C++ : How to ensure that a class member variable is modifiable only within a certain method

Question

I am using C++ 14 with clang on MacOS Sierra. I want to enforce a rule by design. Following is the rule.

I have a member variable in my class say:

unsigned int m_important_num;

There are 4 methods in my class.

fun1();
fun2();
fun3();
fun4();

Objective:
I want only fun2() to be able to change the value of m_important_num.

Question:
Is it possible to make it compiler error if any method other than fun2() changes the variable? One possible way is to declare it const somehow empower fun2() to change const variables? Is this a good solution? Or are their any better solutions?

Secondary question:
Is it a wrong design to try do such a thing?

Answer 1

Sort of, with additional layer:

class S1 {
public:
    void fun2() { /*Modify m_important_num */ }
    unsigned int getImportantNum() const { return m_important_num;}
private:
    unsigned int m_important_num;
};
class S2 : private S1
{
public:
    void fun1();
    using S1::fun2; // or void fun2() {S1::fun2();}
    void fun3();
    void fun4();
};

As Yakk commented, if func2 need access to S2 members, CRTP can solve that:

template <typename Derived>
class S1 {
public:
    void fun2() { asDerived().foo3(); /*Modify m_important_num */ }
    unsigned int getImportantNum() const { return m_important_num;}
private:
    Derived& asDerived() { return stataic_cast<Derived&>(*this); }
private:
    unsigned int m_important_num;
};
class S2 : private S1<S2>
{
    // friend class S1<S2>; // If required.
public:
    void fun1();
    using S1::fun2; // or void fun2() {S1::fun2();}
    void fun3();
    void fun4();
};