Why this code works? (reference to a class's private variable)

This is a discussion on Why this code works? (reference to a class's private variable) within the C++ Programming forums, part of the General Programming Boards category; Code: class B{ public: B(){} void check(const int& y){cout<<y<<endl;} }; class A{ int x; public: A(){x=1;} void foo(){B ob; ob.check(x);} ...

  1. #1
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    Why this code works? (reference to a class's private variable)

    Code:
    class B{                                                                                                                                                                     
    public:                                                                                                                                                                      
        B(){}                                                                                                                                                                    
        void check(const int& y){cout<<y<<endl;}                                                                                                                                 
    };                                                                                                                                                                           
                                                                                                                                                                                 
    class A{                                                                                                                                                                     
        int x;                                                                                                                                                                   
    public:                                                                                                                                                                      
        A(){x=1;}                                                                                                                                                                
        void foo(){B ob; ob.check(x);}                                                                                                                                           
    };                                                                                                                                                                           
                                                                                                                                                                                 
                                                                                                                                                                                 
    int main(){                                                                                                                                                                  
                                                                                                                                                                                 
        A oa;                                                                                                                                                                    
        oa.foo();                                                                                                                                                                
        return 0;                                                                                                                                                                
    }
    x is A's private member, how come B can refer to it (even it's a const) ?

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    B doesn't refer to A's private variable. All it sees is a parameter passed into a function. A can see its own private variable, and can choose to send it wherever it wants.

  3. #3
    C++ Witch laserlight's Avatar
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    x is A's private member, how come B can refer to it (even it's a const) ?
    You are passing an argument, so the B object never actually deals with the member variable named x of A, but its check member function deals with a local variable named y. It so happens that pass by reference is used, so y becomes an alias for x when ob.check(x) is called.
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  4. #4
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    OK, To make my question more clear, I modify the code:

    Code:
    class B{                                                                                                                                                                     
    public:                                                                                                                                                                      
        B(){}                                                                                                                                                                    
        void check( int& y){ y++;}                                                                                                                                               
    };                                                                                                                                                                           
                                                                                                                                                                                 
    class A{                                                                                                                                                                     
        int x;                                                                                                                                                                   
    public:                                                                                                                                                                      
        A(){x=1;}                                                                                                                                                                
        void foo(){                                                                                                                                                              
            cout<<x<<endl;                                                                                                                                                       
            B ob;                                                                                                                                                                
            ob.check(x);                                                                                                                                                         
            cout<<x<<endl;}                                                                                                                                                      
    };                                                                                                                                                                           
                                                                                                                                                                                 
                                                                                                                                                                                 
    int main(){                                                                                                                                                                  
                                                                                                                                                                                 
        A oa;                                                                                                                                                                    
        oa.foo();                                                                                                                                                                
        return 0;                                                                                                                                                                
    }
    Now you see before( and after) the call, A's private member variable actually was changed.

  5. #5
    and the Hat of Guessing tabstop's Avatar
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    Well, apparently class A wanted it changed, since it sent the variable off to a function pass-by-reference. The responsibility for changing the value rests right there, not in class B.

  6. #6
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    Thanks. But x is A's private variable, how would it be changed by a non-friend function?

    Quote Originally Posted by tabstop View Post
    Well, apparently class A wanted it changed, since it sent the variable off to a function pass-by-reference. The responsibility for changing the value rests right there, not in class B.

  7. #7
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    If you pass a value by reference it does NOT matter who owns it, what class it is in. If you pass a variable by reference it is able to be changed by the function regardless of the ownership.

  8. #8
    Captain Crash brewbuck's Avatar
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    "Private" means "Nobody can see this name," not "Nobody can touch this variable."

  9. #9
    C++ Witch laserlight's Avatar
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    But x is A's private variable, how would it be changed by a non-friend function?
    I think you have a fundamental misunderstanding of scope and access control.

    Let's ask a few questions and see how you answer them:
    Code:
    void foo(int& x)
    {
        x = 0;
    }
    
    void bar()
    {
        int y = 1;
        foo(y);
    }
    
    class Z
    {
    private:
        void baz()
        {
            int z = 2;
            foo(z);
        }
    };
    1. Is there a local variable named y in foo(int&)?
    2. Is there a local variable named z in foo(int&)?
    3. Remove the call of foo(y) in bar(). Now, without making any further modification to bar(), can you modify foo(int&) such that it changes the value of y in bar()?
    4. Remove the call of foo(z) in Z::baz(). Now, without making any further modification to Z::baz(), can you modify foo(int&) such that it changes the value of z in Z::baz()?

    The answer to all four questions is no. So, foo(int&) does not have access to y or z. In particular, foo(int&) does not have access to z, but due to pass by reference its parameter x can be an alias (another name) for z if Z::baz() calls foo(z). So, the private access is respected since only a member function (or friend) of Z can access z.
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  10. #10
    and the hat of sweating
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    Your ATM card and pin are private to you.
    If you choose to give me your card and pin #, what's to stop me from emptying your bank account?

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