C++ language some live examples for mutable...

Question

Respected sir ,

I need some help for mutable keyword it is used in a const function and please any body explain for the live example about the mutable and constant function and also diff. for the volatile member and function please help me

in Advance Thank you,

Answer 1

You can use mutable for variables that are allowed to be modified in const object instances. This is called logical constness (opposed to bitwise constness) as the object has not changed from the user's point of view.

You can for example cache the length of a string to increase performance.

class MyString
{
public:
...

const size_t getLength() const
{
    if(!m_isLenghtCached)
    {
         m_length = doGetLength();
         m_isLengthCached = true;
    }

    return m_length;    
}

private:
sizet_t doGetLength() const { /*...*/ }
mutable size_t m_length;
mutable bool m_isLengthCached;
};

Answer 2

You can use mutable on a counter tracking the number of time a Class member is accessed through a const accessor.

Answer 3

I used it once to implement memoization.