Re: [Boost-users] Constructing a shared pointer from a const pointer

Subject: Re: [Boost-users] Constructing a shared pointer from a const pointer
From: Johan Nilsson (r.johan.nilsson_at_[hidden])
Date: 2009-11-11 02:37:51

veerus wrote:
> Hi All,
>
> I have been in trouble for a day solving this seemingly simple
> problem - constructing shared_ptr<X> from const X& without copying
> the X object.
>
> The programming logic does not allow me to construct the shared_ptr
> like this: SX x = SX( new X( x ) );
>
> Consider the following piece of code:
>
> void foo( const X& x )
> {
>  typedef shared_ptr<X> SX;
>  SX px = SX( &x );
>
>  bar( px );
> }
>
> The compiler gives the following error:
> In shared_ptr.hpp at line 184: 'initializing' cannot convert from
> 'const X *' to 'X*'.

To make it compile, you could simply change the typedef to "typedef 
shared_ptr<X const> SX;".

However, the code is suspect - depending on how the caller of foo declares 
and manages the variable used as "x", you're likely to either encounter a 
delete of a (invalid reference to a) stack object sooner or later, or 
perhaps a double delete. If I saw the above during a code review I would say 
foo needs to be redesigned.

If you're relying on identity here, or if X is non-copyable, and want to 
transfer ownership of "x" into "bar" you could define foo as:

void foo(std::auto_ptr<X> px)
{
  bar(shared_ptr<X>(px));
}

[It's hard to tell without knowing the full context]

/ Johan