Re: & operator (in parameters) and const

From:

=?iso-8859-1?Q?Kim Gr=e4sman?= <kim@mvps.org>

Newsgroups:

microsoft.public.vc.language

Date:

Sat, 29 Apr 2006 07:44:23 +0000 (UTC)

Message-ID:

<453b99c6889c58c83988a180693c@news.microsoft.com>

Hi Jack,

#ifdef _DEBUG
virtual void AssertValid() const;
virtual void Dump(CDumpContext& dc) const;
#endif
Could anyone explain with examples how to use (like the above code
fragment)
the & operator and const inside the prototype? (I know & is return by
value
and const is making the function immutable?)

I'm not entirely sure I understand what you mean, but here are some examples
of how to use references and const in methods.

class Person
{
private:
   int m_age;

public:
   Person(int age) : m_age(age)
   {
   }

   void setAge(int age)
   {
      m_age = age;
   }

   int getAge() const
   {
      return m_age;
   }
};

void GrowOlder(Person& person)
{
   person.setAge(person.getAge() + 1);
}

int main()
{
   Person p1(10);
   GrowOlder(p1);
   // p1.m_age is now 11

   const Person p2(100);
   p2.getAge(); // OK, because getAge is const
   p2.setAge(99); // Error, because setAge is non-const and p2 is const
}

So, marking a method as const makes it possible for clients to call it on
const instances of the class. The compiler also checks that a const method
does not modify the state of the object, so you can't write to m_age from
getAge() in the example above.

References are used to let a function modify one of its arguments. By making
the person argument a reference in GrowOlder above, we can modify the actual
person passed in at the call site.

Hope that helps.

--
Best Regards,
Kim Gr?sman