Hello!
I do some experiments on shared_ptr and weak_ptr. So I tried to use them
for a singleton-class, which does not need an explicit
singleton::release_instance method. The singleton-instance will be
destroyed everytime the last external shared_ptr holding the instance
goes out of scope. Is this a valid usage for weak_ptr? Are there any
traps I do not see using these smart_ptrs in this context?
The drawback of using shared_ptr with singleton is the need for a public
destructor. But this is dangerous....it is possible to write
singleton::singleton_ptr first_ptr = singleton::instance();
delete first_ptr.get();
Declaring the destructor as private is only possible when defining
boost::checked_delete as friend of the singleton-class.
( there is a thread on comp.lang.c++.moderated with the topic
'specialization of function template as friend with MSVC6SP5 not
possible' which deals with this problem of friendship... )
But then it is still possible to write the following code
singleton::singleton_ptr first_ptr = singleton::instance();
boost:checked_delete( first_ptr.get() );
OK, it's very unlikely to do it by chance, but it is possible.
Is there a way to allow destruction of the instance only to shared_ptr?
<code>
#include
#include
#include
class singleton : boost::noncopyable
{
public:
typedef boost::shared_ptr< singleton > singleton_ptr;
static singleton_ptr instance()
{
singleton_ptr tmp;
if( s_instance.expired() )
{
tmp.reset( new singleton );
s_instance = tmp;
}
else
{
tmp = boost::make_shared( s_instance );
}
return tmp;
}
void foo()
{
// do something
i++;
}
// like to make it private in order to prohibit delete on it,
// but boost::shared_ptr<> can't handle it....so sad :(
~singleton(){};
private:
typedef boost::weak_ptr< singleton > internal_singleton_ptr;
singleton(){};
int i;
static internal_singleton_ptr s_instance;
};
singleton::internal_singleton_ptr singleton::s_instance;
int main()
{
{
singleton::singleton_ptr first_ptr = singleton::instance();
{
singleton::singleton_ptr second_ptr = singleton::instance();
second_ptr->foo();
}
first_ptr->foo();
}
singleton::singleton_ptr another_ptr = singleton::instance();
another_ptr->foo();
return 0;
}
</code>
Regards, Stephan
--
---------------------------------------------------------------
Dipl.-Inf. (FH) Stephan Born | beusen Solutions GmbH
fon: +49 30 549932-0 | Landsberger Allee 366
fax: +49 30 549932-21 | 12681 Berlin
mailto:stephan.born@beusen.de | Germany
---------------------------------------------------------------
PGP-Key verfügbar | PGP-Key available
---------------------------------------------------------------
