I have this problem in my homework.
I have this 2 classes:
-an user class:
class user
{
protected:
int id;
std::string password;
std::string name;
bool online;
static int usersCounter;
static int onlineCounter;
public:
user(std::string = "user", std::string = "1234");
bool connect(std::string);
void disconnect();
void changePassword(std::string);
void changeName(std::string);
virtual void printInfo();
static int getOnlineNo()
{
return onlineCounter;
}
static int getTotalUsers()
{
return usersCounter;
}
friend class admin;
};
and an admin class:
class admin : public user
{
public:
admin(std::string name = "admin", std::string password = "admin"):
user(name, password){}
void disconnectUser(user&);
void viewUsers( user** );
void printInfo() override;
};
When I try to make a polymorphic object and call a function from admin (a function specific for admin, as disconnectUser(...) or viewUsers(...)) I get errors as "class "user" has no member "viewUsers".
user* usr = new admin();
usr->viewUsers(&usr);//error
Is this something usual and I did a wrong design or It should work and I am doing something wrong?
As pointed out in the comment if you define a class of type A you can access only public methods and property of that class.
In your case, viewUsers is present only in the admin class while you're using the generic user class.
You must use inheritance very careful because is very easy to put the wrong methods inside classes or make a wrong inheritance. For example, are you sure the method viewUsers must be inside the admin class? Usually, if a function doesn't depend on the class is best put it outside the class.
I am getting an issue for retrieving BaseClass correct enum value.
class BaseClass
{
public:
enum EntityId {
EN_NONE = 0,
EN_PLAYER = 1,
EN_PLATFORM,
EN_GROUND,
EN_OBSTACLE,
EN_OTHER
};
void setEntityId(EntityId id) { _Entityid = id; }
EntityId getEntityId() { return _Entityid; }
protected:
EntityId _Entityid;
};
and
class DeriveredClassA : public SomeClass, public BaseClass {....};
class DeriveredClassB : public SomeClass, public BaseClass {....};
The initialization goes like this
DeriveredClassA->setEntityId(BaseClass::EntityId::EN_PLAYER);
DeriveredClassB->setEntityId(BaseClass::EntityId::EN_OBSTACLE);
Which is placed into a different vector list correspoinding to that enum.
However, I am forced to use void* to do static_casts cats...
Like this:
BaseClass* EA = static_cast<BaseClass*>(bodyUserDataA); //bodyUserDataA and bodyUserDataB are both void*
BaseClass* EB = static_cast<BaseClass*>(bodyUserDataB);
And I am trying to retrieve using EA->getEntityId() and EB->getEntityId() so I could check which one is EN_PLAYER, which one is EN_GROUND and etc. So then I could up-class from base into derivered class and do other stuff with it.
Tried using with virtual, however somehow I am receiving 2 copies of _EntityID, which can be either the same or DIFFERENT between my Derivered and BaseClass of that one object.
Moreover, I can't cast right away into DeriveredClass, since the code checking would be huge, due to many different types of DeriveredClass'es (DeriveredClassA, DeriveredClassB, DeriveredClassC, DeriveredClassD) with their corresponding vector list.
My question is that How I need setup correctly both Base and Derivered class, so that I could access _EntityID from Baseclass which is the same of that DeriveredClass? My main problem might is that I used incorectly virtual functions, so I left on default to understand my issue.
P.S. This is mainly my c++ issue, other tags are added due to I am using game engine and physics engine for this case.
I believe that you want your code to look more like this:
class Entity
{
public:
enum Type {
EN_NONE = 0,
EN_PLAYER = 1,
EN_PLATFORM,
EN_GROUND,
EN_OBSTACLE,
EN_OTHER
};
Type getType() { return _type; }
protected:
Entity(Type type): _type(type) {}
private:
const Type _type;
};
Then your derived classes and usage of this base would be more like:
class PlayerEntity: public Entity, public SomeClass
{
public:
PlayerEntity(std::string name): Entity(EN_PLAYER), _name(name) {}
std::string getName() const { return _name; }
private:
std::string _name;
};
class PlatformEntity: public Entity, public SomeClass
{
public:
PlatformEntity(): Entity(EN_PLATFORM) {}
};
Initialization is then done like:
int main()
{
PlatformEntity platform;
std::vector<PlatformEntity> platforms(platform);
std::vector<PlayerEntity> players;
players.emplace_back("Bob");
players.emplace_back("Alice");
players.emplace_back("Ook");
}
Access from user-data could then look like this:
// bodyUserDataA and bodyUserDataB are both void*
Entity* const EA = static_cast<Entity*>(bodyUserDataA);
Entity* const EB = static_cast<Entity*>(bodyUserDataB);
switch (EA->getType())
{
case Entity::EN_PLAYER:
{
PlayerEntity* player = static_cast<PlayerEntity*>(EA);
std::cout << "Found player: " << player->getName();
break;
}
case Entity::EN_OTHER:
...
default:
break;
}
I have the following classes.
// My baseclass
class Item {
public:
virtual const std::string GetItemName() = 0;
};
// My derived class
class Shovel : public Item {
private:
static const std::string _ITEM_NAME = "tool_shovel";
public:
const std::string GetItemName(){
return _ITEM_NAME;
}
}
With this i can access the names of my Item objects like this:
Item* myItem = new Shovel();
myItem.GetItemName(); // Returns "tool_shovel" ofcourse
I would now also like to access the name of an item without having an instance of it like this.
Shovel::GetItemName();
I know it is not possible to implement a virtual static function.
but is there any way to implement this in a 'nice' way or is this more a problem in my concept?
Thanks for the help!
I didn't know it is possible to call a static function directly from an instance, so i solved my problem now with 2 methods.
One public static function, so i can get the name of an item at any time. And another private non-static function to let the baseclass get the name of the current item.
Here is the code:
// My baseclass
class Item {
protected:
virtual const std::string _GetItemName() = 0;
};
// My derived class
class Shovel : public Item {
private:
static const std::string _ITEM_NAME = "tool_shovel";
protected:
const std::string _GetItemName(){
return _ITEM_NAME;
}
public:
static const std::string GetItemName(){
return _ITEM_NAME;
}
};
I hope this helps anyone. If you have questions feel free to ask.
I have a design question. I want custom datatypes implementing an interface. For example, using templates is simply (maybe next design isn't correct -because I can do a generic class instead of the next- but clarifies my goal):
template <typename T>
class IDatatype
{
public:
virtual T getData() const = 0;
virtual void setData(T pData) = 0;
};
class MyChar: public IDatatype<char>
{
public:
void setData(char pData){...}
char getData() const{...}
private:
char _data;
};
class MyInt: public IDatatype<int>
{
public:
void setData(int pData){...}
int getData() const{...}
private:
int _data;
};
IDatatype<int> *data = new MyInt(); // parametrized interface, bad idea :(
data->getData(); // it works ok
From previous classes, it is easy to get the attribute corresponding to each _data class member. My question:
Is there any way (change design, etc.) to implement generic setter and getter in IDatatype
and for any type and thus manipulate the _data attribute of each class
without using templates in the interface?
For example:
class IDatatype
{
public:
// pure virtual getters and setters for specialized _data fields. Here is my design question.
};
class MyChar: public IDatatype
{
public:
void setData(char pData){...};
char getData(){...};
private:
char _data;
};
class MyInt: public IDatatype
{
public:
void setData(int pData){...};
int getData(){...};
private:
int _data;
};
IDatatype *intData = new MyInt(); // no parametrized interface!
intData->getData(); // how can I create this method from IDatatype?
IDatatype *charData = new MyChar();
charData->getData(); // the same here
NOTE: I have no good english, apologize for any errors :)
You could probably achieve this in 3 ways, none as elegant and error free as using a template
Define your data as a union of int/float/char in the base class and act on this union from the set/get methods of the base class. The entire VB (old VB 6) class system works on such a data type called VARIANT.
Return void * from base class and cast and use as appropriate - yuck & good luck!!.
Return the base interface reference itself from the getData which though appearing to be meaningful, has no meaning at all.
4.
In my application, there are 10-20 classes that are instantiated once[*]. Here's an example:
class SomeOtherManager;
class SomeManagerClass {
public:
SomeManagerClass(SomeOtherManager*);
virtual void someMethod1();
virtual void someMethod2();
};
Instances of the classes are contained in one object:
class TheManager {
public:
virtual SomeManagerClass* someManagerClass() const;
virtual SomeOtherManager* someOtherManager() const;
/** More objects... up to 10-20 */
};
Currently TheManager uses the new operator in order to create objects.
My intention is to be able to replace, using plugins, the SomeManagerClass (or any other class) implementation with another one. In order to replace the implementation, 2 steps are needed:
Define a class DerivedSomeManagerClass, which inherits SomeManagerClass [plugin]
Create the new class (DerivedSomeManagerClass) instead of the default (SomeManagerClass) [application]
I guess I need some kind of object factory, but it should be fairly simple since there's always only one type to create (the default implementation or the user implementation).
Any idea about how to design a simple factory like I just described? Consider the fact that there might be more classes in the future, so it should be easy to extend.
[*] I don't care if it happens more than once.
Edit: Please note that there are more than two objects that are contained in TheManager.
Assuming a class (plugin1) which inherits from SomeManagerClass, you need a class hierarchy to build your types:
class factory
{
public:
virtual SomeManagerClass* create() = 0;
};
class plugin1_factory : public factory
{
public:
SomeManagerClass* create() { return new plugin1(); }
};
Then you can assign those factories to a std::map, where they are bound to strings
std::map<string, factory*> factory_map;
...
factory_map["plugin1"] = new plugin1_factory();
Finally your TheManager just needs to know the name of the plugin (as string) and can return an object of type SomeManagerClass with just one line of code:
SomeManagerClass* obj = factory_map[plugin_name]->create();
EDIT: If you don't like to have one plugin factory class for each plugin, you could modify the previous pattern with this:
template <class plugin_type>
class plugin_factory : public factory
{
public:
SomeManagerClass* create() { return new plugin_type(); }
};
factory_map["plugin1"] = new plugin_factory<plugin1>();
I think this is a much better solution. Moreover the 'plugin_factory' class could add itself to the 'factory_map' if you pass costructor the string.
I think there are two separate problems here.
One problem is: how does TheManager name the class that it has to create? It must keep some kind of pointer to "a way to create the class". Possible solutions are:
keeping a separate pointer for each kind of class, with a way to set it, but you already said that you don't like this as it violates the DRY principle
keeping some sort of table where the key is an enum or a string; in this case the setter is a single function with parameters (of course if the key is an enum you can use a vector instead of a map)
The other problem is: what is this "way to create a class"? Unfortunately we can't store pointers to constructors directly, but we can:
create, as others have pointed out, a factory for each class
just add a static "create" function for each class; if they keep a consistent signature, you can just use their pointers to functions
Templates can help in avoiding unnecessary code duplication in both cases.
I have answered in another SO question about C++ factories. Please see there if a flexible factory is of interest. I try to describe an old way from ET++ to use macros which has worked great for me.
ET++ was a project to port old MacApp to C++ and X11. In the effort of it Eric Gamma etc started to think about Design Patterns
I'd create a "base" factory that has virtual methods for creation of all the basic managers, and let the "meta manager" (TheManager in your question) take a pointer to the base factory as a constructor parameter.
I'm assuming that the "factory" can customize the instances of CXYZWManager by deriving from them, but alternatively the constructor of CXYZWManager could take different arguments in the "custom" factory.
A lengthy code example that outputs "CSomeManager" and "CDerivedFromSomeManager":
#include <iostream>
//--------------------------------------------------------------------------------
class CSomeManager
{
public:
virtual const char * ShoutOut() { return "CSomeManager";}
};
//--------------------------------------------------------------------------------
class COtherManager
{
};
//--------------------------------------------------------------------------------
class TheManagerFactory
{
public:
// Non-static, non-const to allow polymorphism-abuse
virtual CSomeManager *CreateSomeManager() { return new CSomeManager(); }
virtual COtherManager *CreateOtherManager() { return new COtherManager(); }
};
//--------------------------------------------------------------------------------
class CDerivedFromSomeManager : public CSomeManager
{
public:
virtual const char * ShoutOut() { return "CDerivedFromSomeManager";}
};
//--------------------------------------------------------------------------------
class TheCustomManagerFactory : public TheManagerFactory
{
public:
virtual CDerivedFromSomeManager *CreateSomeManager() { return new CDerivedFromSomeManager(); }
};
//--------------------------------------------------------------------------------
class CMetaManager
{
public:
CMetaManager(TheManagerFactory *ip_factory)
: mp_some_manager(ip_factory->CreateSomeManager()),
mp_other_manager(ip_factory->CreateOtherManager())
{}
CSomeManager *GetSomeManager() { return mp_some_manager; }
COtherManager *GetOtherManager() { return mp_other_manager; }
private:
CSomeManager *mp_some_manager;
COtherManager *mp_other_manager;
};
//--------------------------------------------------------------------------------
int _tmain(int argc, _TCHAR* argv[])
{
TheManagerFactory standard_factory;
TheCustomManagerFactory custom_factory;
CMetaManager meta_manager_1(&standard_factory);
CMetaManager meta_manager_2(&custom_factory);
std::cout << meta_manager_1.GetSomeManager()->ShoutOut() << "\n";
std::cout << meta_manager_2.GetSomeManager()->ShoutOut() << "\n";
return 0;
}
Here's the solution I thought of, it's not the best one but maybe it will help to think of better solutions:
For each class there would be a creator class:
class SomeManagerClassCreator {
public:
virtual SomeManagerClass* create(SomeOtherManager* someOtherManager) {
return new SomeManagerClass(someOtherManager);
}
};
Then, the creators will be gathered in one class:
class SomeManagerClassCreator;
class SomeOtherManagerCreator;
class TheCreator {
public:
void setSomeManagerClassCreator(SomeManagerClassCreator*);
SomeManagerClassCreator* someManagerClassCreator() const;
void setSomeOtherManagerCreator(SomeOtherManagerCreator*);
SomeOtherManagerCreator* someOtherManagerCreator() const;
private:
SomeManagerClassCreator* m_someManagerClassCreator;
SomeOtherManagerCreator* m_someOtherManagerCreator;
};
And TheManager will be created with TheCreator for internal creation:
class TheManager {
public:
TheManager(TheCreator*);
/* Rest of code from above */
};
The problem with this solution is that it violates DRY - for each class creator I would have to write setter/getter in TheCreator.
This seems like it would be a lot simpler with function templating as opposed to an Abstract Factory pattern
class ManagerFactory
{
public:
template <typename T> static BaseManager * getManager() { return new T();}
};
BaseManager * manager1 = ManagerFactory::template getManager<DerivedManager1>();
If you want to get them via a string, you can create a standard map from strings to function pointers. Here is an implementation that works:
#include <map>
#include <string>
class BaseManager
{
public:
virtual void doSomething() = 0;
};
class DerivedManager1 : public BaseManager
{
public:
virtual void doSomething() {};
};
class DerivedManager2 : public BaseManager
{
public:
virtual void doSomething() {};
};
class ManagerFactory
{
public:
typedef BaseManager * (*GetFunction)();
typedef std::map<std::wstring, GetFunction> ManagerFunctionMap;
private:
static ManagerFunctionMap _managers;
public:
template <typename T> static BaseManager * getManager() { return new T();}
template <typename T> static void registerManager(const std::wstring& name)
{
_managers[name] = ManagerFactory::template getManager<T>;
}
static BaseManager * getManagerByName(const std::wstring& name)
{
if(_managers.count(name))
{
return _managers[name]();
}
return NULL;
}
};
// the static map needs to be initialized outside the class
ManagerFactory::ManagerFunctionMap ManagerFactory::_managers;
int _tmain(int argc, _TCHAR* argv[])
{
// you can get with the templated function
BaseManager * manager1 = ManagerFactory::template getManager<DerivedManager1>();
manager1->doSomething();
// or by registering with a string
ManagerFactory::template registerManager<DerivedManager1>(L"Derived1");
ManagerFactory::template registerManager<DerivedManager2>(L"Derived2");
// and getting them
BaseManager * manager2 = ManagerFactory::getManagerByName(L"Derived2");
manager2->doSomething();
BaseManager * manager3 = ManagerFactory::getManagerByName(L"Derived1");
manager3->doSomething();
return 0;
}
EDIT: In reading the other answers I realized that this is very similar to Dave Van den Eynde's FactorySystem solution, but I'm using a function template pointer instead of instantiating templated factory classes. I think my solution is a little more lightweight. Due to static functions, the only object that gets instantiated is the map itself. If you need the factory to perform other functions (DestroyManager, etc.), I think his solution is more extensible.
You could implement an object factory with static methods that return an instance of a Manager-Class. In the factory you could create a method for the default type of manager and a method for any type of manager which you give an argument representing the type of the Manager-Class (say with an enum). This last method should return an Interface rather than a Class.
Edit: I'll try to give some code, but mind that my C++ times are quite a while back and I'm doing only Java and some scripting for the time being.
class Manager { // aka Interface
public: virtual void someMethod() = 0;
};
class Manager1 : public Manager {
void someMethod() { return null; }
};
class Manager2 : public Manager {
void someMethod() { return null; }
};
enum ManagerTypes {
Manager1, Manager2
};
class ManagerFactory {
public static Manager* createManager(ManagerTypes type) {
Manager* result = null;
switch (type) {
case Manager1:
result = new Manager1();
break;
case Manager2:
result = new Manager2();
break;
default:
// Do whatever error logging you want
break;
}
return result;
}
};
Now you should be able to call the Factory via (if you've been able to make the code sample work):
Manager* manager = ManagerFactory.createManager(ManagerTypes.Manager1);
I would use templates like this as I can't see the point of factories classes:
class SomeOtherManager;
class SomeManagerClass {
public:
SomeManagerClass(SomeOtherManager*);
virtual void someMethod1();
virtual void someMethod2();
};
class TheBaseManager {
public:
//
};
template <class ManagerClassOne, class ManagerClassOther>
class SpecialManager : public TheBaseManager {
public:
virtual ManagerClassOne* someManagerClass() const;
virtual ManagerClassOther* someOtherManager() const;
};
TheBaseManager* ourManager = new SpecialManager<SomeManagerClass,SomeOtherManager>;
You should take a look at the tutorial at
http://downloads.sourceforge.net/papafactory/PapaFactory20080622.pdf?use_mirror=fastbull
It contains a great tutorial on implementing an Abstract factory in C++ and the source code that comes with it is also very robust
Chris
Mh I don't understand a hundred percent, and I am not really into factory stuff from books and articles.
If all your managers share a similar interface you could derive from a base class, and use this base class in your program.
Depending on where the decision which class will be created will be made, you have to use an identifier for creation (as stated above) or handle the decision which manager to instantiate internally.
Another way would be to implement it "policy" like by using templates. So that You ManagerClass::create() returns a specific SomeOtherManagerWhatever instance. This would lay the decision which manager to make in the code which uses your Manager - Maye this is not intended.
Or that way:
template<class MemoryManagment>
class MyAwesomeClass
{
MemoryManagment m_memoryManager;
};
(or something like that)
With this construct you can easily use other managers by only changing the instantiation of MyAwesomeClass.
Also A class for this purpose might be a little over the top. In your case a factory function would do I guess. Well it's more a question of personal preference.
If you plan on supporting plugins that are dynamically linked, your program will need to provide a stable ABI (Application Binary Interface), that means that you cannot use C++ as your main interface as C++ has no standard ABI.
If you want plugins to implement an interface you define yourself, you will have to provide the header file of the interface to plugin programmer and standardize on a very simple C interface in order to create and delete the object.
You cannot provide a dynamic library that will allow you to "new" the plugin class as-is. That is why you need to standardize on a C interface in order to create the object. Using the C++ object is then possible as long as none of your arguments use possibly incompatible types, like STL containers. You will not be able to use a vector returned by another library, because you cannot ensure that their STL implementation is the same as yours.
Manager.h
class Manager
{
public:
virtual void doSomething() = 0;
virtual int doSomethingElse() = 0;
}
extern "C" {
Manager* newManager();
void deleteManager(Manager*);
}
PluginManager.h
#include "Manager.h"
class PluginManager : public Manager
{
public:
PluginManager();
virtual ~PluginManager();
public:
virtual void doSomething();
virtual int doSomethingElse();
}
PluginManager.cpp
#include "PluginManager.h"
Manager* newManager()
{
return new PluginManager();
}
void deleteManager(Manager* pManager)
{
delete pManager;
}
PluginManager::PluginManager()
{
// ...
}
PluginManager::~PluginManager()
{
// ...
}
void PluginManager::doSomething()
{
// ...
}
int PluginManager::doSomethingElse()
{
// ...
}
You didnt talk about TheManager. It looks like you want that to control which class is being used? or maybe you trying to chain them together?
It sounds like you need a abstract base class and a pointer to the currently used class. If you wish to chain you can do it in both abstract class and themanager class. If abstract class, add a member to the next class in chain, if themanager then sort it in order you which to use in a list. You'll need a way to add classes so you'll need an addMe() in themanager. It sounds like you know what your doing so w/e you choose should be right. A list with an addMe func is my recommendation and if you want only 1 active class then a function in TheManager deciding it would be good.
This maybe heavier than you need, but it sounds like you are trying to make a frame work class that supports plugins.
I would break it up into to 3 sections.
1) The FrameWork class would own the plugins.
This class is responsable for publishing interfaces supplied by the plugins.
2) A PlugIn class would own the componets that do the work.
This class is responsable for registering the exported interfaces, and binding the imported interfaces to the components.
3) The third section, the componets are the suppliers and consumers of the interfaces.
To make things extensible, getting things up and running might be broke up into stages.
Create everything.
Wire everything up.
Start everything.
To break things down.
Stop everything.
Destroy everything.
class IFrameWork {
public:
virtual ~IFrameWork() {}
virtual void RegisterInterface( const char*, void* ) = 0;
virtual void* GetInterface( const char* name ) = 0;
};
class IPlugIn {
public:
virtual ~IPlugIn() {}
virtual void BindInterfaces( IFrameWork* frameWork ) {};
virtual void Start() {};
virtual void Stop() {};
};
struct SamplePlugin :public IPlugIn {
ILogger* logger;
Component1 component1;
WebServer webServer;
public:
SamplePlugin( IFrameWork* frameWork )
:logger( (ILogger*)frameWork->GetInterface( "ILogger" ) ), //assumes the 'System' plugin exposes this
component1(),
webServer( component1 )
{
logger->Log( "MyPlugin Ctor()" );
frameWork->RegisterInterface( "ICustomerManager", dynamic_cast( &component1 ) );
frameWork->RegisterInterface( "IVendorManager", dynamic_cast( &component1 ) );
frameWork->RegisterInterface( "IAccountingManager", dynamic_cast( &webServer ) );
}
virtual void BindInterfaces( IFrameWork* frameWork ) {
logger->Log( "MyPlugin BindInterfaces()" );
IProductManager* productManager( static_cast( frameWork->GetInterface( "IProductManager" ) ) );
IShippingManager* shippingManager( static_cast( frameWork->GetInterface( "IShippingManager" ) ) );
component1.BindInterfaces( logger, productManager );
webServer.BindInterfaces( logger, productManager, shippingManager );
}
virtual void Start() {
logger->Log( "MyPlugin Start()" );
webServer.Start();
}
virtual void Stop() {
logger->Log( "MyPlugin Stop()" );
webServer.Stop();
}
};
class FrameWork :public IFrameWork {
vector plugIns;
map interfaces;
public:
virtual void RegisterInterface( const char* name, void* itfc ) {
interfaces[ name ] = itfc;
}
virtual void* GetInterface( const char* name ) {
return interfaces[ name ];
}
FrameWork() {
//Only interfaces in 'SystemPlugin' can be used by all methods of the other plugins
plugIns.push_back( new SystemPlugin( this ) );
plugIns.push_back( new SamplePlugin( this ) );
//add other plugIns here
for_each( plugIns.begin(), plugIns.end(), bind2nd( mem_fun( &IPlugIn::BindInterfaces ), this ) );
for_each( plugIns.begin(), plugIns.end(), mem_fun( &IPlugIn::Start ) );
}
~FrameWork() {
for_each( plugIns.rbegin(), plugIns.rend(), mem_fun( &IPlugIn::Stop ) );
for_each( plugIns.rbegin(), plugIns.rend(), Delete() );
}
};
Here's a minimal factory pattern implementation that I came up with in about 15 minutes. We use a similar one that uses more advanced base classes.
#include "stdafx.h"
#include <map>
#include <string>
class BaseClass
{
public:
virtual ~BaseClass() { }
virtual void Test() = 0;
};
class DerivedClass1 : public BaseClass
{
public:
virtual void Test() { } // You can put a breakpoint here to test.
};
class DerivedClass2 : public BaseClass
{
public:
virtual void Test() { } // You can put a breakpoint here to test.
};
class IFactory
{
public:
virtual BaseClass* CreateNew() const = 0;
};
template <typename T>
class Factory : public IFactory
{
public:
T* CreateNew() const { return new T(); }
};
class FactorySystem
{
private:
typedef std::map<std::wstring, IFactory*> FactoryMap;
FactoryMap m_factories;
public:
~FactorySystem()
{
FactoryMap::const_iterator map_item = m_factories.begin();
for (; map_item != m_factories.end(); ++map_item) delete map_item->second;
m_factories.clear();
}
template <typename T>
void AddFactory(const std::wstring& name)
{
delete m_factories[name]; // Delete previous one, if it exists.
m_factories[name] = new Factory<T>();
}
BaseClass* CreateNew(const std::wstring& name) const
{
FactoryMap::const_iterator found = m_factories.find(name);
if (found != m_factories.end())
return found->second->CreateNew();
else
return NULL; // or throw an exception, depending on how you want to handle it.
}
};
int _tmain(int argc, _TCHAR* argv[])
{
FactorySystem system;
system.AddFactory<DerivedClass1>(L"derived1");
system.AddFactory<DerivedClass2>(L"derived2");
BaseClass* b1 = system.CreateNew(L"derived1");
b1->Test();
delete b1;
BaseClass* b2 = system.CreateNew(L"derived2");
b2->Test();
delete b2;
return 0;
}
Just copy & paste over an initial Win32 console app in VS2005/2008. I like to point out something:
You don't need to create a concrete factory for every class. A template will do that for you.
I like to place the entire factory pattern in its own class, so that you don't need to worry about creating factory objects and deleting them. You simply register your classes, a factory class gets created by the compiler and a factory object gets created by the pattern. At the end of its lifetime, all factories are cleanly destroyed. I like this form of encapsulation, as there is no confusion over who governs the lifetime of the factories.