Re: Placement new and destructor

From:
"peter koch larsen" <peter.koch.larsen@gmail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
11 May 2006 08:44:44 -0400
Message-ID:
<1147299859.239506.136800@i40g2000cwc.googlegroups.com>
dragoncoder skrev:

Hello all,

I am reading C++ Primer 3rd edition by Lippman & Lajoie and come across
the following piece of code as an example demostrating the use of
placement new.


I believe that book was one of the first C++ books I read (after
Stroustrups), and I remember it as a very good book. But it does have
flaws!

Section 8.4.5 (Page 417-418)

#include <iostream>
#include <new>

const int chunk = 16;

class Foo {
public:
  int val() { return _val; }
  Foo() { _val = 0; }
private:
  int _val;
};

char *buf = new char[sizeof (Foo) * chunk];

int main() {
  Foo *pb = new (buf) Foo;
  if ( pb.val() == 0 )
    cout << "new expression worked!" << endl;
  delete[] buf;
  return 0;
}

Looking at the above code, I feel it is incorrect at 2 places.

1. Since pb is a pointer to a Foo object, the expression inside the if
condition should have been pb->val() instead of pb.val(), okay that may
be a printing mistake.

You are correct.

2. The second error is not a typographical error. I see at no place in
the code the destructor of the Foo object is called. Instead delete[]
buf calls the destructors of 16 char objects which is fine but I feel
there should be a call to Foo's destructor before the delete[] buf.
Shouldn't there be a call to pb->~Foo() before delete[] ?


I agree once again. This time it is not a direct error - you don't have
to call destructors. And there is no leak - the buffer is deleted all
right. So perhaps we should forgive Lippman and Lajoie considering that
they aim to demonstrate placement new.

Please tell me, if I am missing something as this book is rated highly
recommended at accu.org.


Then probably it is a very good book. No book is without errors and
Lippman and Lajoie are both very knowledgeable - and they are fine
writers.

I have a feeling there might be a third error, however. The character
vector is allocated via new char[]. Is new char guaranteed to allocate
memory that is properly aligned for any datatype? So far as I
understand it, the buffer is only guaranteed to be properly aligned for
characters.

/Peter

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