Re: Anonymous union rules against constructors & assignment

From:
James Dennett <jdennett@acm.org>
Newsgroups:
comp.std.c++
Date:
Wed, 7 Jun 2006 11:09:16 CST
Message-ID:
<4BChg.102685$iU2.73289@fed1read01>
Crosbie Fitch wrote:

<johnchx2@yahoo.com> wrote in message
news:1149633364.561734.56320@i39g2000cwa.googlegroups.com...

memcpy doesn't work on non-PODs.
This is because they aren't guaranteed to be laid out anything like you
think they will be.


'memcpy doesn't work on non-PODs' !!!

Wow! That's bad news.

So, let's say I had this:

class A {... }* a;

foo()
{void* p=malloc(sizeof(A));

 new(p) A();

 a=(A*)malloc(sizeof(A));

 memcpy(a,p,sizeof(A)); // Doesn't work if A is non-POD?

 free(p);

 a->method();

 delete a;
}

If memcpy doesn't work on non-POD objects then something truly fundamental
has changed, e.g. C++ has become C#.


It is not valid to use memcpy on non-PODs. If you
choose to do it, you cannot portably cast the resulting
memory to the non-POD type and expect to use it.

I think that it's legal for an implementation to add
a hidden "my address" member to a non-POD object, and
to validate that it's consistent with the object's
address on each access to the object.

This is nothing new, and C++ still isn't C#. It's not
C either; C++'s object model is layered on top of C's,
but non-PODs are a whole world of their own.

-- James

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