an intrusive smart pointer

From:
Mathieu Lacage <mathieu.lacage@sophia.inria.fr>
Newsgroups:
comp.lang.c++.moderated
Date:
Fri, 28 Dec 2007 13:36:25 CST
Message-ID:
<fl3geu$17f$1@aioe.org>
hi,

I am using an adhoc intrusive templated smart pointer implementation and
I stumbled upon a small implementation quirk.

details:
  - intrusive: requires Ref and Unref to be defined on target type
  - assumes refcount is initialized to one by constructor of target type

The above details make it impossible for me to write code like this:

Ptr<T> p = new T ();

because the above code initializes the refcount to 1 in T::T and invokes
Ptr<T>::Ptr (T *p) which increments the refcount a second time. Instead,
I have to write:

Ptr<T> p = Ptr<T> (new T (), false); // avoid adding an extra refcount

or, the following:

Ptr<T> p = Create<T> ();

which hides the slightly scary bool argument behind a homemade Create
method.

However, I really would like to support the direct new syntax:

Ptr<T> p = new T ();

A simple way to support this would be to initialize the refcount to zero
in T::T. However, when I attempted to do this, I found the following
issue. If you write:

T::T ()
{
  DoSomething (this);
}
void
DoSomething (Ptr<T> p)
{}

The call to DoSomething is going to increment and decrement the refcount
to one and back to zero, hence, triggering a delete before the object is
fully constructed.

So, I really wonder if it is possible to support both the direct new
syntax and calls to DoSomething from within the constructor.

regards,
Mathieu

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