Hi,

Given:

template <typename T>
struct Y {
   T x;
};


Is it safe to assume that the address of any Y object is the
same as the address of its member x?

Thanks,
Joel de Guzman


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joel de guzman wrote:
>
> Hi,
>
> Given:
>
> template <typename T>
> struct Y {
>    T x;
> };
>
> Is it safe to assume that the address of any Y object is the
> same as the address of its member x?

No. According to 9.2p17, "A pointer to a POD-struct object, suitably
converted using a reinterpret_cast, points to its initial member (of if
that member is a bit-field, then to the unit in which it resides) and
vice versa." Therefore, the guarantee doesn't hold if T is a type that
renders Y<T> a non-POD struct. For instance, if T was a member pointer,
or a reference type, a non-POD struct or non-POD union type, or a
function type, then there's no guarantee. In particular, the address
can't possibly be the same as the address of x, if T is a function type,
since in that case x would be a member function.

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In article <9b36q3$7lvre$1@ID-58159.news.dfncis.de>, joel de guzman says...
>
>Hi,
>
>Given:
>
>template <typename T>
>struct Y {
>   T x;
>};
>
>
>Is it safe to assume that the address of any Y object is the
>same as the address of its member x?
>
>Thanks,
>Joel de Guzman

Each class Y<T> is a POD, and the first member of a POD
has the same address as the structure itself. (9.2/17)

So the answer is yes.

Adding copy assignment ctors and dtors, or non-POD types to the class
may break this relation.

Regards,

--
Michiel Salters
Consultant Technical Software Engineering
CMG Trade, Transport & Industry
Michiel.Salters@cmg.nl

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joel de guzman wrote:
>
> Given:
>
> template <typename T>
> struct Y {
>    T x;
> };
>
> Is it safe to assume that the address of any Y object is the
> same as the address of its member x?
>

Only if Y<T> is a POD-struct, which in this case requires that T be a POD type.

The exact definition of a POD (Plain Old Data) type is a bit
complicated. Any data type that can be declared in C is a POD type.
If Y or T has C++ features, you need to refer to the C++ standard,
section 9 paragraph 4, which in turn refers to you section 8.5.1,
for full details.

--
Steve Clamage, stephen.clamage@sun.com

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Michiel Salters wrote:
>
> In article <9b36q3$7lvre$1@ID-58159.news.dfncis.de>, joel de guzman says...
> >
> >Hi,
> >
> >Given:
> >
> >template <typename T>
> >struct Y {
> >   T x;
> >};
> >
> >
> >Is it safe to assume that the address of any Y object is the
> >same as the address of its member x?
> >
> >Thanks,
> >Joel de Guzman
>
> Each class Y<T> is a POD, and the first member of a POD
> has the same address as the structure itself. (9.2/17)

 struct Z{
  int i;
  Z(int in) i(in) {};
 };

 union U{
  int j;
  Z z;
 };

None of the following are POD classes: Y<int&>, Y<Z>, Y<U>, Y<int Z::*>.
And how about:

 typedef void void_func();

Y<void_func>::x is a member function, rather than a data member; as
such, it can't have the same address as the structure itself.

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In article <3AD6F375.43220D23@wizard.net>, James Kuyper Jr. says...
>
>Michiel Salters wrote:
>>
>> In article <9b36q3$7lvre$1@ID-58159.news.dfncis.de>, joel de guzman says...
>> >
>> >Hi,
>> >
>> >Given:
>> >
>> >template <typename T>
>> >struct Y {
>> >   T x;
>> >};
>> >
>> >Is it safe to assume that the address of any Y object is the
>> >same as the address of its member x?
>> >
>> >Thanks,
>> >Joel de Guzman
>>
>> Each class Y<T> is a POD, and the first member of a POD
>> has the same address as the structure itself. (9.2/17)

>None of the following are POD classes: Y<int&>, Y<Z>, Y<U>, Y<int Z::*>.

You're right, I should have added "if and only if T is a POD itself."

>And how about:
>
> typedef void void_func();
>
>Y<void_func>::x is a member function, rather than a data member; as
>such, it can't have the same address as the structure itself.

I don't think so. Y<void_func>::x is a pointer to a non-member function.
And because it is a pointer, it can have offset of 0 in the structure.

I can't think of a way to create a member function other than writing
class { // or struct
Return_Type MethodName (Argument_opt) // { optional implementation }
}

--
Michiel Salters
Consultant Technical Software Engineering
CMG Trade, Transport & Industry
Michiel.Salters@cmg.nl

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