CODEWITHSUNDEEP
c++
Adrian
Adrian

Reputation: 20068

C++ OOP quiz question

I'm doing a C++ quiz and I need to say whats wrong in the following code:

class Base {
    friend class SubClass;
    int n;
    virtual int getN()
    {
        return n;
    }
};

class SubClass: public Base {
public:
    SubClass() {}
    SubClass(const SubClass& s) {}
};

int main()
{
    SubClass sc;
    if(sc.getN() <= 5)
    {
        int x = sc.getN();
    }
    return 0;
}

Besides that n is uninitilized and maybe the object should be created through a Base class pointer, what else could be wrong?
When I run it I get this error:

 error: 'virtual int Base::getN()' is private

Upvotes: 6

Views: 1878

Answers (6)

omar al zaben
omar al zaben

Reputation: 1

you can not access to getN() before make it as public

Upvotes: 0

Sarfaraz Nawaz
Sarfaraz Nawaz

Reputation: 361462

As by default every member of a class1 is private, getN in the base class is declared private.

Make getN public as:

class Base {
    friend class SubClass;
    int n;
public: //<--------------------- you forgot this
    virtual int getN()
    {
        return n;
    }
};

1. I mean, a class defined with the keyword class. Note that class can be defined with the keyword struct and union as well, according to the C++ Standard.

EDIT:

If you think because SubClass is a friend of Base, so it can access private members of Base from outside, then that is wrong. friend means member functions of SubClass can access private members of Base class.

However, if you make main() friend of Base, then your code would work:

 class Base {
        friend int main(); //make main() friend of Base
        //...
    };

Now from main(), any private members of Base can be accessed!

See this demo : http://www.ideone.com/UKkCF

Upvotes: 15

johnsyweb
johnsyweb

Reputation: 141810

Your compiler should give you some clues...

% g++ -Wall -Wextra -Wshadow -Weffc++ test.cpp
test.cpp:1: warning: ‘class Base’ has virtual functions but non-virtual destructor
test.cpp:10: warning: ‘class SubClass’ has virtual functions but non-virtual destructor
test.cpp: In constructor ‘Base::Base()’:
test.cpp:1: warning: ‘Base::n’ should be initialized in the member initialization list
test.cpp: In constructor ‘SubClass::SubClass()’:
test.cpp:12: note: synthesized method ‘Base::Base()’ first required here 
test.cpp: In copy constructor ‘SubClass::SubClass(const SubClass&)’:
test.cpp:13: warning: base class ‘class Base’ should be explicitly initialized in the copy constructor
test.cpp: At global scope:
test.cpp:13: warning: unused parameter ‘s’
test.cpp: In function ‘int main()’:
test.cpp:4: error: ‘virtual int Base::getN()’ is private
test.cpp:19: error: within this context
test.cpp:4: error: ‘virtual int Base::getN()’ is private
test.cpp:21: error: within this context
test.cpp:21: warning: unused variable ‘x’

Upvotes: 4

relaxxx
relaxxx

Reputation: 7824

class Base {
    friend class SubClass;
    int n;
public:
    virtual int getN()
    {
        return n;
    }
};

class members are private as default

Upvotes: 2

Jesse Emond
Jesse Emond

Reputation: 7490

the function 

virtual int getN() { /*...*/ }

is private(by default) within your Base class. Since it's not overloaded within you SubClass, it stays private when you call it within your main function.

If you want to make it legal, make it public within the class:

public:
    virtual int getN() { /*...*/ }

Upvotes: 2

Alexander Gessler
Alexander Gessler

Reputation: 46607

The error message says it all: getN is implicitly private, which is the default for classes in C++.

Use

class Base {
 // ...

public:
   virtual int getN();
};

to make it a publicly accessible member. The facts that SubClass is a friend of Base and that getN is called through an instance of Subclass don't matter here - getN is simply not accessible from within main(), where it is called from. You could make main a friend of Base, however.

Upvotes: 3

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