Why can a const member function modify a static data member?

Question

In the following C++ program, modifying a static data member from a const function is working fine:

class A  {   public:        static int a; // static data member      void set() const     {         a = 10;     } }; 

But modifying a non-static data member from a const function does not work:

class A  {   public:        int a; // non-static data member      void set() const     {         a = 10;     } }; 

Why can a const member function modify a static data member?

Answer 1

It's the rule, that's all. And for good reason.

The const qualifier on a member function means that you cannot modify non-mutable non-static class member variables.

By way of offering some rationalisation, the this pointer in a const qualified member function is a const type, and this is inherently related to an instance of a class. static members are not related to a class instance. You don't need an instance to modify a static member: you can do it, in your case, by writing A::a = 10;.

So, in your first case, think of a = 10; as shorthand for A::a = 10; and in the second case, think of it as shorthand for this->a = 10;, which is not compilable since the type of this is const A*.

Answer 2

According to the C++ Standard (9.2.3.2 Static data members)

1 A static data member is not part of the subobjects of a class...

And (9.2.2.1 The this pointer)

1 In the body of a non-static (9.2.1) member function, the keyword this is a prvalue expression whose value is the address of the object for which the function is called. The type of this in a member function of a class X is X*. If the member function is declared const, the type of this is const X*,...

And at last (9.2.2 Non-static member functions)

3 ... if name lookup (3.4) resolves the name in the id-expression to a non-static non-type member of some class C, and if either the id-expression is potentially evaluated or C is X or a base class of X, the id-expression is transformed into a class member access expression (5.2.5) using (*this) (9.2.2.1) as the postfix-expression to the left of the . operator.

Thus in this class definition

class A  {   public:        static int a;       void set() const     {         a = 10;     } }; 

the static data member a is not a subobject of an object of the class type and the pointer this is not used to access the static data member. So any member function, non-static constant or non-constant, or a static member function can change the data member because it is not a constant.

In this class definition

class A  {   public:        int a;       void set() const     {         a = 10;     } }; 

the non-static data member a is an subobject of an object of the class type. To access it in a member function there is used either a member access syntax of this syntax is implied. You may not use a constant pointer this to modify the data member. And the pointer this is indeed has type const A * within the function set because the function is declared with the qualifier const. If the function had no the qualifier in this case the data member could be changed.