From: Weiqi Gao <weiqigao AT a DOT crl DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: no copy-ctor for temps
Date: Fri, 01 Oct 1999 20:41:40 -0500
Organization: CRL Network Services
Lines: 64
Message-ID: <37F562D4.57291365@a.crl.com>
References: <eb8J3.1214$%K6 DOT 36 AT firefly>
NNTP-Posting-Host: a116022.stl1.as.crl.com
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
X-Mailer: Mozilla 4.51 [en] (X11; I; Linux 2.2.5-15 i586)
X-Accept-Language: en
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Reply-To: djgpp AT delorie DOT com

"Wendy E. McCaughrin" wrote:
> 
>  The following example shows that 'gxx' will not invoke a copy-ctor
>  for temporaries, only for variables.

There's no need to.

> #include <iostream.h>
> #include <string.h>
> 
>  class Overflow
>  { char mssg[80];
>    public:
>       Overflow( char* ccp ) { strcpy(mssg,ccp); }
>       Overflow( const Overflow& ovfl )   // must be 'const' !
>       { cerr << "copy ctor: ";  strcpy(mssg,ovfl.mssg); }
>       void Report() { cerr << mssg; }
>  };
> 
>  void TstCpy( Overflow );    // call by value
> 
>  int main()
>  { Overflow of = " I am a variable\n";
>    TstCpy(of);   // passing a variable: copy-ctor invoked
>    TstCpy(Overflow(" I am a temporary\n"));  // passing temp: no
>                                              // copy-ctor

The object is constructed on TstCpy()'s stack.  Remember that argument
passing is like initialization.  Therefore

  TstCpy(of);

is akin to

  Overflow temp = of;

which invokes the copy constructor, whereas

  TstCpy(Overflow(" I an a temporary\n"));

is akin to

  Overflow temp = Overflow(" I an a temporary\n");

which does not invoke the copy constructor, but merely constructs the
new object at the place where temp is allocated.

>    return 0;
>  }
> 
>  void TstCpy(Overflow ovrflw)
>  { ovrflw.Report(); }
> 
> 
>  When compiled and run, the output is:
> 
>  copy ctor: I am a variable  (indicating call of copy-ctor)
>  I am a temporary            (defaults to bit-wise copy)

It's not a bit-wise copy after all.

-- 
Weiqi Gao
weiqigao AT a DOT crl DOT com