12 Classes [class]

12.2 Class members [class.mem]

12.2.2 Non-static member functions [class.mfct.non-static]

12.2.2.1 The this pointer [class.this]

In the body of a non-static 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*, if the member function is declared volatile, the type of this is volatile X*, and if the member function is declared const volatile, the type of this is const volatile X*.
[Note
:
Thus in a const member function, the object for which the function is called is accessed through a const access path.
end note
]
[Example
:
struct s {
  int a;
  int f() const;
  int g() { return a++; }
  int h() const { return a++; } // error
};

int s::f() const { return a; }
The a++ in the body of s​::​h is ill-formed because it tries to modify (a part of) the object for which s​::​h() is called.
This is not allowed in a const member function because this is a pointer to const; that is, *this has const type.
end example
]
Similarly, volatile semantics apply in volatile member functions when accessing the object and its non-static data members.
A cv-qualified member function can be called on an object-expression only if the object-expression is as cv-qualified or less-cv-qualified than the member function.
[Example
:
void k(s& x, const s& y) {
  x.f();
  x.g();
  y.f();
  y.g();                        // error
}
The call y.g() is ill-formed because y is const and s​::​g() is a non-const member function, that is, s​::​g() is less-qualified than the object-expression y.
end example
]
Constructors and destructors shall not be declared const, volatile or const volatile.
[Note
:
However, these functions can be invoked to create and destroy objects with cv-qualified types, see [class.ctor] and [class.dtor].
end note
]