Question on use of "placement" new

[letz]

Hi,

We have a class whose objects are to be allocated in shared memory. To
do that a ShmManager base class is defined so that operator new and
delete are redefined to allocate segments in shared memory. A typical
class "Foo" then inherit from ShmManager to have get this behaviour.
class ShmManager {

void* operator new(size_t size);
void operator delete(void* p, size_t size);

};

class Foo : public ShmManager
{

int fData1;
Barr fData2[16];

};

In the previous Foo example, the size of fData2 array is known at
compilation time, but we need to make this size "dynamic", but keeping
the object memory layout "flat".

We are using the "placement" new syntax doing:

class Foo1 : public ShmManager
{

int fData1;
Barr fData2[]; // will be extented using "placement" new

};

ShmManager* ptr = ShmManager::ope rator new(sizeof(Foo1 ) + num *
sizeof(Barr));
Foo1* = new(ptr) Foo1();

So that Foo1 object nows gets a dynamic "num" number of elements. This
seems to work, but is this safe to do that? Are we obliged to put the
fData2 fied as the *last* element in the Foo1? Is there any better/
safer manner to implement the same pattern?

Thanks

Stéphane Letz

[Ian Collins]

Frank Birbacher wrote:

Hi!

Ian Collins schrieb:

>You don't need boost for that, all you have to do is provide an
appropriate allocator to manage a memory pool in shared memory.

Right. My point was: it works, you can use vectors in shared memory. The
OP seems to oppose to this solution. I cannot agree on some fragile
solution using the variable-sized-object-and-int[]-hack (which is
illegal in C++) when there is a cleaner solution (using a vector with
shmem alloc).

I agree. The only real taboos with hared memory objects are static data
members and virtual functions.

--
Ian Collins.

[peter koch]

On 19 Maj, 01:01, Ian Collins <[e-mail] mwrote:

Frank Birbacher wrote:

Hi!

Paavo Helde schrieb:

I guess with this you meant to say that e.g. std::vector is outlawed.

Well, boost allows vectors, maps, and other stuff in shared memory:
http://www.boost.org/doc/libs/1_35_0...ess/quick_guid...

You don't need boost for that, all you have to do is provide an
appropriate allocator to manage a memory pool in shared memory.

This requires the memory pool to have the same adress in every
process. Apparantly this is not the case for the OP.

/Peter