RE: [boost] smart pointer puzzle

From: E. Gladyshev (egladysh_at_[hidden])
Date: 2002-03-25 22:44:06

> Boost shared_ptr as the shared_ptr does not have
> total ownership of the
> object. The definition of Boost's shared pointer
> does not allow for your
> style of shared ownership.

The requirements don't force you to use
the boost's implementation of smart pointers.
On contrary it says "implement a smart pointer".

> 
> 2) You need a much smarter shared_ptr that can
> reconnect to other copies
> from the stored pointer, and if there is none to
> remember that it does not
> own it. A normal shared_ptr will not be smart enough
> to do this due to the
> overhead that will be added, as it requires the
> keeping of a map of some
> sort of values under controlled by all shared_ptrs
> in the program.

This is a real solution that conforms to 
all the requirements.  The requirements
don't say anything about the overhead.

But we can append our solution with something
like this.

The new real smart shared_pointer<> implies
some overhead.  The significance of the 
overhead depends on the context where
the shared_pointer<> is used.
Assuming that our implementation of
shared_pointer<> uses a logarithmic
container like STL::map<> to map the plain pointers,
the overhead can be estimated as X*log(N), where
N is the average number of new objects created
by the system per second (the new objects
are controlled with shared_pointer<>),
and X is the average number of new shared_pointer<>
objects per second.  This way you'll get 
average overhead time per second. 

As you can see for a huge class of application
the overhead is not significant.

> 
> 4) You fire the designer who gave the impossible
> specs.

I really like this solution, LOL.
But unfortunately we cannot justify it,
the spec is possible.  You proved it yourself.

> > -----Original Message-----
> > From: boost-admin_at_[hidden]
> > [mailto:boost-admin_at_[hidden]]On
> > Behalf Of E. Gladyshev
> > Sent: Monday, March 25, 2002 4:38 PM
> > To: boost_at_[hidden]
> > Subject: [boost] smart pointer puzzle
> >
> >
> >
> > I've decided to start another thread,
> > since my original "plain pointers and shared_ptr"
> > caused some confusion.
> >
> > To understand what I was talking about,
> > here is a puzzle :).
> >
> > Objective:
> >    implement a smart pointer
> >    shared_pointer<int> and function foo( int* px )
> >    that does something on *px.  foo() can
> >    use shared_pointer<int> but only internally.
> >
> > Requirements:
> > 1. There must be only one version of foo() without
> >    any conditional compiling.
> >
> > 2. The foo() client and foo() can be running
> >    in different threads.
> >
> > 3. It should be obvious how to implement
> > shared_pointer<int>
> >    in multi-thread environment.
> >
> > 4. foo() has to work correctly if the client uses
> >    shared_pointer<int> to share the 'px' object.
> >    In other words, if shared_pointer<int> goes out
> >    of scope on the client side while foo() is
> still
> >    using the object pointed by 'px' (can happen 
> in
> > multi-thread
> >    applications), the program should not crash.
> >    foo() has to call the object destructor when
> >    it exits and nobody else references the object.
> >
> > 5. foo() has to work consistently if the client
> >    is not using shared_pointer<int> ( a legacy
> >    C-code).
> >
> > 6. Following solutions are not accepted.
> >     - Solution 1. it is not possible.
> >     - Solution 2. change the foo() interface.
> >     - Solution 3. create multiple versions of
> foo().
> >
> >
> > Here are some foo() usages:
> >
> > main()
> > {
> >    {
> >       shared_pointer<int> x( new int );
> >       foo( x.get() );
> >       //make sure that foo() is started
> >       ...
> >    } //forget about 'x' here
> >
> >    //do something else
> >    ...
> >
> >    //make sure that foo() is done.
> >    ...
> >    return 0;
> > }
> >
> > main()
> > {
> >    int x;
> >    foo( &x );
> >    //wait when foo() is done.
> > }
> >
> > main()
> > {
> >     int *px = new x;
> >     foo( px );
> >
> >     //wait when foo() is done.
> >     delete px;
> > }
> >
> >
> > See a proposed solution in my first
> > "plain pointers and shared_ptr" message.
> >
> > Anybody has a better idea?
> >
> 
> 
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