访问从一个类到另一个与 C++ (OOP) 中的前一个类无关的私有字段

Question

I have 2 classes - A and B . Class A has a member list<int> and a method addData() that adds a number to this list. The second class B has a function displayContent() - it iterates through the list of the class A and prints its content (the added numbers). The problem I face is that I cannot access the private (cannot be changed to public) field list of class A from the newly instantiated object (from class B ). I would like to ask how can this be achieved with getters/setters , or a copy constructor ?

Answer 1

In displayContent_a, your identified 'a' is undefined, because within that function or class you have never declared a variable 'a'. A simple solution is to pass an object of type A to the function which determines what A object to display the list of:

void displayContent_a(A& a) {
        //this uses the a from the parameter
        for (auto x = a.a.begin(); x != a.a.end(); x++) { //use a.a because we want to access the list 
            cout << *x << endl;
        }

    }

However, this won't be enough, as B can't access private data of A. To fix this, you have to make B a friend of A. This can be done in the following way:

class B; //forward declare B so it can be used in friend declarations in A

class A
{
//... your content

    friend class B;
}

Answer 2

In good code classes are never entangled. This means that if class B uses class A than class A shouldn't have any knowledge about B . So adding a friendship is just a workaround which makes code entangled.

It is hard to say what is best approach without knowing more context, but there are couple possibilities.

You can add such API to A:

class A {
public:
    void forEach(std::function<void(int)> f) const {
        for(auto x : mData) {
            f(x);
        }
    }

private:
    list<int> mData;
};

class B {
public:
    void displayContent(std::ostream &out) {
        a.forEach([&out](int x){ out << x<< ", "; });
    }

private:
    A a;
};

Advantage is that you can change lots of thing in A (for example change list to vector) and you do not have to fix anything in B .

There are more possibilities. Simple getter could solve issue:

class A {
public:
    list<int> getData() const {
        return mData;
    }

private:
    list<int> mData;
};

Answer 3

To access variable 'a' of class A, it is not possible with current implementation.

Firstly, make class B as the derived class of base class A

Even then, it is not possible to access the private variable of class A from anywhere outside even from derived class.

Try to implement by declaring variable as protected.

Another implementation can be by declaring class B as friend of class A. That way, private variables can be accessed from class B

Answer 4

A getter is a member function which allows the member attribute List<int> L to be read from outside of the object. In class A define:

public:
   List<int> getList(){
      return L;
     } 

Now you can add In Class B:

A a(constructor args);
List<int>=a.getList();

in the same measure you can use setters to change the value of private attributes eg

 public:
    void setList(List<int> data){
        L=data;
       }