Re: [boost] plain pointers and shared_ptr

From: E. Gladyshev (egladysh_at_[hidden])
Date: 2002-03-22 19:36:12

> An even easier solution is to simply use
> shared_ptr<> everywhere without
> exception, it is as thread safe as any code is
> likely to get at present.

It is not always true and sometimes not possible.
What if you need to make a simple C- style API.
so that the foo(int *px) function can be called
from C and C++.  It could be and export from a DLL 
for example.

Another requirement is that foo() has to work
correctly independently whether the caller
is using shared_ptr<> or not.
In other words foo() has to share objects
properly when the caller is using shared_ptr<> and not

to screw things up when the caller is not using
shared_ptr<>.

> management. Why are you
> trying to apply them to things that they cannot
> possibly handle?

"cannot possible handle" is a strong statement
maybe they can or there is a better solution.
Are you 100% sure? 
I am never 100% sure in anything :).

--- Ken Shaw <ken_at_[hidden]> wrote:
> 
> ----- Original Message -----
> From: "E. Gladyshev" <egladysh_at_[hidden]>
> To: <boost_at_[hidden]>
> Sent: Friday, March 22, 2002 1:10 PM
> Subject: [boost] plain pointers and shared_ptr
> 
> 
> >
> > In my previous post, I looked at using
> shared_ptr<>
> > with plain pointers.
> > The problem is that if a caller passes an object
> to a
> > function with a
> > plain pointer, the caller has to make sure that
> the
> > object is
> > valid while the function is using this object.
> > In other words a function cannot claim a shared
> > owenership of an object
> > if the object is passed in with a plain pointer. 
> This
> > issue
> > is especially important in multi-threaded
> > applications.
> > The following code won't work of course.
> >
> > void foo( int* p )
> > {
> > shared_ptr<int> my_int( p );  //claim shared
> > ownership
> > }
> >
> > main()
> > {
> > shared_ptr<int> x( new int );
> > foo( x.get() );
> > }  //!!CRASH  shared_ptr<> will try to delete an
> > object that has been deleted by foo().
> >
> >
> > A very easy solution is to use a global object
> map.
> > STL has a logarithmic <map> class.  The following
> > code shows how to do that.  This is just a test
> code.
> > Do you think if it is a valid idea to be included
> in
> > boost?
> >
> 
> An even easier solution is to simply use
> shared_ptr<> everywhere without
> exception, it is as thread safe as any code is
> likely to get at present.
> 
> >
> > #include <map>
> > using namespace std;
> >
> > typedef map<void*, int*> object_map;
> >
> > class smart_ptr_base
> > {
> > protected:
> > static object_map _map;
> > };
> >
> > template <typename T>
> > class smart_ptr : public smart_ptr_base
> > {
> > public:
> >
> > smart_ptr( T* d )
> > {
> > pair<object_map::iterator, bool>  p;
> > p = _map.insert( object_map::value_type( d, new
> int
> > ) );
> > _it = p.first;
> > if( p.second ) *(p.first->second) = 1;
> > else ++*(p.first->second);
> > }
> >
> > virtual ~smart_ptr()
> > {
> > if( !--*(_it->second) )
> > {
> > delete (T*)(_it->first);
> > _map.erase( _it );
> > }
> > }
> >
> > //all other standard smart pointer stuff goes here
> > //...
> >
> > protected:
> > object_map::iterator _it;
> > };
> >
> > object_map smart_ptr_base::_map;
> >
> >
> >
> > void foo( A* p )
> > {
> > smart_ptr<A> tmp( p );  //claim shared ownership
> > }
> >
> > int main(int argc, char* argv[])
> > {
> > A *p = new A();
> > smart_ptr<A> tmp( p );
> > foo( p );
> > return 0;
> > }
> >
> > One problem that I'd like to solve is to deal with
> > pointers
> > to static objects.
> > I am not sure if there are any smart pointer
> > implementations that
> > can reference static objects.  For example:
> >
> > int g_x;
> > int main()
> > {
> > share_ptr<int> tmp(&g_x)
> > } //CRASH
> >
> 
> smart pointers are intended to simplify heap memory
> management. Why are you
> trying to apply them to things that they cannot
> possibly handle?
> 
> Ken Shaw
> 
> 
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