Path: utzoo!utgpu!watserv1!watmath!att!dptg!ulysses!andante!alice!ark
From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c++
Subject: Re: constructor called within constructor???
Message-ID: <11017@alice.UUCP>
Date: 5 Jul 90 15:00:50 GMT
References: <965@unet.UUCP>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 62

In article <965@unet.UUCP>, noemi@sugarfoot.net.com (Me) writes:

> I'm trying to call a constructor routine (1) from withing another
> constructor routine (2).  Essentially, I want constructor 2 to do
> exactly what constructor 1 does, plus one extra thing:
> 
> 	public:
> 		classname();		/* constructor 1 */
> 		classname(int arg);	/* constructor 2 */

> 	/* constructor 1 */
> 	classname:classname()
> 	{
> 		/* lots of things */
> 	}

> 	/* constructor 2 */
> 	classname:classname(int arg)
> 	{
> 		classname();		/* call constructor 1 */
> 		privatevar = arg;	/* "one extra thing" */
> 	}

Of course this doesn't work: the call

	classname();

means `please construct an object of type `classname' using its
empty constructor and then throw it away.'

In fact, there is no way to call a constructor explicitly (yes, I
know this statement is not completely true, but it is true for
this purpose); the way to do what you want is to reorganize your
program slightly:

	public:
		classname();		// constructor 1
		classname(int);		// constructor 2
	private:
		void build();		// do the real work


	classname::classname()		// constructor1
	{
		build();
	}

	classname::classname(int arg)	// constructor2
	{
		build();
		privatevar = arg;
	}

	void classname::build()
	{
		// the stuff that was in constructor1
	}

If you're worried about overhead, make the `build' function inline.
-- 
				--Andrew Koenig
				  ark@europa.att.com