mon, Jul 8, 2013 at 3:20 PM, Marco Vassallo
> >
> <address@hidden<mailto:address@hidden><mailto:address@hidden<mailto:address@hidden>>>
> wrote:
> > Hi all,
> >
> > for the fem-fenics pkg I'm creating a new class, and I thus add as
> > private members
> >
> >    DECLARE_OCTAVE_ALLOCATOR;
> >    DECLARE_OV_TYPEID_FUNCTIONS_AND_DATA;
> >
> > and also the DEFINE counterpart,
> >
> > which requires the class to have a default constructor (right?)
> >
> > The problem is that one of the parent class from which I derive my
> own class
> > doesn't provide a default constructor. Example
> >
> > class B: public octave_base_value, public A
> > {
> >    B(): octave_base_value(), A(), B() {} //error: A() not available
> >
> >    private:
> >    A my_element;
> >    DECLARE_OCTAVE_ALLOCATOR;
> >    DECLARE_OV_TYPEID_FUNCTIONS_AND_DATA;
> > }
> >
> > How can I manage it?
> >
> > I'm not sure I understand your construct correctly, but is it
> > intentional that B inherits from A *and* contains a A object
> > (my_element)? Can't you get rid of the inheritance from A and just have
> > the my_element member?
> >
> > Michael.
>
> This class should be a wrapper for the A class.
> Even if I avoid deriving from A,
>
>   class B: public octave_base_value
> {
>   B(): octave_base_value() {} //error: default constructor for
> my_element not available
>
>   private:
>   A my_element;
>   DECLARE_OCTAVE_ALLOCATOR;
>   DECLARE_OV_TYPEID_FUNCTIONS_AND_DATA;
>   }
>
> I still need a constructor for my_elements, and as it is an object of
> type A I
> need to initialize it somehow.
>
> Why don't you do the following instead for the default constructor of
> class B:
>
> B(): octave_base_value(), my_element(<whatever_argument>) {}
>
This is exactly what I'm doing, but the <whatever_argument> is something which
I added to the class only for this purpose and which should not be there.
In my class there should be nothing else then

A my_element;

To be honest, it's hard to help without more concrete information. But if class A does not provide a public/protected constructor, maybe it provides another constructor that accepts argument. If it does not, then it means class A was not intended to be used directly and you're misusing it.

Michael.