cv-qualifier ::= const | volatile
cv-qualifier-seq ::= const | volatile | const volatile | volatile const

The const keyword can be used as a qualifier when declaring objects, types, or member functions. When qualifying an object, using const means that the object cannot be the target of an assignment, and you cannot call any of its const member functions. When qualifying the target of a pointer, it means the destination cannot be modified. When member functions of a const object are called, this is a const pointer. When qualifying a member function, using the const qualifier means that within the member function, this is a const pointer, and the member function can be called for const objects. (Member functions without the const modifier cannot be called for a const object.)

When declaring pointers and references, be sure to distinguish between a const pointer (for which the pointer cannot be assigned, but what it points to can be modified) and a pointer to const (for which the pointer can be assigned, but what it points to cannot be modified).

Example

struct rect {
  rect(int x, int y) : x_(x), y_(y) {}
  void x(int x) { x_ = x; }
  void y(int y) { y_ = y; }
  int x(  ) const { return x_; }
  int y(  ) const { return y_; }
  int area(  ) const;
private:
  int x_, y_;
};
int rect::area() const { return x() * y(  ); }
const rect zero(0, 0);
const rect unit(1, 1);
rect p(4, 2), q(2, 4);
const rect* ptr1 = &zero;     // OK: pointer to const
ptr1->x(42);                  // Error: *ptr is const
ptr1 = &p;                    // Error: p is not const
ptr1 = &unit;                 // OK: unit is const
rect* const ptr2 = &p;        // OK: const pointer
ptr2 = &q;                    // Error: cannot set ptr2
ptr2->x(42);                  // OK: *ptr2 is not const

See Also

const_cast, declaration, mutable, type, volatile, Chapter 2