Re: [Boost-users] Copy a smart pointer into allocated memory

Subject: Re: [Boost-users] Copy a smart pointer into allocated memory
From: Tiago Alves (talves_at_[hidden])
Date: 2009-06-15 09:47:25

Thanks. I never used that placement new before. Always leaning... :)

Also, thanks for the heads up about the destructor. I am destroying the 
weak_ptr() in the lua garbage collector.

Tiago Alves

Peter Dimov escreveu:
> Tiago Alves:
>> Hi, and sorry for the probable newbieness of this question!
>>
>> I want to put weak_ptr into a piece of memory previously allocated.
>>
>> this is my code:
>>
>> // construct a weak_ptr from a share_ptr
>> boost::weak_ptr<Obj>* cameraPtr = new 
>> boost::weak_ptr<Obj>(obj_share_ptr);
>> // lua_newuserdata allocs memory and returns a pointer to it, just 
>> like malloc.
>> boost::weak_ptr<Obj>* camera = 
>> static_cast<boost::weak_ptr<Obj>*>(lua_newuserdata(pLuaState,sizeof(boost::weak_ptr<Obj>))); 
>>
>>
>> Now i want camera to be a weak_ptr of ObjPtr.
>> I tried different ways to do this, such as
>> *camera = boost::weak_ptr<Obj>(obj_share_ptr);
>> or
>> camera->swap(*cameraPtr);
>> or
>> *camera = *cameraPtr;
>>
>> I think none of these work because in fact they all do a swap() 
>> between camera and cameraPtr. Since camera is just an malloc, it will 
>> try to swap dirty memory and segfault sometimes (yes, sometimes this 
>> worked).
>
> In C++, the way to construct an object of type X into uninitialized 
> memory (pointed to by a pointer p) is called placement new:
>
>    #include <new>
>
>    new( p ) X;
>
> In your case, you'd do:
>
> void * p = lua_newuserdata( pLuaState, sizeof( boost::weak_ptr<Obj> ) );
> new( p ) boost::weak_ptr<Obj>( obj_share_ptr );
>
> The last line constructs a new weak_ptr in the memory at p from the 
> obj_share_ptr argument (same as cameraPtr in your original example, 
> but without the allocation.)
>
> Later (probably in your 'gc' metamethod, whatever that is :-) ) you'll 
> also have to dispose of the weak_ptr object; this is done by invoking 
> the destructor:
>
> p->~weak_ptr<Obj>();
>
> In weak_ptr's case, you can also cheat and just reset() it, as its 
> empty state consists of two NULL pointers and can be garbage-collected 
> with impunity, but in principle, the proper way to end the lifetime of 
> such an object is to call the destructor.
>
>