C++ - Calling virtual function from constructor

<!-- X-Subject-Header-End--> <!-- X-Head-of-Message-->

• From : Rob Newberry <robnewberry at grouplogic dot com>
• To : Eddy Ilg <eddy at fericom dot net>
• Cc : gcc-help at gcc dot gnu dot org
• Date : Thu, 22 Nov 2001 12:58:25 -0500 (EST)
• Subject : Re: C++ - Calling virtual function from constructor

<!-- X-Head-of-Message-End--><!-- X-Head-Body-Sep-Begin-->

---

<!-- X-Head-Body-Sep-End--><!-- X-Body-of-Message-->

The reason you don't call virtual methods inside a constructor is because
you don't know what kind of object you are -- while you're in the process
of being constructed, you're still in a _somewhat_ indeterminate state.
Only after you are constructed are you a real object.

Here's an example:

	class	a
	{
		public:
			a();

			virtual void v_meth();
	};

	class	b : public a
	{
		public:
			b();

			virtual void v_meth();
	};


	a::a()
	{
		// this will _always_ call a::v_meth, because
		// at this point, that's what we are -- even
		// if we're in the process of constructing a 
		// subclass of class a
		v_meth();
	}

	void a::v_meth()
	{
		printf("a::v_meth\n");
	}

	b::b()
	{
		// likewise, this will always call b::v_meth,
		// because at this point, that's what we are
		v_meth();
	}

	void b::v_meth()
	{
		printf("b::v_meth\n");
	}

	main()
	{
		printf("constructing an a:\n");
		a	an_a;

		printf("constructing a b:\n");
		b	a_b;
	}
	
The output of this, with gcc, is:

	constructing an a:
	a::v_meth
	constructing a b:
	a::v_meth
	b::v_meth

As you can see, when you are constructing the 'b' object a_b, the
constructor for 'a' and 'b' get called in succession.  But the constructor
for 'a' calls 'v_meth', it ONLY calls the 'a' method, because at that
point, that's all the object it is.  This might not be the behavior you
want to have, since you're constructing a 'b' object.  That's why the
compiler complains about you calling a virtual method in a constructor.

Rob

---------------------------------------------------------------------
Rob Newberry
Director of Fajita Technology
Group Logic, Inc.