Re: [boost] Variant/Any and the never-empty guarantee

From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2007-01-26 12:59:55

Steven Watanabe wrote:

> As mentioned, that solution causes can cause an unresolvable race condition.
> For example this class could not be used in a variant then.
> 
> class C {
> public:
>     explicit C(int i = 0) : n(i) {
>        register_object(this);
>     }
>     ~C() {
>        unregister_object(this);
>     }
>     static void increment_all() {
>        std::for_each(objects.begin(), objects.end(), ++*_1);
>     }
>     C& operator++() {
>        boost::mutex::scoped_lock l(m2);
>        ++n;
>        return(*this);
>     }
> private:
>     static std::set<C*> objects;
>     static boost::mutex m1;
>     void register_object(C* c) {
>        boost::mutex::scoped_lock l(m1);
>        objects.insert(c);
>     }
>     void unregister_object(C* c) {
>        boost::mutex::scoped_lock l(m1);
>        objects.erase(c);
>     }
>     boost::mutex m2;
>     int n;
> };

I see the problem now.
If you move the object, then the pointer in the set doesn't point to a 
valid object anymore.

Would a move constructor that replaces the pointer in the set completely 
solve the problem?