Xref: utzoo comp.lang.c++:12124 comp.std.c++:713
Path: utzoo!news-server.csri.toronto.edu!rutgers!usc!samsung!uunet!pdn!tscs!tct!chip
From: chip@tct.uucp (Chip Salzenberg)
Newsgroups: comp.lang.c++,comp.std.c++
Subject: Re: distinguishing operator[] on left and right
Message-ID: <27D7F747.39E@tct.uucp>
Date: 8 Mar 91 20:42:46 GMT
References: <1991Mar2.000705.3496@mathcs.sjsu.edu> <1991Mar2.212017.13885@world.std.com> <1991Mar7.062634.7598@am.dsir.govt.nz>
Organization: Teltronics/TCT, Sarasota, FL
Lines: 36

According to robert@am.dsir.govt.nz (Robert Davies):
>I suggested having two versions of the operator[]
>      char &operator[](int i)
>      char operator[](int i) const

I've done this.

>      char c = ((const String)g)[3];

Perhaps you meant

       char c = ((const String &)g)[3];

That's less likely to create a temporary.

>I suggest that it would be reasonable for the second version to be the
>default if the compiler can tell that the operation won't affect the value
>of g.

There's the rub.  How's the compiler supposed to know that?  We may
know from reading the class definition what's meant, but the compiler
hasn't got a prayer at figuring out when to call the const function
even though you're not operating on a const object.

A workaround would be to create a const reference to the object in
question, and use it for access:

     String s;
     String &r = s;
     char c = s[0];    // slow
     char d = r[0];    // fast

-- 
Chip Salzenberg at Teltronics/TCT     <chip@tct.uucp>, <uunet!pdn!tct!chip>
 "Most of my code is written by myself.  That is why so little gets done."
                 -- Herman "HLLs will never fly" Rubin