From 6509750638859412349 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,c0e61b6a667823c0 X-Google-Attributes: gidf78e5,public From: Bruce Visscher Subject: Re: no const operator[] in STL map? Date: 1999/01/25 Message-ID: <36ABD140.DAC0F0AA@mindspring.com>#1/1 X-Deja-AN: 436520948 Content-Transfer-Encoding: 7bit Approved: Fergus Henderson References: <916993358.766341@aloomba.aaii.oz.au> X-Original-Date: Sun, 24 Jan 1999 21:04:48 -0500 X-Accept-Language: en X-Server-Date: 25 Jan 1999 02:00:44 GMT Content-Type: text/plain; charset=us-ascii X-Complaints-To: news@news.unimelb.edu.au X-Trace: izvestia.its.unimelb.edu.au 917231670 9740 128.250.29.16 (25 Jan 1999 02:34:30 GMT) Organization: MindSpring Enterprises X-Auth: PGPMoose V1.1 PGP comp.std.c++ iQBFAgUANqvYHeEDnX0m9pzZAQFlJwGAjPBEIjJuS1C26xajkeQS3kNxrn6agG9f wVONWZMnrjymKahf8pZZ+snuEjzZCFbn =mvye Mime-Version: 1.0 NNTP-Posting-Date: 25 Jan 1999 02:34:30 GMT Newsgroups: comp.std.c++ Siemel Naran wrote: > On 22 Jan 1999 15:51:17 GMT, Anthony Shipman wrote: > > >The version of STL supplied for IRIX 6.5 does not have the > > const T& operator[](const key_type&) const > >operator that is listed in the STL tutorial by Musser and Saini. > > > >Has it been removed from the standard or is it a fault with the SGI STL? > > No, the standard does not have such an operator. > I wish it did. Me too. > [snip] > > But there is a logical alternative if const operator[] can't > find the object with key_type 'key' in the map. Just throw an > exception. > I agree that it would have been nice had the standard provided a const operator[]. However, I don't agree that it should throw. To me there is an obvious alternative: return by value rather than by reference and return mapped_type() if the key is not found. One useful application for map suggested in [Musser & Sauni] is that of a sparse array. With a const overload implemented like this you would get consistent behavior (m[x] returns mapped_type() if x is not found whether or not m is const). In fact, the const overload would be the one to use if wherever possible for a sparse vector. You wouldn't want to allocate 10000000 elements if you did: struct T {/*...*/}; map const& g(); void f(map const&); const map& m=g(); for(int i=0; i<10000000; ++i) f(m);. > There is already a precedent for such behaviour. There is precedence for my suggestion too (typing the following in from my pdf copy, I hope I don't make any mistakes): 26.3.2 Template class valarray [...] template class valarray { [...] // 26.3.2.3 element access: T operator[](size_t) const; T& operator[](size_t); [...] It's not that unusual in my experience to see non-const overload of operator[] return by reference while the const overload returns by value. This seems to bother some, though, so you could instead return by reference to const and statically allocate a default constructed mapped_type when needed instead (i.e., the first time map::operator[] const was invoked for a non-existant key). Bruce Visscher --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://reality.sgi.com/austern_mti/std-c++/faq.html ]