This blog post explains the importance and use of the “C++ mutable keyword”. You will learn here when and how to use the mutable keyword in C++ programming with some example codes. But before going in depth-first we need to know that mutable is a storage class specifier,  like static, thread_local, and extern.

What is the mutable keyword in C++?

The mutable specifier can only be applied to a non-static data member whose type is neither const-qualified nor a reference type. If a data member is declared mutable, then it is legal to assign a value to this data member from a const member function. For example,

class Data
{
    mutable const int* ptr1; // OK

    mutable int* const ptr2; // ill-formed because ptr2 is const
};

When need to use the mutable keyword?

As we know that we can not modify data members of class/struct through the const method. Let’s see the example code, where I am modifying the value of data member “m_accessCount” in  GetData. The GetData is const method.

#include <iostream>
using namespace std;


class Data
{
public:
    Data():m_accessCount(0)
    {
    }
    int GetData() const  //const method
    {
        return (++m_accessCount);
    }
private:
    int m_accessCount; //data member
};


int main()
{
    Data obj;

    cout << obj.GetData()<<endl;

    return 0;
}

Output:

But sometimes we need to modify one or more data members of class/struct through the const method by preventing to modify other data members. This task can be easily performed by using the mutable keyword.

The mutable specifier on a class data member nullifies a const specifier applied to the containing class/struct object and permits modification of the mutable class member even though the rest of the object is const.

Let’s consider the same above discussed example but I am using a mutable keyword with m_accessCount.

#include <iostream>
using namespace std;


class Data
{
public:
    Data():m_accessCount(0)
    {
    }
    int GetData() const  //const method
    {
        return (++m_accessCount);
    }
private:
    mutable int m_accessCount; //data member with mutable
};


int main()
{
    Data obj;

    cout << obj.GetData()<<endl;

    return 0;
}

Output: 1

What will happens if try to modify a non-mutable data member with a const object?

Except that any class member declared mutable can be modified, any attempt to modify a const object during its lifetime results in undefined behavior. Let’s see some example codes to understand this statement.

Example 1:

In the below example, const object “y” tries to modify the mutable and non-mutable data member. We will get here compiler error because we can not modify the non-mutable data member with const object.

#include <iostream>
using namespace std;


class X
{
public:
    X() {}
    //mutable member
    mutable int i;
    int j;
};


class Y
{
public:
    //object of x
    X x;
    Y():x()
    {

    }
};


int main()
{
    const Y y;

    y.x.i++; // well-formed: mutable member can be modified

    y.x.j++; // ill-formed: const-qualified member modified

    return 0;
}

Output:

Example 2:

In the below example forcefully we are trying to change the value of non-mutable data members through a const object. In that situation, we will get undefined behavior. Let’s see the example code,

#include <iostream>
using namespace std;


class X
{
public:
    X() {}
    //mutable member
    mutable int i;
    int j;
};


class Y
{
public:
    //object of x
    X x;
    Y():x()
    {

    }
};


int main()
{
    const Y y;

    Y* p = const_cast<Y*>(&y); // cast away const-ness of y

    p->x.i = 99; // well-formed: mutable member can be modified

    p->x.j = 99; // undefined: modifies a const subobject

    return 0;
}

Output:

The behavior of the code will be undefined.

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