Re: Storing pointer as a key in std::map

From:

Alberto Ganesh Barbati <AlbertoBarbati@libero.it>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 21 Jun 2007 22:20:53 CST

Message-ID:

<_yFei.45125$%k.170454@twister2.libero.it>

Carl Barron ha scritto:

Stl containers own the pointers, and not the data pointed to, so
without an explicit deletion of the pointers, destruction of the
container leaks that memory.

You are assuming that every pointer is a custodial pointer to some heap
allocated object. Although that is a very common scenario, it's not
always the case. For example, a quick way to track all live instances of
a certain class is the following:

class A
{
   // let's assume no multi-threading for sake of exposition
   static std::set<A*> sRegistry;

public:
   A()
   {
     sRegistry.insert(this);
   }

   ~A()
   {
     sRegistry.erase(this);
   }
};

Notice that instances of A can be allocated either on the heap or on the
stack. You could also allow static instances, if you ensure A::sRegistry
is constructed before every instance of A (replacing sRegistry with a
static function+local static variable might help in that case).

HTH,

Ganesh

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