From -7728577387036781133 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,ec4b60e80d49f4fe X-Google-Attributes: gidf78e5,public From: Valentin Bonnard Subject: Re: Constant expression (Was: Re: Loop invariance) Date: 1998/08/06 Message-ID: <35CA360C.3EF7@pratique.fr>#1/1 X-Deja-AN: 378667718 X-NNTP-Posting-Host: cyber31-9.paris.imaginet.fr Content-Transfer-Encoding: 7bit Approved: stephen.clamage@sun.com (comp.std.c++) References: <35C1FD0B.6EF2@bean.jpl.nasa.gov> <35C67E63.69530B1C@ix.netcom.com> <6q7q3c$omn$1@nnrp1.dejanews.com> <35C8A8B3.77BC@pratique.fr> <35C8C39D.41C6@wizard.net> To: James Kuyper X-UID: 0000000001 X-Status: $$$T Content-Type: text/plain; charset=us-ascii Organization: Ecole Normale Superieure, Paris, France Mime-Version: 1.0 Reply-To: bonnardv@pratique.fr Newsgroups: comp.std.c++ Originator: clamage@taumet James Kuyper writes: > Valentin Bonnard wrote: > > > > AllanW@my-dejanews.com wrote: > > > > > Five years ago, I would write code like this: > > > const int days_in_Q1 = 91; // 31+28+31; > > > instead of: > > > const int days_in_Q1 = 31+28+31; > > > because I knew that some compilers would do the addition at runtime. > > > > A conforming C++ compiler is required to compute the sum. > > Computation of integral constant expressions can't be > > defered to runtime. But there is no such requirement in > > the following example: > > > > const float sum = 2. + 4.; > > Citations please? How could a conforming program tell? It can't. Explanation follows. > The standard does > a very precise definition of tranlation phases, but seldom explicitly > distiguishes run-time from compile-time. The principle of the least punishement: an implementor doesn't make extra efforts (takes extra work/ressources) to punish his customers. For example, if I hear someone saying that 'identifiers with the letter "j" in them should be avoided because a compiler could slow down accesses to such identifiers', I will answer that it's wrong under the least punishement principle: it would require extra work to add this slowdown (the compiler would test if the letter j is used, then punish the programmer with the slowdown). Does this explanation satisfies you ? -- Valentin Bonnard mailto:bonnardv@pratique.fr info about C++/a propos du C++: http://pages.pratique.fr/~bonnardv/ [ 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 ]