Related
I'm trying to use C++ to emulate something like dynamic typing. I'm approaching the problem with inherited classes. For example, a function could be defined as
BaseClass* myFunction(int what) {
if (what == 1) {
return new DerivedClass1();
} else if (what == 2) {
return new DerivedClass2();
}
}
The base class and each derived class would have the same members, but of different types. For example, BaseClass may have int xyz = 0 (denoting nothing), DerivedClass1 might have double xyz = 123.456, and DerivedClass2 might have bool xyz = true. Then, I could create functions that returned one type but in reality returned several different types. The problem is, when ere I try to do this, I always access the base class's version of xyz. I've tried using pointers (void* for the base, and "correct" ones for the derived classes), but then every time I want to access the member, I have to do something like *(double*)(obj->xyz) which ends up being very messy and unreadable.
Here's an outline of my code:
#include <iostream>
using std::cout;
using std::endl;
class Foo {
public:
Foo() {};
void* member;
};
class Bar : public Foo {
public:
Bar() {
member = new double(123.456); // Make member a double
};
};
int main(int argc, char* args[]) {
Foo* obj = new Bar;
cout << *(double*)(obj->member);
return 0;
};
I guess what I'm trying to ask is, is this "good" coding practice? If not, is there a different approach to functions that return multiple types or accept multiple types?
That is not actually the way to do it.
There are two typical ways to implement something akin to dynamic typing in C++:
the Object-Oriented way: a class hierarchy and the Visitor pattern
the Functional-Programming way: a tagged union
The latter is rather simple using boost::variant, the former is well documented on the web. I would personally recommend boost::variant to start with.
If you want to go down the full dynamic typing road, then things get trickier. In dynamic typing, an object is generally represented as a dictionary containing both other objects and functions, and a function takes a list/dictionary of objects and returns a list/dictionary of objects. Modelling it in C++ is feasible, but it'll be wordy...
How is an object represented in a dynamically typed language ?
The more generic representation is for the language to represent an object as both a set of values (usually named) and a set of methods (named as well). A simplified representation looks like:
struct Object {
using ObjectPtr = std::shared_ptr<Object>;
using ObjectList = std::vector<ObjectPtr>;
using Method = std::function<ObjectList(ObjectList const&)>;
std::map<std::string, ObjectPtr> values;
std::map<std::string, Method> methods;
};
If we take Python as an example, we realize we are missing a couple things:
We cannot implement getattr for example, because ObjectPtr is a different type from Method
This is a recursive implementation, but without the basis: we are lacking innate types (typically Bool, Integer, String, ...)
Dealing with the first issue is relatively easy, we transform our object to be able to become callable:
class Object {
public:
using ObjectPtr = std::shared_ptr<Object>;
using ObjectList = std::vector<ObjectPtr>;
using Method = std::function<ObjectList(ObjectList const&)>;
virtual ~Object() {}
//
// Attributes
//
virtual bool hasattr(std::string const& name) {
throw std::runtime_error("hasattr not implemented");
}
virtual ObjectPtr getattr(std::string const&) {
throw std::runtime_error("gettattr not implemented");
}
virtual void setattr(std::string const&, ObjectPtr) {
throw std::runtime_error("settattr not implemented");
}
//
// Callable
//
virtual ObjectList call(ObjectList const&) {
throw std::runtime_error("call not implemented");
}
virtual void setcall(Method) {
throw std::runtime_error("setcall not implemented");
}
}; // class Object
class GenericObject: public Object {
public:
//
// Attributes
//
virtual bool hasattr(std::string const& name) override {
return values.count(name) > 0;
}
virtual ObjectPtr getattr(std::string const& name) override {
auto const it = values.find(name);
if (it == values.end) {
throw std::runtime_error("Unknown attribute");
}
return it->second;
}
virtual void setattr(std::string const& name, ObjectPtr object) override {
values[name] = std::move(object);
}
//
// Callable
//
virtual ObjectList call(ObjectList const& arguments) override {
if (not method) { throw std::runtime_error("call not implemented"); }
return method(arguments);
}
virtual void setcall(Method m) {
method = std::move(m);
}
private:
std::map<std::string, ObjectPtr> values;
Method method;
}; // class GenericObject
And dealing with the second issue requires seeding the recursion:
class BoolObject final: public Object {
public:
static BoolObject const True = BoolObject{true};
static BoolObject const False = BoolObject{false};
bool value;
}; // class BoolObject
class IntegerObject final: public Object {
public:
int value;
}; // class IntegerObject
class StringObject final: public Object {
public:
std::string value;
}; // class StringObject
And now you need to add capabilities, such as value comparison.
You can try the following design:
#include <iostream>
using std::cout;
using std::endl;
template<typename T>
class Foo {
public:
Foo() {};
virtual T& member() = 0;
};
class Bar : public Foo<double> {
public:
Bar() : member_(123.456) {
};
virtual double& member() { return member_; }
private:
double member_;
};
int main(int argc, char* args[]) {
Foo<double>* obj = new Bar;
cout << obj->member();
return 0;
};
But as a consequence the Foo class already needs to be specialized and isn't a container for any type anymore.
Other ways to do so, are e.g. using a boost::any in the base class
If you need a dynamic solution you should stick to using void* and size or boost::any. Also you need to pass around some type information as integer code or string so that you can decode the actual type of the content.
See also property design pattern.
For example, you can have a look at zeromq socket options https://github.com/zeromq/libzmq/blob/master/src/options.cpp
I'm saving and reloading a bunch of different objects all derived from a common base to a file, and obviously I need to store the class name (or something similar) in order to create the correct object type on reloading.
Saving is easy:
class Base
{
virtual string className() const = 0;
void saveToFile()
{
write(className());
... other writing stuff
}
}
class Derived1: public Base
{
string className() const { return "Derived1"; };
...
}
class Derived2: public Base
{
string className() const { return "Derived2"; };
...
}
and, loading is easy if you don't mind duplicating the strings...
static Base * Base::factory(const String &cname)
{
if (cname == "Derived1")
return new Derived1;
else if (cname == "Derived2")
return = new Derived2;
else ...
}
void load()
{
String cname = readString();
Base * obj(Base::factory(cname);
obj->readIt();
}
But, the duplicated strings offends my sense of DRY: Ideally, className() could be static virtual but that isn't allowed. I have a feeling that I'm missing an obvious 'clean' way round this, but I can't see it yet. Any suggestions?
Note: OK, code slightly tweaked using a factory method. Note that this doesn't actually answer the problem!
Note #2: The code above isn't trying to be a perfect representation of the ultimate factory pattern. I'm not concerned with unwanted linkage between base and derived classes, or potential difficulties in extending the hierarchy. I'm looking for answers that will simplify the code rather than complicate it.
That's about the best you could do, you might clean it up a bit though by wrapping the if in a factory class.
Two things here. First, to avoid having to write out the name twice,
I've used something like the following in the past:
class Derived : public Base
{
// ...
static char const* className() { return "Derived"; }
virtual char const* getClassName() const { return className(); }
// overrides virtual function in the base class...
};
In addition, if you want to be able to read the class from an external
source, you'll need some sort of static factory function, which
registers itself with a map of such functions. I'd go ahead and do this
in Base:
class Base
{
// ...
protected:
class Factory
{
protected:
Factory( std::string const& type );
public:
virtual Base* constructFromFile( std::istream const& source ) const = 0;
};
typedef std::map <std::string, Factory const*> FactoryMap;
static FactoryMap& factories();
template <typename Derived>
class ConcreteFactory : public Factory
{
public:
ConcreteFactory() : Factory( Derived::className() ) {}
virtual Base* constructFromFile( std::istream const& source ) const
{
return new Derived( source );
}
};
public:
static Base* readFromFile( std::istream& source );
};
Base::FactoryMap&
Base::factories()
{
static FactoryMap theOneAndOnly;
return theOneAndOnly;
}
Base::Factory::Factory( std::string const& type )
{
std::pair <FactoryMap::iterator, bool> results
= factories().insert( std::make_pair( type, this ) );
assert (results.second);
}
Base* Base::readFromFile( std::istream& source )
{
std::string type = readType( source );
FactoryMap::const_iterator factory = factories().find( type );
if ( factory == factories().end() ) {
throw UnknownType(...);
}
return factory->second->constructFromFile( std::istream& source );
}
Finally, for each derived class, you'll have to define a constructor
taking an std::istream&, and a static instance of
Base::ConcreteFactory <Derived>. (As written above, this must be a
static member.)
You can use a Factory Method (AKA Virtual Constructor) This is explained in the Design Patterns book and in many places throughout the Internet -- you can do a Google search on these terms. There is probably already a discussion of this in StackOverflow.
You obviously need the Factory pattern.
Read more here
http://www.codeproject.com/Articles/363338/Factory-Pattern-in-Cplusplus
Id' make map of strings to functions, that create classes:
std::hash_map<std::string, std::function<Base*()> creators;
then, you can create function to fill map
template <typename T> void add()
{
creators.insert(std::pair(T::class_name(), []()-> Base* { return new T(); }));
}
usage is simple:
//factory constructor
add<Derived1>();
add<Derived2>();
//creation
Base* r = 0;
auto it = creators.find(string);
if (it != creators.end()) {
r = (*it)();
}
I came across the same problem and found a pretty neat answer in this article:
Industrial Strenght Pluggable Factories
Close enough to James Kanze's answer, but I suggest you read this and try it yourself.
I want to create a class factory and I would like to use reflection for that. I just need to
create a object with given string and invoke only few known methods.
How i can do that?
You will have to roll your own. Usually you have a map of strings to object creation functions.
You will need something like the follwing:
class thing {...};
/*
class thing_A : public thing {...};
class thing_B : public thing {...};
class thing_C : public thing {...};
*/
std::shared_ptr<thing> create_thing_A();
std::shared_ptr<thing> create_thing_C();
std::shared_ptr<thing> create_thing_D();
namespace {
typedef std::shared_ptr<thing> (*create_func)();
typedef std::map<std::string,create_func> creation_map;
typedef creation_map::value_type creation_map_entry;
const creation_map_entry creation_map_entries[] = { {"A", create_thing_A}
, {"B", create_thing_B}
, {"C", create_thing_C} };
const creation_map creation_funcs(
creation_map_entries,
creation_map_entries + sizeof(creation_map_entries)
/ sizeof(creation_map_entries[0] );
}
std::shared_ptr<thing> create_thing(const std::string& type)
{
const creation_ma::const_iterator it = creation_map.find(type);
if( it == creation_map.end() ) {
throw "Dooh!"; // or return NULL or whatever suits you
}
return it->second();
}
There are other ways to do this (like having a map of strings to objects from which to clone), but I think they all boil down to having a map of strings to something related to the specific types.
There is no reflection in C++, directly supported by the standard.
However C++ is sufficiently low-level that you can implement some minimal support for reflection to complete the task at hand.
For the simple task of creating a Factory, you usually use the Prototype approach:
class Base
{
public:
virtual Base* clone() const = 0;
virtual ~Base();
};
class Factory
{
public:
std::unique_ptr<Base> get(std::string const& name);
void set(std::string const& name, std::unique_ptr<Base> b);
private:
boost::ptr_map<std::string,Base> mExemplars;
};
Of course, those "known methods" that you are speaking about should be defined within the Base class, which acts as an interface.
There is no reflection in C++, so you should restate your question trying to explain what are the requirements that you would have fulfilled with the reflection part of it.
Depending on your actual constraints and requirements, there are a few things that you can do. The first approach that I would take would be creating an abstract factory where concrete factories can register and provide a simple interface:
class Base {}; // shared base by all created objects
class ConcreteFactoryBase {
public:
virtual ~ConcreteFactoryBase() {}
virtual Base* create() const = 0; // actual construction
virtual std::string id() const = 0; // id of the types returned
};
class AbstractFactory
{
typedef std::map<std::string, ConcreteFactory* > factory_map_t;
public:
void registerFactory( ConcreteFactoryBase* factory ) {
factories[ factory->id() ] = factory;
}
Base* create( std::string const & id ) const {
factory_map_t::const_iterator it = factories.find( id );
if ( it == factories.end() ) {
return 0; // or throw, or whatever makes sense in your case
}
return (*it)->create();
}
~AbstractFactory(); // ensure that the concrete factories are deleted
private:
std::map<ConcreteFactoryBase*> factories;
};
The actual concrete factories can be implemented manually but they can probably be templated, unless the constructors for the different types require different arguments:
template <typename T>
class ConcreteFactory : public ConcreteFactoryBase {
public:
ConcreteFactory( std::string const & id ) : myid(id) {}
virtual Base* create() const {
return new T;
}
virtual std::string id() const {
return myid;
}
private:
std::string myid;
};
class Test : public Base {};
int main() {
AbstracFactory factory;
factory.register_factory( new ConcreteFactory<Test>("Test") );
}
Optionally you could adapt the signatures so that you can pass arguments to the constructor through the different layers.
Then again, by knowing the actual constraints some other approaches might be better. The clone() approach suggested elsewhere is good (either by actually cloning or by creating an empty object of the same type). That is basically blending the factory with the objects themselves so that each object is a factory of objects of the same type. I don't quite like mixing those two responsabilities but it might be one of the simplest approaches with less code to write.
You could use typeid & templates to implement the factory so you won't need strings at all.
#include <string>
#include <map>
#include <typeinfo>
//***** Base *****
class Base
{
public:
virtual ~Base(){} //needs to be virtual to make typeid work
};
//***** C1 *****
class C1 : public Base
{};
//***** Factory *****
class Factory
{
public:
template <class T>
Base& get();
private:
typedef std::map<std::string, Base> BaseMap;
BaseMap m_Instances;
};
template <class T>
Base& Factory::get()
{
BaseMap::const_iterator i = m_Instances.find(typeid(T).name());
if(i == m_Instances.end()) {
m_Instances[typeid(T).name()] = T();
}
return m_Instances[typeid(T).name()];
}
//***** main *****
int main(int argc, char *argv[])
{
Factory f;
Base& c1 = f.get<C1>();
return 0;
}
I have an application that has several objects (about 50 so far, but growing). There is only one instance of each of these objects in the app and these instances get shared among components.
What I've done is derive all of the objects from a base BrokeredObject class:
class BrokeredObject
{
virtual int GetInterfaceId() = 0;
};
And each object type returns a unique ID. These IDs are maintained in a header file.
I then have an ObjectBroker "factory". When someone needs an object, then call GetObjectByID(). The boker looks in an STL list to see if the object already exists, if it does, it returns it. If not, it creates it, puts it in the list and returns it. All well and good.
BrokeredObject *GetObjectByID(int id)
{
BrokeredObject *pObject;
ObjectMap::iterator = m_objectList.find(id);
// etc.
if(found) return pObject;
// not found, so create
switch(id)
{
case 0: pObject = new TypeA; break;
case 1: pObject = new TypeB; break;
// etc.
// I loathe this list
}
// add it to the list
return pObject;
}
What I find painful is maintaining this list of IDs and having to have each class implement it. I have at least made my consumer's lives slightly easier by having each type hold info about it's own ID like this:
class TypeA : public BrokeredObject
{
static int get_InterfaceID() { return IID_TYPEA; }
int GetInterfaceID() { return get_InterfaceID(); }
};
So I can get an object like this:
GetObjectByID(TypeA::get_InterfaceID());
Intead of having to actually know what the ID mapping is but I still am not thrilled with the maintenance and the potential for errors. It seems that if I know the type, why should I also have to know the ID?
What I long for is something like this in C#:
BrokeredObject GetOrCreateObject<T>() where T : BrokeredObject
{
return new T();
}
Where the ObjectBroker would create the object based on the type passed in.
Has C# spoiled me and it's just a fact of life that C++ can't do this or is there a way to achieve this that I'm not seeing?
Yes, there is a way. A pretty simple even in C++ to what that C# code does (without checking for inheritance though):
template<typename T>
BrokeredObject * GetOrCreateObject() {
return new T();
}
This will work and do the same as the C# code. It is also type-safe: If the type you pass is not inherited from BrokeredObject (or isn't that type itself), then the compiler moans at the return statement. It will however always return a new object.
Singleton
As another guy suggested (credits to him), this all looks very much like a fine case for the singleton pattern. Just do TypeA::getInstance() to get the one and single instance stored in a static variable of that class. I suppose that would be far easier than the above way, without the need for IDs to solve it (i previously showed a way using templates to store IDs in this answer, but i found it effectively is just what a singleton is).
I've read that you will leave the chance open to have multiple instances of the classes. One way to do that is to have a Mingleton (i made up that word :))
enum MingletonKind {
SINGLETON,
MULTITON
};
// Singleton
template<typename D, MingletonKind>
struct Mingleton {
static boost::shared_ptr<D> getOrCreate() {
static D d;
return boost::shared_ptr<D>(&d, NoopDel());
}
struct NoopDel {
void operator()(D const*) const { /* do nothing */ }
};
};
// Multiton
template<typename D>
struct Mingleton<D, MULTITON> {
static boost::shared_ptr<D> getOrCreate() {
return boost::shared_ptr<D>(new D);
}
};
class ImASingle : public Mingleton<ImASingle, SINGLETON> {
public:
void testCall() { }
// Indeed, we have to have a private constructor to prevent
// others to create instances of us.
private:
ImASingle() { /* ... */ }
friend class Mingleton<ImASingle, SINGLETON>;
};
class ImAMulti : public Mingleton<ImAMulti, MULTITON> {
public:
void testCall() { }
// ...
};
int main() {
// both do what we expect.
ImAMulti::getOrCreate()->testCall();
ImASingle::getOrCreate()->testCall();
}
Now, you just use SomeClass::getOrCreate() and it cares about the details. The custom deleter in the singleton case for shared_ptr makes deletion a no-op, because the object owned by the shared_ptr is allocated statically. However, be aware of problems of destruction order of static variables: Static initialization order fiasco
The way I would solve this problem is using what I would call the Static Registry Pattern, which in my mine mind is the C++ version of dependency injection.
Basically you have a static list of builder objects of a type that you use to build objects of another type.
A basic static registry implementation would look like:
template <class T>
class StaticRegistry
{
public:
typedef std::list<T*> Container;
static StaticRegistry<T>& GetInstance()
{
if (Instance == 0)
{
Instance = new StaticRegistry<T>;
}
return *Instance;
}
void Register(T* item)
{
Items.push_back(item);
}
void Deregister(T* item)
{
Items.remove(item);
if (Items.empty())
{
delete this;
Instance = 0;
}
}
typedef typename Container::const_iterator const_iterator;
const_iterator begin() const
{
return Items.begin();
}
const_iterator end() const
{
return Items.end();
}
protected:
StaticRegistry() {}
~StaticRegistry() {}
private:
Container Items;
static StaticRegistry<T>* Instance;
};
template <class T>
StaticRegistry<T>* StaticRegistry<T>::Instance = 0;
An implementation of BrokeredObjectBuilder could look like this:
class BrokeredObjectBuilderBase {
public:
BrokeredObjectBuilderBase() { StaticRegistry<BrokeredObjectBuilderBase>::GetInstance().Register(this); }
virtual ~BrokeredObjectBuilderBase() { StaticRegistry<BrokeredObjectBuilderBase>::GetInstance().Deregister(this); }
virtual int GetInterfaceId() = 0;
virtual BrokeredObject* MakeBrokeredObject() = 0;
};
template<class T>
class BrokeredObjectBuilder : public BrokeredObjectBuilderBase {
public:
BrokeredObjectBuilder(unsigned long interface_id) : m_InterfaceId(interface_id) { }
virtual int GetInterfaceId() { return m_InterfaceId; }
virtual T* MakeBrokeredObject() { return new T; }
private:
unsigned long m_InterfaceId;
};
class TypeA : public BrokeredObject
{
...
};
// Create a global variable for the builder of TypeA so that it's
// included in the BrokeredObjectBuilderRegistry
BrokeredObjectBuilder<TypeA> TypeABuilder(TypeAUserInterfaceId);
typedef StaticRegistry<BrokeredObjectBuilderBase> BrokeredObjectBuilderRegistry;
BrokeredObject *GetObjectByID(int id)
{
BrokeredObject *pObject(0);
ObjectMap::iterator = m_objectList.find(id);
// etc.
if(found) return pObject;
// not found, so create
BrokeredObjectBuilderRegistry& registry(BrokeredObjectBuilderRegistry::GetInstance());
for(BrokeredObjectBuilderRegistry::const_iterator it = registry.begin(), e = registry.end(); it != e; ++it)
{
if(it->GetInterfaceId() == id)
{
pObject = it->MakeBrokeredObject();
break;
}
}
if(0 == pObject)
{
// userinterface id not found, handle this here
...
}
// add it to the list
return pObject;
}
Pros:
All the code that knows about creating the types is seperated out into the builders and the BrokeredObject classes don't need to know about it.
This implementation can be used in libraries and you can control on a per project level what builders are pulled into a project using a number of different techniques.
The builders can be as complex or as simple (like above) as you want them to be.
Cons:
There is a wee bit of infrastructure involved (but not too much).
The flexability of defining the global variables to include what builders to include in your project does make it a little messy to work with.
I find that people find it hard to understand this pattern, I'm not sure why.
It's sometimes not easy to know what is in the static registry at any one time.
The above implementation leaks one bit of memory. (I can live with that...)
The above implementation is very simple, you can extend it in lots of different ways depending on the requirements you have.
Use a template class as the broker.
Make the instance a static member of the function. It will be created on first use and automagically-destroyed when the program exits.
template <class Type>
class BrokeredObject
{
public:
static Type& getInstance()
{
static Type theInstance;
return theInstance;
}
};
class TestObject
{
public:
TestObject()
{}
};
int main()
{
TestObject& obj =BrokeredObject<TestObject>::getInstance();
}
Instead of GetInterfaceId() in the BrokeredObject base class, you could define that pure virtual method:
virtual BrokeredObject& GetInstance()=0;
And in the derived classes you'll return from that method the instance of the particular derived class, if it's already created, if not, you'll first create it and then return it.
It doesn't look like you need the global object to do the management, so why not move everything into the classes themselves?
template <class Type>
class BrokeredObject
{
protected:
static Type *theInstance;
public:
static Type *getOrCreate()
{
if (!theInstance) {
theInstance = new Type();
}
return theInstance;
}
static void free()
{
delete theInstance;
}
};
class TestObject : public BrokeredObject<TestObject>
{
public:
TestObject()
{}
};
int
main()
{
TestObject *obj = TestObject::getOrCreate();
}
If you have RTTI enabled, you can get the class name using typeid.
One question, why are you using a factory rather than using a singleton pattern for each class?
Edit: OK, so you don't want to be locked into a singleton; no problem. The wonderful thing about C++ is it gives you so much flexibility. You could have a GetSharedInstance() member function that returns a static instance of the class, but leave the constructor public so that you can still create other instances.
If you always know the type at compile time there is little point in calling BrokeredObject* p = GetObjectByID(TypeA::get_InterfaceID()) instead of TypeA* p = new TypeA or TypeA o directly.
If you on the other hand don't know the exact type at compile time, you could use some kind of type registry.
template <class T>
BrokeredObject* CreateObject()
{
return new T();
}
typedef int type_identity;
typedef std::map<type_identity, BrokeredObject* (*)()> registry;
registry r;
class TypeA : public BrokeredObject
{
public:
static const type_identity identity;
};
class TypeB : public BrokeredObject
{
public:
static const type_identity identity;
};
r[TypeA::identity] = &CreateObject<TypeA>;
r[TypeB::identity] = &CreateObject<TypeB>;
or if you have RTTI enabled you could use type_info as type_identity:
typedef const type_info* type_identity;
typedef std::map<type_identity, BrokeredObject* (*)()> registry;
registry r;
r[&typeid(TypeA)] = &CreateObject<TypeA>;
r[&typeid(TypeB)] = &CreateObject<TypeB>;
Each new class could of course, in any case, be self-registering in the registry, making the registration decentralized instead of centralized.
You should almost certainly be using dependency injection.
Why not this?
template
BrokeredObject* GetOrCreateObject()
{
return new T();
}
My use-case tended to get a little more complex - I needed the ability to do a little bit of object initialization and I needed to be able to load objects from different DLLs based on configuration (e.g. simulated versus actual for hardware). It started looking like COM and ATL was where I was headed, but I didn't want to add the weight of COM to the OS (this is being done in CE).
What I ended up going with was template-based (thanks litb for putting me on track) and looks like this:
class INewTransModule
{
public:
virtual bool Init() { return true; }
virtual bool Shutdown() { return true; }
};
template <typename T>
struct BrokeredObject
{
public:
inline static T* GetInstance()
{
static T t;
return &t;
}
};
template <>
struct BrokeredObject<INewTransModule>
{
public:
inline static INewTransModule* GetInstance()
{
static INewTransModule t;
// do stuff after creation
ASSERT(t.Init());
return &t;
}
};
class OBJECTBROKER_API ObjectBroker
{
public:
// these calls do configuration-based creations
static ITraceTool *GetTraceTool();
static IEeprom *GetEeprom();
// etc
};
Then to ensure that the objects (since they're templated) actually get compiled I added definitions like these:
class EepromImpl: public BrokeredObject<EepromImpl>, public CEeprom
{
};
class SimEepromImpl: public BrokeredObject<SimEepromImpl>, public CSimEeprom
{
};
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.