CODEWITHSUNDEEP
c++oopfriend
Jiahao Cai
Jiahao Cai

Reputation: 1250

How to access the class member

Sorry for the bad title... I think the solution may exist on this site, but I cannot find it.

class A {
};

class B {
private:
    int _b;
};

class C {
private:
    A a; // a: I want to access _b in b
    B b;
};

Let's say I have 3 classes like the code above, now the object a in class C wants to access the member _b in object b. Is there any method to achieve it?

I tried using friend, I wonder if I haven't used it in a proper way, because it made the code very complex, like this:

class B {
private:
    int _b;
public:
    B() : _b(5) {}
    int get_b() {
        return _b;
    }
};

class A {
public:
    int get_a(B& b) {
        cout << b.get_b();
    }
};

class C {
private:
    friend class A;
    A a;
    B b;
public:
    A& get_A() {
        return a;
    }
    B& get_B() {
        return b;
    }
};

int main() {
    C c;
    c.get_A().get_a(c.get_B());
}

Thank you in advance.

EDIT

Sorry about the confusing code above, actually I want to implement a compiler using OO style. I think a compiler is made of lexer, parser, and symbol table and other things. So I think the relationship is:

class compiler {
private:
    lexer l;
    parser p;
    symbol_table st;
...
};

And the parser and lexer need to access the symbol_table, that is why this question is put forward. I think this design is resemble to the real compiler "in my opinion", but it seems hard to implement... Any advice is appreciated.

Upvotes: 1

Views: 80

Answers (2)

adamfowlerphoto
adamfowlerphoto

Reputation: 2751

Your code looks really confused. I'm not sure what you are trying to do in the second piece of code. The only place you can enable access to members of class B is in class B. You can do this either through making those members public or protected, adding accessor functions for the members or setting another class to be a friend class of class B by adding a friend class declaration inside class B.

Personally I would go with accessor functions as this means you are reducing class to class dependencies. Any class that has internal access to class B, by being made a friend or having direct access to member variables because they are public, is then dependent on the structure of class B.

Upvotes: 0

skypjack
skypjack

Reputation: 50550

In you example, A must be a friend of B and not of C to let it access members of B:

class A;

class B {
    friend class A;
    //...
};

// ....

In other terms, it should look like this:

#include<iostream>

using namespace std;

class A;

class B {
    friend class A;
private:
    int _b;
public:
    B() : _b(5) {}
};

class A {
public:
    int get_a(B& b) {
        cout << b._b;
    }
};

class C {
private:
    A a;
    B b;
public:
    A& get_A() {
        return a;
    }
    B& get_B() {
        return b;
    }
};

int main() {
    C c;
    c.get_A().get_a(c.get_B());
}

That being said, you should probably reconsider your design to avoid such a strong dependency between your classes and get rid of friends.

Upvotes: 1

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