I have these classes:
class Field{
public:
int X;
void validate(){
validator->validate(this);
}
void setValidator(Validator* v){
validator = v;
}
private:
Validator* validator;
};
class DerivedField : public Field{
public:
int Y;
}
class Validator {
public:
virtual void validate(Field*); // do something with Field.X
};
class DerivedValidator : public Validator {
virtual void validate(Field*); //do something with DerivedField.Y
};
I want to do this:
DerivedValidator* v = new DerivedValidator();
DerivedField* f = new DerivedFiled();
f->setValidator(v);
f->validate(); // Error, Validator::validate called instead of DerivedValidator::validate
Since It doesnt work, what can I do to avoid this:
class DerivedValidator{
void validate(Field* f){
DerivedField* dv = dynamic_cast<DerivedField*>(f);
// do something with dv.Y
}
};
Thank you.
Edit: Ok, all 'bugs' fixed now.
First, by changing the method signature, you've created a new method that will hide the base class's. You need to have DerivedValidator's validate() method take a Field* parameter only.
Then, once you've added the virtual keyword to your methods, when you pass a DerivedField in, the correct method should be called.
You still won't be able to access the inherited values of DerivedField as you haven't told the base class anything about the derived class. ie, Field knows all about Validator types, but has no clue what a DerivedValidator is. Whatever you want to access in the derived class has to go via the base class signature.
It's good that it doesn't work, since DerivedField doesn't inherit from Field. If you want to hack, you can just use C-style casts.
Rather than creating a parallel class hierarchy for fields and validators, you might be better off with a design something like this:
class Field{
public:
int X;
virtual bool validate(Validator* v){
return (v->isValid(X));
}
}
class DerivedField : public Field{
public:
int Y;
virtual bool validate(Validator* v){
return (v->isValid(X) && v->isValid(Y));
}
}
Each of your derived Validator classes can have different ideas of what is valid and what isn't.
You code is full of small errors. First, your classes son't derive from anything, I assume DerivedField inherits from Field and DerivedValidator from Validator. Second, you need to write validator->validate, as it's a pointer. Third, Field::validate has no argument but you call f->validate(v). If these errors are corrected, then the only thing you got to do is make Validator::validate virtual and use your proposed solution (with the dynamic_cast).
you need to make Validator::validate() virtual.
Related
I have some issues to find a relevant solution to my problem.
I have to return some data from a class, and the type of data kind of depends on the class.
My first solution was this :
class Base
{
virtual QVector<Data*> getData() const = 0;
};
class Derived : public Base
{
virtual QVector<DerviedData*> getData() const = 0;
};
But I know this is impossible, even if DerivedData extends Data, because of invalid covariant return types.
So I came up with another idea which implies template. What I did is I turned the Base class into a template class :
template<class T>
class Base
{
virtual QVector<T*> getData() const = 0;
}
And then I could write a Derivedconstructor like that :
Derived::Derived() : Base<DerivedData>() {}
But know I have another problem. Suppose that I write another class, which has a method taking any Base class in parameters.
void Foo::doSomething(Base* b) {
b->getData();
}
This does not compile and says
invalid use of template-name 'Base' without an argument list
which I understand perfectly.
Supposing that my code will look like that :
DerivedClass1 d1;
DerivedClass2 d2;
DerivedClass3 d3;
this->doSomething(&d1);
this->doSomething(&d2);
this->doSomething(&d3);
What are my solutions here ? May I do something like "templating" the method doSomething ?
this->doSomething<DerivedData>(&d1);
With a protoype like
template<class T>
void doSomething(Base<T>* b);
Is that possible ? Is that a good way of thinking ?
Coming from Java, I used to resolve such problems by using wildcards
abstract List<? extends Data> getData();
But I heard there is stricly speaking no such things in C++ (more or less simulable with such things as std::is_base_of).
Thank you for your time.
You can let Derived::getData() return QVector<Data*>. When you need to use it, find out if the pointers in QVector is to Data or DerivedData, using dynamic_cast or similar method.
I'm working on a very, very simple data access layer (DAL) featuring two classes: DataTransferObject (DTO) and DataAccessObject (DAO). Both classes are abstract base classes and need to be inherited and modified for a specific use case.
class DataTransferObject {
protected:
//protected constructor to prevent initialization
};
class DataAccessObject {
public:
virtual bool save(DataTransferObject o) = 0;
virtual DataTransferObject* load(int id) = 0;
};
in case of a House class from the business logic layer, the implementation of the DAL classes would read something along these lines:
class Dto_House : public DataTransferObject {
public:
int stories;
string address; //...which are all members of the House class...
Dto_House(House h);
};
class Dao_House : public DataAccessObject {
public:
bool save(Dto_House h) { /*...implement database access, etc...*/ }
Dto_House* load(int id) {/*...implement database access, etc...*/ }
};
EDIT: Of course, the derived classes know about the structure of the House class and the data storage.
Simple, nice, okidoke.
Now I wanted to provide a method toObject() in the DTO class in order to quickly convert the Dto_House into a House object. I then read about the automatic return type deduction in C++14 and tried:
class DataTransferObject {
public:
virtual auto toObject() = 0;
};
But I had to discover: No automatic return type deduction for virtual functions. :(
What are your ideas about implementing a "virtual function with deduced return type" for this specific case? I want a general toObject() function in my DTO "interface".
The only thing that came to my mind was something like:
template <typename T>
class DataTransferObject {
virtual T toObject() = 0;
};
class Dto_House : public DataTransferObject<House> {
public:
int stories;
string address;
House toObject() {return House(stories, address);}
};
EDIT:
A possible use case would be:
House h(3, "231 This Street");
h.doHouseStuff();
//save it
Dto_House dtoSave(h);
Dao_House dao;
dao.save(dtoSave); //even shorter: dao.save(Dto_House(h));
//now load some other house
Dto_House dtoLoad = dao.load(id 2);
h = dtoLoad.toObject();
h.doOtherHouseStuff();
But the house does not know it can be saved and loaded.
Of course, the abstract DAO class may be derived to further refine it for the use with, e.g. Sqlite, XML files or whatever... I just presented the very basic concept.
How about setting an empty abstract class - practically, an interface, then have both of your types implement it and set this as the toObject returning reference type?
class Transferable
{
virtual ~Transferable() = 0;
}
And then:
class DataTransferObject {
public:
//Return a reference of the object.
virtual Transferable& toObject() = 0;
};
Dto_House : public DataTransferObject, Transferable { /*...*/ }
House : public DataTransferObject, Transferable { /*...*/ }
The example above is to get my point.
Even better, you can use the DataTransferObject for this cause as your returning reference type, and no other abstract class:
class DataTransferObject {
public:
virtual DataTransferObject& toObject() = 0;
};
Dto_House : public DataTransferObject { /*...*/ }
House : public DataTransferObject { /*...*/ }
Update: If you want to have the classes separated apart, separating any association between data and operations by convention, you could set the name of the base class on something that represents the data i.e.: Building, Construction etc, and then use it for the reference type in toObject.
You can also have the class manipulating those operations on the API of data manipulation.
In general, you can not have a virtual function returning different types in different subclasses, as this violates the whole concept of statically typed language: if you call DataTransferObject::toObject(), the compiler does not know what type it is going to return until runtime.
And this highlight the main problem of your design: why do you need a base class at all? How are you going to use it? Calling DataTransferObject::toObject(), even if you use some magic to get it work (or use a dynamically typed language), sounds like a bad idea since you can not be sure what the return type is. You will anyway need some casts, or some ifs, etc, to get it working — or you will be using only the functionality common for all such objects (House, Road, etc.) — but then you just need a common base class for all of them.
In fact, there is one exception to the same return type rule: if you return a pointer to a class, you can use the Covariant return type concept: a subclass may override a virtual function to return a subclass of the original return type. If all your "objects" have a common base class, you may use something along the lines of
struct DataTransferObject {
virtual BaseObject* toObject() = 0;
};
struct Dto_House : public DataTransferObject {
virtual House* toObject() { /*...*/ } // assumes that House subclasses BaseObject
};
However, this will still leave the same problem: if all you have in your code is DataTransferObject, even if you (but not the compiler) know it is a Dto_House, you will need some cast, which might be unreliable.
On the other hand, you template solution seems quite good except that you will not be able to explicitly call DataTransferObject::toObject() (unless you know the type of the object), but that's a bad idea as I have explained.
So, I suggest you think on how you are going to actually use the base classes (even write some sample code), and make your choice based on that.
I have certain functionality encapsulated in classes which I use in another class. I think this is called composition.
class DoesSomething01
{
public:
DoesSomething01();
void functionality01();
void functionality02();
};
class DoesSomething02
{
public:
DoesSomething02();
void functionality01();
void functionality02();
};
class ClassA
{
public:
ClassA();
private:
DoesSomething01 *m_doesSomething01;
DoesSomething02 *m_doesSomething02;
};
If I have now a ClassB which "knows" ClassA and have to use/execute functionality01 and/or functionality02 of classes DoesSomething01 and/or DoesSomething02 I see two possibilities:
a) Add methods like this to ClassA to provide ClassB direct access to DoesSomething01 and/or DoesSomething02:
DoesSomething01 *getDoesSomething01() { return *m_doesSomething01; }
DoesSomething02 *getDoesSomething02() { return *m_doesSomething02; }
ClassB could then do something like this:
m_classA->getDoesSomething01()->functionality01();
b) Add (in this case four) methods to ClassA which forwards the method calls to DoesSomething01 and DoesSomething02 like this:
void doesSomething01Functionality01() { m_doesSomething01->functionality01(); }
void doesSomething01Functionality02() { m_doesSomething01->functionality02(); }
void doesSomething02Functionality01() { m_doesSomething02->functionality01(); }
void doesSomething02Functionality02() { m_doesSomething02->functionality02(); }
Which option is better and why?
What are the advantages/disadvantages of each option?
First option can be considered a code smell. According to Robert C. Martin's 'Clean Code' it is "Transitive Navigation" and should be avoided. Quoting the author:
In general we don’t want a single module to know much about its
collaborators. More specifically, if A collaborates with B, and B
collaborates with C, we don’t want modules that use A to know about C.
(For example, we don’t want a.getB().getC().doSomething();.)
Second option looks better. It is classical use of Facade pattern. And it is better, because it hides other functionalities of classes DoesSomthing01 and DoesSomthing02. Then you ve'got simplified view of it which is easier to use than 1st option.
Edit: there is also one more thing. You've got two classes which have the same functionalites and are aggregated by other class. You should consider using Stratey pattern here. The your code will look like this:
class DoesSomething
{
public:
virtual void functionality01() = 0;
virtual void functionality02() = 0;
}
class DoesSomething01 : DoesSomething
{
public:
DoesSomething01();
void functionality01();
void functionality02();
};
class DoesSomething02 : DoesSomething
{
public:
DoesSomething02();
void functionality01();
void functionality02();
};
class ClassA
{
public:
ClassA();
DoesSomething* doesSomething(); // Getter
void doesSomething(DoesSomething* newDoesSomething); // Setter
// ...
private:
DoesSomething *m_doesSomething;
};
Then you will need only two method instead of four:
void doesFunctionality01() { m_doesSomething->functionality01(); }
void doesFunctionality02() { m_doesSomething->functionality02(); }
The first scenario is a violation of law of Demeter, which says that a class can only talk to its immediate friends. Basically the problem with the first approach is that any change in the inner classes DoSomething01 and DoSomething02 will trigger a change in Class A as well as Class B because both classes are now directly dependent on these inner classes.
The second option is better as it encapsulates the class B from inner classes but a side effect of this solution is that now class A has a lot of methods that does nothing fancy except for delegating to its inner classes. This is fine but imagine if DoSomething01 has an inner class DoSomething03 and class B needs to access its functionality without directly knowing about it than the class A would need to have another method that would delegate to DoSomething01 that would in turn delegate to DoSomething03. In this case I think it is better to let class B directly know about DoSomething01 otherwise class A is going to have a huge interface that simply delegates to its inner classes.
If there are many classes and/or many methods to be called it makes sense to invent
an interface in the form of an abstract parent class:
class SomeInterface
{
public:
SomeInterface(){}
virtual void functionally01() = 0;
virtual void functionally02() = 0;
}
DoesSomthing01 and other classes would then inherit this class:
class DoesSomthing01 : public SomeInterface
and implement the methods.
If it make sense to associate a key with the instantiation of such a class
you could store these objects in ClassA e.g. using a map (here I
use an integer as the key):
class ClassA
{
private:
std::map<int, SomeInterface*> m_Interfaces;
public:
SomeInterface* getInterface(const int key)
{
std::map<int, SomeInterface*>::iterator it(m_Interfaces.find(key));
if (it != m_Interfaces.end())
return it->second;
else
return NULL;
}
};
From ClassB you could then access them
int somekey = ...;
SomeInterface *myInter = m_classA->getInterface(somekey);
if (myInter)
myInter->functionally01();
This way you have just one access method (getInterface()) independent
of the number of objects.
In order to encode the access to the methods using a key you could
create a map which maps a key onto a closure or a simple switch statement:
in SomeInterface:
public:
void executeMethod(const int key)
{
switch(key)
{
case 1: functionally01(); break;
case 2: functionally01(); break;
default:
// error
}
int methodKey = ...;
int objectKey = ...;
SomeInterface *myInter = m_classA->getInterface(objectKey);
if (myInter)
myInter->executeMethod(methodKey);
Looks like a good case for a Mediator Pattern.
This pattern manage communication between 2 objects that he owns.
I have something like this:
class Base
{
public:
static int Lolz()
{
return 0;
}
};
class Child : public Base
{
public:
int nothing;
};
template <typename T>
int Produce()
{
return T::Lolz();
}
and
Produce<Base>();
Produce<Child>();
both return 0, which is of course correct, but unwanted. Is there anyway to enforce the explicit declaration of the Lolz() method in the second class, or maybe throwing an compile-time error when using Produce<Child>()?
Or is it bad OO design and I should do something completely different?
EDIT:
What I am basically trying to do, is to make something like this work:
Manager manager;
manager.RegisterProducer(&Woot::Produce, "Woot");
manager.RegisterProducer(&Goop::Produce, "Goop");
Object obj = manager.Produce("Woot");
or, more generally, an external abstract factory that doesn't know the types of objects it is producing, so that new types can be added without writing more code.
There are two ways to avoid it. Actually, it depends on what you want to say.
(1) Making Produce() as an interface of Base class.
template <typename T>
int Produce()
{
return T::Lolz();
}
class Base
{
friend int Produce<Base>();
protected:
static int Lolz()
{
return 0;
}
};
class Child : public Base
{
public:
int nothing;
};
int main(void)
{
Produce<Base>(); // Ok.
Produce<Child>(); // error :'Base::Lolz' : cannot access protected member declared in class 'Base'
}
(2) Using template specialization.
template <typename T>
int Produce()
{
return T::Lolz();
}
class Base
{
public:
static int Lolz()
{
return 0;
}
};
class Child : public Base
{
public:
int nothing;
};
template<>
int Produce<Child>()
{
throw std::bad_exception("oops!");
return 0;
}
int main(void)
{
Produce<Base>(); // Ok.
Produce<Child>(); // it will throw an exception!
}
There is no way to override a static method in a subclass, you can only hide it. Nor is there anything analogous to an abstract method that would force a subclass to provide a definition. If you really need different behaviour in different subclasses, then you should make Lolz() an instance method and override it as normal.
I suspect that you are treading close to a design problem here. One of the principals of object-oriented design is the substitution principal. It basically says that if B is a subclass of A, then it must be valid to use a B wherever you could use an A.
C++ doesn't support virtual static functions. Think about what the vtable would have to look like to support that and you'll realize its a no-go.
or maybe throwing a compile-time error when using Produce<Child>()
The modern-day solution for this is to use delete:
class Child : public Base
{
public:
int nothing;
static int Lolz() = delete;
};
It helps avoid a lot of boilerplate and express your intentions clearly.
As far as I understand your question, you want to disable static method from the parent class. You can do something like this in the derived class:
class Child : public Base
{
public:
int nothing;
private:
using Base::Lolz;
};
Now Child::Lolz becomes private.
But, of course, it's much better to fix the design :)
EDIT: minor fixes (virtual Print; return mpInstance) following remarks in the answers.
I am trying to create a system in which I can derive a Child class from any Base class, and its implementation should replace the implementation of the base class.
All the objects that create and use the base class objects shouldn't change the way they create or call an object, i.e. should continue calling BaseClass.Create() even when they actually create a Child class.
The Base classes know that they can be overridden, but they do not know the concrete classes that override them.
And I want the registration of all the the Child classes to be done just in one place.
Here is my implementation:
class CAbstractFactory
{
public:
virtual ~CAbstractFactory()=0;
};
template<typename Class>
class CRegisteredClassFactory: public CAbstractFactory
{
public:
~CRegisteredClassFactory(){};
Class* CreateAndGet()
{
pClass = new Class;
return pClass;
}
private:
Class* pClass;
};
// holds info about all the classes that were registered to be overridden
class CRegisteredClasses
{
public:
bool find(const string & sClassName);
CAbstractFactory* GetFactory(const string & sClassName)
{
return mRegisteredClasses[sClassName];
}
void RegisterClass(const string & sClassName, CAbstractFactory* pConcreteFactory);
private:
map<string, CAbstractFactory* > mRegisteredClasses;
};
// Here I hold the data about all the registered classes. I hold statically one object of this class.
// in this example I register a class CChildClass, which will override the implementation of CBaseClass,
// and a class CFooChildClass which will override CFooBaseClass
class RegistrationData
{
public:
void RegisterAll()
{
mRegisteredClasses.RegisterClass("CBaseClass", & mChildClassFactory);
mRegisteredClasses.RegisterClass("CFooBaseClass", & mFooChildClassFactory);
};
CRegisteredClasses* GetRegisteredClasses(){return &mRegisteredClasses;};
private:
CRegisteredClasses mRegisteredClasses;
CRegisteredClassFactory<CChildClass> mChildClassFactory;
CRegisteredClassFactory<CFooChildClass> mFooChildClassFactory;
};
static RegistrationData StaticRegistrationData;
// and here are the base class and the child class
// in the implementation of CBaseClass::Create I check, whether it should be overridden by another class.
class CBaseClass
{
public:
static CBaseClass* Create()
{
CRegisteredClasses* pRegisteredClasses = StaticRegistrationData.GetRegisteredClasses();
if (pRegisteredClasses->find("CBaseClass"))
{
CRegisteredClassFactory<CBaseClass>* pFac =
dynamic_cast<CRegisteredClassFactory<CBaseClass>* >(pRegisteredClasses->GetFactory("CBaseClass"));
mpInstance = pFac->CreateAndGet();
}
else
{
mpInstance = new CBaseClass;
}
return mpInstance;
}
virtual void Print(){cout << "Base" << endl;};
private:
static CBaseClass* mpInstance;
};
class CChildClass : public CBaseClass
{
public:
void Print(){cout << "Child" << endl;};
private:
};
Using this implementation, when I am doing this from some other class:
StaticRegistrationData.RegisterAll();
CBaseClass* b = CBaseClass::Create();
b.Print();
I expect to get "Child" in the output.
What do you think of this design? Did I complicate things too much and it can be done easier? And is it OK that I create a template that inherits from an abstract class?
I had to use dynamic_pointer (didn't compile otherwise) - is it a hint that something is wrong?
Thank you.
This sort of pattern is fairly common. I'm not a C++ expert but in Java you see this everywhere. The dynamic cast appears to be necessary because the compiler can't tell what kind of factory you've stored in the map. To my knowledge there isn't much you can do about that with the current design. It would help to know how these objects are meant to be used. Let me give you an example of how a similar task is accomplished in Java's database library (JDBC):
The system has a DriverManager which knows about JDBC drivers. The drivers have to be registered somehow (the details aren't important); once registered whenever you ask for a database connection you get a Connection object. Normally this object will be an OracleConnection or an MSSQLConnection or something similar, but the client code only sees "Connection". To get a Statement object you say connection.prepareStatement, which returns an object of type PreparedStatement; except that it's really an OraclePreparedStatement or MSSQLPreparedStatement. This is transparent to the client because the factory for Statements is in the Connection, and the factory for Connections is in the DriverManager.
If your classes are similarly related you may want to have a function that returns a specific type of class, much like DriverManager's getConnection method returns a Connection. No casting required.
The other approach you may want to consider is using a factory that has a factory-method for each specific class you need. Then you only need one factory-factory to get an instance of the Factory. Sample (sorry if this isn't proper C++):
class CClassFactory
{
public:
virtual CBaseClass* CreateBase() { return new CBaseClass(); }
virtual CFooBaseClass* CreateFoo() { return new CFooBaseClass();}
}
class CAImplClassFactory : public CClassFactory
{
public:
virtual CBaseClass* CreateBase() { return new CAImplBaseClass(); }
virtual CFooBaseClass* CreateFoo() { return new CAImplFooBaseClass();}
}
class CBImplClassFactory : public CClassFactory // only overrides one method
{
public:
virtual CBaseClass* CreateBase() { return new CBImplBaseClass(); }
}
As for the other comments criticizing the use of inheritance: in my opinion there is no difference between an interface and public inheritance; so go ahead and use classes instead of interfaces wherever it makes sense. Pure Interfaces may be more flexible in the long run but maybe not. Without more details about your class hierarchy it's impossible to say.
Usually, base class/ derived class pattern is used when you have an interface in base class, and that interface is implemented in derived class (IS-A relationship). In your case, the base class does not seem to have any connection with derived class - it may as well be void*.
If there is no connection between base class and derived class, why do you use inheritance? What is the benefit of having a factory if factory's output cannot be used in a general way? You have
class CAbstractFactory
{
public:
virtual ~CAbstractFactory()=0;
};
This is perfectly wrong. A factory has to manufacture something that can be used immediately:
class CAbstractFactory
{
public:
virtual ~CAbstractFactory(){};
public:
CBaseClass* CreateAndGet()
{
pClass = new Class;
return pClass;
}
private:
CBaseClass* pClass;
protected:
CBaseClass *create() = 0;
};
In general, you're mixing inheritance, virtual functions and templates the way they should not be mixed.
Without having read all of the code or gone into the details, it seems like you should've done the following:
make b of type CChildClass,
make CBaseClass::Print a virtual function.
Maybe I'm wrong but I didn't find any return statement in your CBaseClass::Create() method!
Personally, I think this design overuses inheritance.
"I am trying to create a system in which I can derive a Child class from any Base class, and its implementation should replace the implementation of the base class." - I don't know that IS-A relationships should be that flexible.
I wonder if you'd be better off using interfaces (pure virtual classes in C++) and mixin behavior. If I were writing it in Java I'd do this:
public interface Foo
{
void doSomething();
}
public class MixinDemo implements Foo
{
private Foo mixin;
public MixinDemo(Foo f)
{
this.mixin = f;
}
public void doSomething() { this.mixin.doSomething(); }
}
Now I can change the behavior as needed by changing the Foo implementation that I pass to the MixinDemo.