Encapsulation — C++ Tutorial

Access Control and Friend Functions

Encapsulation restricts direct access to class internals and provides controlled access through public methods. C++ also has the friend keyword for granting access.

Example

class Temperature {
private:
    double celsius;

public:
    Temperature(double c) : celsius(c) {
        if (c < -273.15) throw invalid_argument("Below absolute zero");
    }

    double getCelsius() const { return celsius; }
    double getFahrenheit() const { return celsius * 9.0 / 5.0 + 32; }
    double getKelvin() const { return celsius + 273.15; }

    void setCelsius(double c) {
        if (c < -273.15) throw invalid_argument("Below absolute zero");
        celsius = c;
    }

    // Friend function can access private members
    friend ostream& operator<<(ostream& os, const Temperature& t) {
        os << t.celsius << "°C";
        return os;
    }
};

Temperature t(100);
cout << t << endl;              // "100°C"
cout << t.getFahrenheit() << endl; // 212