Re: Controlling/releasing ownership in shared_ptr
* digz:
In the code below , a simplified version
I need to pass a smart pointer to function f which I control .I
cannot use std::auto_ptr as a parameter to f because
it releases ownership on copy construction and I still need the
pointer after f returns.( may be I could return the auto_ptr
again ?? )
after f does its stuff , i need to pass it to g which expects a raw
pointer ( I cannot change that API )
I know g does maintain its own ptr_deque which takes care of
ownership etc.. ,
I am forced to do a get() on the shared_ptr ( instead of an ideal
release() on auto_ptr which would free the ptr from smart
management )
Now shared_ptr and ptr_deque desturctors cause a double delete
leading to UB.
How do i workaround this problem ?
#include<boost/shared_ptr.hpp>
#include<boost/ptr_container/ptr_deque.hpp>
boost::ptr_deque<int> l;
"l" is not a good name because it's easily confused with "1".
void f(boost::shared_ptr<int> t = boost::shared_ptr<int>())
{}
void g(int* i){
l.push_back(i);
}
int h(){
boost::shared_ptr<int> i(new int);
f(i); //need to pass aroung i, so cant use std::auto_ptr
g(i.get()); //cant release from boost::shared_ptr ownership
}
You can use std::auto_ptr for exception safety and ownership transfer.
Off-the-cuff, fix errors if any:
void f( int& ) {}
void transferOwnershipToG( std::auto_ptr<int> p )
{
int* pRaw = p.get();
p.release();
g( pRaw );
}
int h()
{
std::auto_ptr<int> p( new int );
f( *p ); // If f throws, std::auto_ptr deallocates.
transferOwnershipToG( p );
}
Or, with the current boost::shared_ptr interface you can release a
boost::shared_ptr by replacing its delete function. I think the
function to gain access to the deleter is named get_deleter. However,
this could be a bit dangerous, because with a shared_ptr at hand one
expects that it will handle deallocation and can be copied and stored
safely, which will not be the case in your code.
Cheers, & hth.,
- Alf
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