From:	DavMac AT iname DOT com (Davin McCall)
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: no copy-ctor for temps
Date:	Sat, 02 Oct 1999 01:53:32 GMT
Organization:	Monash Uni
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On Fri, 01 Oct 1999 19:45:14 GMT, wemccaug AT prairienet DOT org (Wendy E.
McCaughrin) wrote:

> The following example shows that 'gxx' will not invoke a copy-ctor
> for temporaries, only for variables.

Are you saying that's bad?

The code you point out as not calling the copy-ctor instantiates a
temporary instance of the class as a function parameter. The reason
that the copy-ctor is not called is because no copy operation is
performed - the temporary object is created directly on the stack
before the function is called.

The alternative, to construct the object and then copy it to the
stack, would be less efficient.

I am aware that this sort of thing can get confusing - I've been
caught out by things like this myself.

Davin.



>
>#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
>   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)

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