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  • INTERVIEW QUESTIONS LANGUAGES C++ DETAILS
    Question :
    mutable" Keyword - What is "mutable"?


    Category C++ Interview Questions
    Rating (0.4) By 490 users
    Added on 7/16/2010
    Views 1412
    Rate it!
    Answers:

    Answer1.
    "mutable" is a C++ keyword. When we declare const, none of its data members can change. When we want one of its members to change, we declare it as mutable.

    Answer2.
    A "mutable" keyword is useful when we want to force a "logical const" data member to have its value modified. A logical const can happen when we declare a data member as non-const, but we have a const member function attempting to modify that data member. For example:

    class Dummy {
    public:
    bool isValid() const;
    private:
    mutable int size_ = 0;
    mutable bool validStatus_ = FALSE;
    // logical const issue resolved
    };

    bool Dummy::isValid() const
    // data members become bitwise const
    {
    if (size > 10) {
    validStatus_ = TRUE; // fine to assign
    size = 0; // fine to assign
    }
    }


    Answer3.
    "mutable" keyword in C++ is used to specify that the member may be updated or modified even if it is member of constant object. Example:

    class Animal {
    private:
    string name;
    string food;
    mutable int age;
    public:
    void set_age(int a);
    };

    void main() {
    const Animal Tiger(’Fulffy’,'antelope’,1);
    Tiger.set_age(2);
    // the age can be changed since its mutable
    }



    This keyword can only be applied to non-static and non-const data members of a class. If a data member is declared mutable, then it is legal to assign a value to this data member from a const member function.

    // mutable.cpp
    class X
    {
    public:
    bool GetFlag() const
    {
    m_accessCount++;
    return m_flag;
    }
    private:
    bool m_flag;
    mutable int m_accessCount;
    };

    int main()
    {
    }



    Posted by: prasanthkumar.s    

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