Re: [Boost-users] newbie question on preferred method to use shared_ptrs

From: Rush Manbert (rush_at_[hidden])
Date: 2006-02-15 20:05:07

Perry Smith wrote:
> Hi,
> 
> I often have to "fight" with my C++ code and I must be missing a  
> basic design characteristic of C++.
> 
> Suppose there is a shared_ptr that has already been created.  I now  
> want to create an object with new and "give" it to shared_ptr to  
> manage.  I tend to do this:
> 
> std::tr1::shared_ptr<int> dog;
> 
> int fun(void)
> {
>      int *temp = new int;
>      dog = temp;
>      // ...
> }
> 
> This does not work.  I've discovered two ways to achieve (I think)  
> what I'm looking for:
> 
> int fun(void)
> {
>      std::tr1::shared_ptr<int> temp(new int);
>      dog = temp;
>      // ...
> }

Almost. What you want to do is this:

        std::trl::shared_ptr<int> dog(new int);

which instantiates the object of interest and gives its address to the 
shared ptr.
> 
> or ...
> 
> int fun(void)
> {
>      foo *temp = new int;
>      dog.reset(temp);
>      // ...
> }
> 
This works, but is clumsy. I believe the intent of reset is to change
the contained pointer value (and also decrement the use count for the 
original contained pointer). I have also used it in the following way:
{
     boost::shared_ptr<myObjectType> pMine;	// Contains a NULL pointer value

     // Do some stuff

     if (test some boolean that doing some stuff created)
     {	// Because this is true we create a new object
        pMine.reset (new myObjectType (my constructor args));
     }

     // Do some more stuff

     if (pMine)
     {	// I created an object
     }
     else
     {	// I didn't create an object
     }
}

In case you haven't yet, take a look at:
http://www.boost.org/libs/smart_ptr/shared_ptr.htm

Hope it helps,
Rush