Creation of objects only in Manager class - c++

Question

I want to create a manager class which shall manage all the created objects of one type. Naturally this class should also create these objects. Therefore the client isn't allowed to create objects by himself, but instead must always use the manager class to do it.
Additionally the client is allowed to define his own classes which will be managed by one Manager.
template<class Type>
class Manager{
//...
Type* createInstance(){
Type* ptr = new Type();
//do sommething
return ptr;
}
};
The problem is: How do I restrict the creation of instances only to the manager class?
One possibility would be to declare the constructor as private and the Manager class as a friend class:
class A{
friend class Manager<A>;
private:
A(){}
~A(){}
};
Since the client is allowed to define own classes he could just declare the constructor public and the manager would still work without any problems. But the client would be able to create instances of those classes without the manager class witnessing it, resulting in bad behaviour/ressource management of the framework.
Is there a way to overcome this problem?

Answer 1

Managers of this sort are usually a bad pattern. The following code sample shows why.
It's the same problem as the one that says all destructors should be virtual. I use Manager to create a B, which derives from A, assign the object into an A* pointer. If it's destroyed as such, it goes through Manager::destroy(), which will probably not end well since it was created via Manager::make()
#include <iostream>
using namespace std;
template<class Type>
class Manager
{
public:
Manager(char *type)
{
mytype = type;
}
Type *make()
{
Type *ptr = new Type();
cout << "in Manager<" << mytype << ">::make() ptr->mytype is " << ptr->mytype << endl;
return ptr;
}
void destroy(Type *ptr)
{
cout << "in Manager<" << mytype << ">::destroy() ptr->mytype is " << ptr->mytype << endl;
delete ptr;
}
private:
char *mytype;
};
class A
{
friend class Manager<A>;
protected:
A()
{
mytype = "A";
cout << "in A()" << endl;
}
virtual ~A()
{
cout << "in ~A() mytype is " << mytype << endl;
}
char *mytype;
};
class B : public A
{
friend class Manager<B>;
protected:
B()
{
mytype = "B";
cout << "in B()" << endl;
}
virtual ~B()
{
cout << "in ~B() mytype is " << mytype << endl;
}
};
int main()
{
Manager<A> ma("A");
Manager<B> mb("B");
B *b = mb.make();
A *a = b;
ma.destroy(a); // This line calls Manager<A>::destroy. It should call Manager<B>::destroy.
return 0;
}
which produces the following as output:
in A()
in B()
in Manager<B>::make() ptr->mytype is B
in Manager<A>::destroy() ptr->mytype is B -- Oops - wrong destroy.
in ~B() mytype is B
in ~A() mytype is B
Which in turn means you can't use inheritance to it's full capability, which really defeats the purpose of using an OO language in the first place.
Depending on why you think you need a manager, there is probably a better solution.
If it's for memory management (unlikely given your original code snippet), overriding new and delete is worth taking a look at.
If it's for keeping track of all instances for processing purposes (e.g. objects in a game that get updated every game tick) then while it's a bit of a mental gearshift, a better solution is to integrate the manager into the class itself, as a set of static member functions and variables. Managers of this sort are almost invariably singletons, so making them static members / functions gets you to the same place semantically.
So you might have:
static set<A *> s_collection;
static void Heartbeat() // in class A
{
// lock s_collection here
for (auto it = s_collection.begin, end = s_collection.end() it != end; ++it)
{
// process *it
}
// unlock s_collection here
}
and then during A::A() you insert this into s_collection, and likewise during A::~A() you erase this.
Just be careful to use suitable synchronization primitives if you're multithreaded since most stl containers are not inherently thread safe.

Answer 2

You could use some variation of the passkey pattern to achieve this:
http://coliru.stacked-crooked.com/a/257860767a74a15e