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
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To:	djgpp AT delorie DOT com
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"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

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