In c++, if a member pointer point to some data, how to protect that data from being modified? -

class { public:     a(){ val = 0; p = new int; *p = 0; }     //void fun_1()const{ val = 1; }  not allowed     void fun_2()const{ *p = 1; }     void display()const{ cout<< val <<' '<< *p <<endl; } private:     int val;     int * p; };  int main() {     const a;     a.fun_2(); } 

change member data in const member function fun_1()const not allowed. however, when data not directly member of object, allocated storage , assigned inside object, const function can't protect it. fun_2()const can change data p point although it's const function example.

is there way protect data p point ?

it's relatively straightforward cause compiler protect pointed-to object, not done automatically because isn't correct thing do.

template<typename t> class constinator_ptr {     t* p; public:     explicit constinator_ptr( t* p_init ) : p (p_init) {}      t*& ptr() { return p; } // use reassign, or define operator=(t*)      t* operator->() { return p; }     const t* operator->() const { return p; }     t& operator*() { return *p; }     const t& operator*() const { return *p; } }; 

just use in place of raw pointer, this:

class { public:     a() : val{0}, p{new int(0)} {}     //void fun_1()const{ val = 1; }  not allowed     void fun_2()const{ *p = 1; } // causes error     void display()const{ cout<< val <<' '<< *p <<endl; } private:     int val;     constinator_ptr<int> p; };