pointer to member conversion to a pointer to member that is a member's base class

[Vladimir_petter]

Dear All,

There is the problem in nutshells (see the program bellow):

It is ok to convert pointer to F<T> to the pointer to I.
Now if I have pointer to member "F<T> entity::*" can I convert it to the "I
entity::*"?
Compiler does not let me to do that (I've tried on VC 7.1 and Comeau 4.3.3).
If I do reinterpret_cas t then the program will be compiled and runs as
expected all though I would like to know possible implications of this
approach.

The goal here to create a static array of fields for a class and be able to
execute algorithms on this array (like the for loop in the main function).

Thanks,
Vladimir.

#include <string>

using namespace std;

struct I
{
virtual char const * foo()=0;
};

template <typename T>
struct F : public I
{
char const * foo()
{
return typeid(T).name( );
}

T v_;
};

struct entity
{
typedef I entity::* mem_t;

static mem_t fields[4];

F<int> f1_;
F<char> f2_;
F<string> f3_;
F<double> f4_;
};

entity::mem_t entity::fields[4] = {
reinterpret_cas t<mem_t>(&entit y::f1_),
reinterpret_cas t<mem_t>(&entit y::f2_),
reinterpret_cas t<mem_t>(&entit y::f3_),
reinterpret_cas t<mem_t>(&entit y::f4_),
};
int main(int, char **)
{
entity e;
for(int i=0; i<4; ++i)
{
printf("%s\n", (e.*(entity::fi elds[i])).foo());
}

return 0;
}

[Victor Bazarov]

Alexander Grigoriev wrote:

"Victor Bazarov" <v.********@com Acast.net> wrote in message
news:uN******** ******@TK2MSFTN GP09.phx.gbl...

Alexander Grigoriev wrote:

"Victor Bazarov" <v.********@com Acast.net> wrote in message
news:eM***** *********@tk2ms ftngp13.phx.gbl ...
"Alexande r Grigoriev" <al***@earthlin k.net> wrote...
>Use static_cast. It allows for those explicit convertions, if the

original
>and target types are compatible.

Are they compatible? How do you define "compatible "?

If there exists static_cast from B* to D * and vice versa (B being base
class of D), then there exist both implicit conversion from pointer to
member of base B to pointer to member of derived D (4.11/2) and explicit
static_cas t to another direction (5.2.9/9).

Did you miss the fact that it's not a member of B or member of D the OP
was asking about? They both are members of 'T', but one is of class B
and the other is of class D. B T::* and D T::*. Can we simply drop the
fact that they are pointers to members of the same type? I submit that
we cannot.

Victor

Yes, such conversion doesn't seem possible, other than brute-force
reinterpret_cas t.

reinterpret_cas t is not the "catch all" method. It still must behave
according to the language rules. The Standard specifies a strict set of
conversions _permitted_ to be done using reinterpret_cas t (see 5.2.10)
and this one isn't one of them either. So, "brute force" will result in
_undefined_beha viour_.

I think the language could be changed to permit that conversion (even to
let it be implicit), but it's a issue to be resolved in comp.std.c++.

Victor

[Victor Bazarov]

Vladimir_petter wrote:

Dear All,

There is the problem in nutshells (see the program bellow):
[..]

I posted your message along with my summary to comp.std.c++. If and
when there are replies, I'll report back here.

Victor

[Vladimir_petter]

Hello Viktor,

There is the problem in nutshells (see the program bellow):
[..]

I posted your message along with my summary to comp.std.c++. If and
when there are replies, I'll report back here.

Thanks for your efforts. I actually posted my question to the
comp.lang.c++.m oderated, but might be I've picked up a wrong group or the
way I put the problem is not right, but so far there was no response.
Anyways I've marked your posting in comp.std.c++ and will monitor it.

Thanks,
Vladimir.