Re: I'm confused about the space a class takes in a structure?

 red floyd <>
Fri, 17 Aug 2007 09:14:52 -0700
On Aug 17, 8:33 am, Pep <> wrote:

Juha Nieminen wrote:

Pep wrote:

So if you do a sizeof(foo) you get back 8 as a result and a
sizeof(bar) returns 16. However the class foo has methods as well. Now
if I was to do any form of binary copy on bar such as using memcpy for
example using the sizeof(bar) as the length, will I definitely be
copying the complete data in bar or will the operation be thrown out
by things like internal members in foo created by the compiler, like
the vtbl in foo?

  How can foo have a virtual table when it has no virtual methods?
If it had a virtual table you would see it in the size of foo:
Instead of being 8 (ie. 4 2-byte shorts) it would probably be
12 (or 16 if you are compiling in a 64-bit system). That's because
a class with virtual methods has (in most compiler implementations)
a pointer to the virtual table inside it.

Yep, it was more a badly worded question as to "what should I look out
for" to refresh my memory as usually I no longer have to worry about
the sort of things any more.

  Even if it had virtual methods, it doesn't really matter. The pointer
to the virtual table will be the same for all objects of the same type.
Thus if you copy an object of type foo to an object of type foo, the
virtual table pointer will be the same.

  Exactly what kind of incomplete data would you expect memcpy() to copy
in this case?
  Of course you should take into account that memcpy() skips
constructors, copy constructors and assignment operators. Assuming you
don't have any of those, I suppose it doesn't matter.

Yep, I don;t really know if they are using memcpy in the database code
or not as they are very cagey when we talk to them about their
product. All I do know is that they are definitely doing binary copy
(of some form) on the data we send which they expect to be a struct

In that case, your best bet may be to create a POD type that mirrors
your class, copy into the POD, and forward to the database class for

Alternatively, if you can rewrite some of your code....

struct PODdata
   // all the POD data to be serialized goes here

class Myclass : private PODdata // note private inheritance
   // methods, constructors, etc....
    PODdata* as_PODdata() { return this; }

And pass the result of as_PODdata() to your serializer.

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