Re: Placement new and destructor
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
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]