C++ (sort of) factory - c++

Question

I've seen a number of posts regarding C++ factories, but so far I haven't seen a solution that solves my problem. (Though I may be missing something.)
Example console app:
#include <memory>
#include <map>
#include <iostream>
using namespace std;
class ResourceManager;
/// abstract base class
class Identity
{
public:
int Id() const { return _id; }
/// make this an abstract class
virtual ~Identity() = 0 {}
protected:
Identity() { _id = _nextId++; }
private:
int _id;
static int _nextId;
};
int Identity::_nextId = int();
/// derived classes
class Component : public Identity
{
friend class ResourceManager;
public:
~Component() { }
};
class Entity : public Identity
{
friend class ResourceManager;
public:
~Entity() { }
};
class ResourceManager
{
public:
template<typename T>
T& Create()
{
auto ptr = std::make_shared<T>();
auto id = ptr->Id();
_resources[id] = std::move(ptr);
return *dynamic_pointer_cast<T>(_resources[id]);
}
private:
std::map<int, std::shared_ptr<Identity>> _resources;
};
int main(int argc, char *argv[])
{
cout << "Factory test" << endl;
ResourceManager r;
auto& e = r.Create<Entity>();
cout << "e.id = " << e.Id() << endl;
Entity e2;
cout << "e2.id = " << e2.Id() << endl;
Component c;
cout << "c.id = " << c.Id() << endl;
std::getchar();
}
I need to make sure that only ResourceManager can instantiate Entity, Component and any classes that derive from them.
I've looked and adding ResourceManager as a friend class to Identity, and making the constructors private or protected, with no success. (This could be a blind alley, or just an implementation problem on my end.)
Any suggestions?
Update and Edit
Replaced code with compilable example. Although the constructor for Identity() is protected, I can still directly instantiate derived classes.

Answer 1

Following should work:
friend class ResourceManager; should be in each derivated classes.
(friend is not inherited).
class ResourceManager;
/// abstract base class
class Identity
{
public:
int Id() const { return _id; }
/// make this an abstract class
virtual ~Identity() = 0;
// Forbid any copy
Identity(const Identity&) = delete;
Identity(const Identity&&) = delete;
Identity& operator = (const Identity&) = delete;
Identity& operator = (Identity&&) = delete;
protected:
Identity() { _id = _nextId++; }
private:
int _id;
static int _nextId;
};
// empty destructor
Identity::~Identity() {}
int Identity::_nextId = 0;
/// derived classes
class Component : public Identity
{
friend class ResourceManager;
public:
~Component() { }
protected:
Component() = default;
};
class Entity : public Identity
{
friend class ResourceManager;
public:
~Entity() { }
protected:
Entity() = default;
};
class ResourceManager
{
public:
template<typename T>
T& Create()
{
std::unique_ptr<T> ptr(new T);
T& res = *ptr;
_resources[ptr->Id()] = std::move(ptr);
return res;
}
/// TODO: need to make sure that resource ID is actually of type T
/// and that _resources contains ID.
template<typename T>
T* Get(int id)
{
auto it = _resources.find(id);
if (it == _resources.end()) {
return nullptr;
}
return dynamic_cast<T*>(it->second.get());
}
private:
std::map<int, std::unique_ptr<Identity>> _resources;
};
Note that since ResourceManager owns the resource I change std::shared_ptr to std::unique_ptr.
I fixed ResourceManager::Get with invalid id.

Answer 2

Have you tried a protected construtor?
class Identity
{
friend class ResourceManager;
public:
int Id() { return _id; }
virtual ~Identity() = 0;
protected:
Identity() {
_id = _nextId++;
}
private:
static int _nextId;
// do not forget to put "int Identity::_nextId = 0;" in a source file
};
Identity::~Identity()
{
}
But you need to repeat this pattern in every derived class. Thus making ResourceManager a friend and making the constructor private or protected.