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Thomas Thiriez
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 Posted: Wed Aug 27, 2003 9:38 am Post subject: Empty POD as a base |
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Consider this struct:
struct B
{
};
Obviously, it's a POD.
Therefore, I am allowed to copy instances of it with:
void CopyB(B &b1, const B &b2)
{
memcpy(reinterpret_cast<unsigned char *>(&b1),
reinterpret_cast<const unsigned char *>(&b2),
sizeof(B));
}
Let's inherit from B:
struct D : B
{
char c;
};
On my implementation that performs the EBO, I have sizeof(D) == 1.
Now, the question is:
Am I allowed to use CopyB to copy the base subobject of D ?
Let's try:
void f()
{
D d1, d2;
d1.c = 'A';
d2.c = 'B';
CopyB(d1, d2);
cout <
}
Obviously, there is something wrong.
I guess either a POD is not allowed to be empty,
or a base subobject can't be considered as a POD anymore.
--
Thomas
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Francis Glassborow
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| Posted: Wed Aug 27, 2003 3:48 pm Post subject: Re: Empty POD as a base |
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In article <3f4bddca$0$6205$626a54ce (AT) news (DOT) free.fr>, Thomas Thiriez
<thomas.thiriez (AT) free (DOT) fr> writes
| Quote: | Consider this struct:
struct B
{
};
Obviously, it's a POD.
|
And the solution is to make such objects non-PODs. I may be wrong but I
think that they serve no useful purpose in C while being useful in C++
(e.g. as types for exception objects)
--
Francis Glassborow ACCU
64 Southfield Rd
Oxford OX4 1PA +44(0)1865 246490
All opinions are mine and do not represent those of any organisation
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Ben Hutchings
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| Posted: Thu Aug 28, 2003 10:33 am Post subject: Re: Empty POD as a base |
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In article <3f4bddca$0$6205$626a54ce (AT) news (DOT) free.fr>, Thomas Thiriez wrote:
| Quote: | Consider this struct:
struct B
{
};
Obviously, it's a POD.
Therefore, I am allowed to copy instances of it with:
void CopyB(B &b1, const B &b2)
{
memcpy(reinterpret_cast<unsigned char *>(&b1),
reinterpret_cast<const unsigned char *>(&b2),
sizeof(B));
}
Let's inherit from B:
struct D : B
{
char c;
};
On my implementation that performs the EBO, I have sizeof(D) == 1.
Now, the question is:
Am I allowed to use CopyB to copy the base subobject of D ?
snip |
No, you are only allowed to copy *complete* POD objects this way (standard
section 3.9 paragraph 2). You just found a reason for the limitation -
this kind of copying conflicts with the empty base optimisation.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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johnchx
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| Posted: Thu Aug 28, 2003 2:48 pm Post subject: Re: Empty POD as a base |
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Thomas Thiriez <thomas.thiriez (AT) free (DOT) fr> wrote
| Quote: | Obviously, there is something wrong.
I guess either a POD is not allowed to be empty,
or a base subobject can't be considered as a POD anymore.
|
The resolution to DR 43 (included in TC1) says that you can't memcpy a
base class subobject.
http://std.dkuug.dk/jtc1/sc22/wg21/docs/cwg_defects.html#43
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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James Dennett
Guest
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| Posted: Thu Aug 28, 2003 3:04 pm Post subject: Re: Empty POD as a base |
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Thomas Thiriez wrote:
| Quote: | Consider this struct:
struct B
{
};
Obviously, it's a POD.
Therefore, I am allowed to copy instances of it with:
void CopyB(B &b1, const B &b2)
{
memcpy(reinterpret_cast<unsigned char *>(&b1),
reinterpret_cast<const unsigned char *>(&b2),
sizeof(B));
}
|
So long as it is a complete object, std::memcpy will work.
| Quote: | Let's inherit from B:
struct D : B
{
char c;
};
On my implementation that performs the EBO, I have sizeof(D) == 1.
Now, the question is:
Am I allowed to use CopyB to copy the base subobject of D ?
|
No.
| Quote: | Let's try:
void f()
{
D d1, d2;
d1.c = 'A';
d2.c = 'B';
CopyB(d1, d2);
cout <
}
Obviously, there is something wrong.
|
The code :)
| Quote: | I guess either a POD is not allowed to be empty,
or a base subobject can't be considered as a POD anymore.
|
Or (c), it's a POD, and it's empty, but you can't rely on
copying POD subobjects.
-- James.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Dave Harris
Guest
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| Posted: Fri Aug 29, 2003 10:56 am Post subject: Re: Empty POD as a base |
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[email]francis (AT) robinton (DOT) demon.co.uk[/email] (Francis Glassborow) wrote (abridged):
| Quote: | struct B
{
};
Obviously, it's a POD.
And the solution is to make such objects non-PODs. I may be wrong
but I think that they serve no useful purpose in C while being
useful in C++ (e.g. as types for exception objects)
|
Possible uses in C include as a placeholder (eg, when members may be
added to B in the next version), and in conjunction with:
(B *) malloc( sizeof(B) );
as a way of getting a unique, statically typed identifier.
-- Dave Harris, Nottingham, UK
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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