Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!elroy.jpl.nasa.gov!decwrl!world!wmm
From: wmm@world.std.com (William M Miller)
Newsgroups: comp.lang.c++
Subject: Re: Incorrect handling of destructors in Turbo C++
Keywords: Turbo C++, destructors
Message-ID: <1991Mar25.034657.20995@world.std.com>
Date: 25 Mar 91 03:46:57 GMT
References: <546@humu.NOSC.Mil>
Organization: The World Public Access UNIX, Brookline, MA
Lines: 36

kent@humu.nosc.mil (Kent K. Kuriyama) writes:
> void main()
> {
>   A a;
>
>   printf("before call\n");
>   sub(a);
>   printf("after call\n");
> }
> ------------
>
> When this program is executed:
>
> ------------
> Constructor called
> before call
> in sub
> after call
> Destructor called
> Destructor called
> ------------
>
> Am I wrong to think that the destructor should only be called once?

Yes.  C++ has, by default, the call-by-value semantics of C.  When you
called "sub(a)", an object was created to hold the copy of "a" that was
actually passed to "sub()."  Since you didn't provide a constructor that
could be used for this purpose (it's called a "copy constructor" and is
declared as "A::A(const A&)"), the compiler generated a default one for you
that just did a bit-for-bit copy.  Naturally, the default one didn't have a
printf() call in it, so its execution didn't show up in your output.  Of the
two destructors, one is for the copy passed to "sub()," the other is for
"a."

-- William M. Miller, Glockenspiel, Ltd.
   wmm@world.std.com