«

class base {
public:
base(void *) {}
};

class der : base {
public:
der() :base(this) {}
};

Does this have any defined value when used in a base class initialiser?
Only 1 out of 3 compilers have given a warning.

Fraser.

I think it points to an uninitialised sub-object. It needs to be used
with some care.

Fraser.

Fraser Ross wrote:
> class base {
> public:
> base(void *) {}
> };
>
> class der : base {
> public:
> der() :base(this) {}
> };
>
> Does this have any defined value when used in a base class initialiser?

Absolutely.

It points to the memory where the respective object resides (or will
reside once initialised).

> Only 1 out of 3 compilers have given a warning.

You are not prohibited from using 'this' in the initialiser list. Just
be careful, the object hasn't been fully initialised yet.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

On Jun 25, 7:25 pm, "Fraser Ross" <z...@zzzzzz.com> wrote:
> class base {
> public:
> base(void *) {}
> };

> class der : base {
> public:
> der() :base(this) {}
> };

> Does this have any defined value when used in a base class
> initialiser?

Yes, but it might not be what is wanted. The implicit
conversion to void* is very dangerous here; not only can base
not use the pointer in its constructor, it must convert it to
der* in order to use it later.

--
James Kanze (GABI Software) email:
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
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On Jun 25, 9:29 pm, Victor Bazarov <v.Abaza...@comAcast.net> wrote:
> Fraser Ross wrote:
> > class base {
> > public:
> > base(void *) {}
> > };

> > class der : base {
> > public:
> > der() :base(this) {}
> > };

> > Does this have any defined value when used in a base class
> > initialiser?

> Absolutely.

> It points to the memory where the respective object resides
> (or will reside once initialised).

It points to the memory where the der object will reside. The
only way the base class can correctly use it is by reconverting
it to a der*.

--
James Kanze (GABI Software) email:
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

>"James Kanze"
>On Jun 25, 7:25 pm, "Fraser Ross"
>> class base {
>> public:
>> base(void *) {}
>> };

>> class der : base {
>> public:
>> der() :base(this) {}
>> };

>> Does this have any defined value when used in a base class
>> initialiser?

>Yes, but it might not be what is wanted. The implicit
>conversion to void* is very dangerous here; not only can base
>not use the pointer in its constructor, it must convert it to
>der* in order to use it later.


It could also be converted later to base*.

I think my real code might violate 12.7/3. Where it says "might use
path E* -> D*-> A*" is it implying that conversions from E* to C* and D*
are equally good because they are directly inherited base classes? If I
remove C, B and X from the example and force the conversions of E* ->
D*-> A* the Comeau compiler gives no diagnostics but the standard is
saying its undefined behaviour.

struct A { };

struct D : virtual A {
D(A*);
};

struct E : D {
E() : D(this) {}// undefined: upcast from E* to A* might use path E* !
D* ! A*
};

Fraser.

On Jun 26, 2:20 pm, "Fraser Ross" <z...@zzzzzz.com> wrote:
> >"James Kanze"
> >On Jun 25, 7:25 pm, "Fraser Ross"
> >> class base {
> >> public:
> >> base(void *) {}
> >> };
> >> class der : base {
> >> public:
> >> der() :base(this) {}
> >> };
> >> Does this have any defined value when used in a base class
> >> initialiser?
> >Yes, but it might not be what is wanted. The implicit
> >conversion to void* is very dangerous here; not only can base
> >not use the pointer in its constructor, it must convert it to
> >der* in order to use it later.

> It could also be converted later to base*.

If you don't mind undefined behavior. The only legal use of a
void* is converting it back to the original type. Here, the
original type is der*, not base*.

> I think my real code might violate 12.7/3. Where it says
> "might use path E* -> D*-> A*" is it implying that conversions
> from E* to C* and D* are equally good because they are
> directly inherited base classes? If I remove C, B and X from
> the example and force the conversions of E* -> D*-> A* the
> Comeau compiler gives no diagnostics but the standard is
> saying its undefined behaviour.

You'll have to show use your real code. My comments concerning
undefined behavior mainly concerned the passage through a void*.
The examples in 12.7/3 don't have any void*. §12.7/2 does
guarantee that you could pass your this to a base*, but it
doesn't say anything about a void*.

> struct A { };

> struct D : virtual A {
> D(A*);
> };

> struct E : D {
> E() : D(this) {}// undefined: upcast from E* to A* might use path E* !
> D* ! A*
> };

I'm not sure vis-a-vis the standard, but I do know that this
fails with certain compilers.

Curiously, you can call member functions of A in the same
context (and calling such functions requires the same
conversion). And the member function can return its this
pointer.

--
James Kanze (GABI Software) email:
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34