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5. How is called the usage of an object of a class as a field of another class?

Posted on 2015-01-23
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Last Modified: 2015-01-24
How is called the usage of an object of a class as a field of another class?
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Question by:Nusrat Nuriyev
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5 Comments
 
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Expert Comment

by:jkr
ID: 40567467
You basically call that a class member, nothing else. E.g.

class A {

// ...
};

class B {


// ...

public:

  A myA; // here, an instance of 'class A' is a member of every instace of 'class B'
};

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Author Comment

by:Nusrat Nuriyev
ID: 40567613
Sorry, I mean this question:
5. How the usage of an object of a class as a field of another class is named?

Probably, this question deals with keyword friend, I have read about that but, don't have enough knowledges.
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LVL 86

Expert Comment

by:jkr
ID: 40567626
Ahh, I see. The C++ 'friend' relationship allos the classes that are declared as 'friend' to access all data in the declaring class. There's the catch phrase "In C++, your friends have access to your privates" which humorously illustrates that. You can also declare single functions or operators as friends, which sometimes  is neccessary when you e.g. want to sort a sequence based on a protected or private value. Wikipedia outlines that nicely: http://en.wikipedia.org/wiki/Friend_class
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Author Comment

by:Nusrat Nuriyev
ID: 40568086
#include <iostream>

using namespace std;

class Temperature
{
    private:
        double kelvin;
        double celcius;
        double fahrenheit;
    public:
        // Required constructors
        Temperature()
        {
            kelvin = 273.15;
            celcius = 0;
            fahrenheit = 32;
        }
        Temperature(double k, double c, double f)
        {
            kelvin = k;
            celcius = c;
            fahrenheit = f;
        }

        friend ostream& operator<<(ostream &output, const Temperature &T)
        {
            output << "K: " << T.kelvin << " C: " << T.celcius << " F: " << T.fahrenheit;
            return output;

        }

        friend istream& operator>>(istream &input, Temperature &T)
        {
            input >> T.kelvin >> T.celcius >> T.fahrenheit;
            return input;
        }

};

int main()
{
    Temperature t1;
    cin >> t1;
    cout << t1;
    return 0;
}

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Could you explaing why it will won't work without friend keyword even if Temperature class data members are public?
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LVL 86

Accepted Solution

by:
jkr earned 500 total points
ID: 40568180
That's because this example is badly grouped, making the impresion that these operators are in fact class members, whereas they are in fact global operators that aren't members of the class. Took me a while to spot that, You can see that both operators take a 'Temperature&' argument, which would not be necessary if they were members. That example should better be written like

#include <iostream>

using namespace std;

class Temperature
{
        friend ostream& operator<<(ostream&, const Temperature&);
        friend istream& operator>>(istream&, Temperature&);
    private:
        double kelvin;
        double celcius;
        double fahrenheit;
    public:
        // Required constructors
        Temperature()
        {
            kelvin = 273.15;
            celcius = 0;
            fahrenheit = 32;
        }
        Temperature(double k, double c, double f)
        {
            kelvin = k;
            celcius = c;
            fahrenheit = f;
        }
};

ostream& operator<<(ostream &output, const Temperature &T)
{
    output << "K: " << T.kelvin << " C: " << T.celcius << " F: " << T.fahrenheit;
    return output;
}

istream& operator>>(istream &input, Temperature &T)
{
    input >> T.kelvin >> T.celcius >> T.fahrenheit;
    return input;
}

int main()
{
    Temperature t1;
    cin >> t1;
    cout << t1;
    return 0;
}
                                          

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