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Question

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?

Answer 1

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(); }

Answer 2

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.

Answer 3

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 eg 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);

Answer 4

Looks like a good case for a Mediator Pattern.

This pattern manage communication between 2 objects that he owns.