Re: 'placement' new, and allocation in the constructor.

From:

"Victor Bazarov" <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Sat, 6 May 2006 22:33:22 -0400

Message-ID:

<GamdnT9psb3vwcDZnZ2dnUVZ_u-dnZ2d@comcast.com>

Shea Martin wrote:

If I have a class Foo like this:

class Foo
{
public:
  Foo()
  {
    mData = new char[ time() % 1000 ];
  }
  ~Foo()
  {
    delete [] mData;
  }
private:
  char* mData;
};

Now if I create a Foo object using the 'placement' new like so:
void* raw = malloc( sizeof(Foo()) );
Foo *foo = new(raw) Foo();

foo.mData will not be taken from my memory pool, correct?

Which one is "your" memory pool? Essentially both 'malloc' and 'new'
(or 'new[]') perform allocations in what is known as "free store". For
all we know, it's one and the same "memory pool" for both of them.

There is no
way to know that how much memory Foo() will allocate. But it *would*
be nice to have the mData memory in the same pool as my foo object.

Would it? Really? Why?

Any tips or tricks that I am overlooking, that would allow me to do
this (without access to the Foo class)?

You _could_ provide your own 'malloc' and 'new[]'. If you know for sure
that 'Foo' uses "new[]", then you just need to overload the function
called '::operator new[]' and do what you need there.

V
--
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