Mutable keyword in C++

Mutable data members are those whose values can be changed in runtime even if the object’s type is constant. It is the polar opposite of constant.

It is sometimes necessary to modify one or more data members of a class or struct using a const function, even if you do not want the function to update other members of the class or struct. The mutable keyword makes it simple to complete this task.

Example:

#include <iostream>
using namespace std;
class Coderz {
   public:
      int a;
   mutable int b;
   Coderz(int x=0, int y=0) {
      a=x;
      b=y;
   }
   void seta(int x=0) {
      a = x;
   }
   void setb(int y=0) {
      b = y;
   }
   void disp() {
      cout<<endl<<"a: "<<a<<" b: "<<b<<endl;
   }
};
int main() {
   const Coderz t(10,20);
   cout<<t.a<<" "<<t.b<<"\n";
   // t.a=30; //Error occurs because a can not be changed, because object is constant.
   t.b=100; //b still can be changed, because b is mutable.
   cout<<t.a<<" "<<t.b<<"\n";
   return 0;
}

Output:

10 20
10 100

If we uncomment the commented statements, then an error occurs, i.e,

/tmp/V7dAvdWEIa.cpp: In function 'int main()':
/tmp/V7dAvdWEIa.cpp:24:7: error: assignment of member 'Coderz::a' in read-only object
   24 |    t.a=30; //Error occurs because a can not be changed, because object is constant.
      |    ~~~^~~

Note: also read about const keyword in C++

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