Newsgroups: comp.lang.c++
Path: utzoo!utgpu!cunews!csi.uottawa.ca!news
From: hitz@sim5.csi.uottawa.ca (Martin Hitz)
Subject: Re: Constructor question
Message-ID: <1991Apr12.150506.4069@csi.uottawa.ca>
Summary: Is expensive
Sender: news@csi.uottawa.ca
Nntp-Posting-Host: sim5
Organization: University of Ottawa
References: <1991Apr2.110623.22219@and.cs.liv.ac.uk> <20164@alice.att.com> <17400@sunquest.UUCP>
Date: Fri, 12 Apr 91 15:05:06 GMT

In article <17400@sunquest.UUCP> francis@sunquest.UUCP (Francis Sullivan) writes:
>>> Can I call one constructor to class X explicitly from within another
>>> constructor to class X ?
>
>Yes, by using operator=
>
>#include <stdio.h>
>
>class X {
> int j;
>
>public:
> X(int i) : j(i) { printf("Called X(int=%d): j=%d\n", i, j); }      //1
> X(char *cp, int i) {
>    *this = X(i);
>    printf("Called X(char *cp= '%s', int=%d): j=%d\n", cp, i, j);   //2
> }
>};
>
>main() {
> X x("test", 5);
>}

Although this works in principle, one should be aware of the fact
that X(i) on the right hand side of the assignment usually generates
a temporary anonymous instance of X which is copied to the lhs and
then destroyed. So this might be an expensive way of doing initialization.
However, some compilers might optimize it. Exchanging the lines //1 and //2
by

 X(int i) : j(i) { printf("Called X(int=%d): j=%d this=%ld\n", i, j, this); }//1
 printf("Called X(char *cp= '%s', int=%d): j=%d this=%ld\n", cp, i, j, this);//2

yields

	Called X(int=5): j=5 this=84813140
	Called X(char *cp= 'test', int=5): j=5 this=84813136
	
using Zortech (2 distinct objects are involved) but results in

	Called X(int=5): j=5 this=-134218552
	Called X(char *cp= 'test', int=5): j=5 this=-134218552

using g++ (only 1 object).

Martin Hitz (hitz@csi.UOttawa.CA)