From 3633163082817280918 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,ec4b60e80d49f4fe X-Google-Attributes: gidf78e5,public From: Jerry Leichter Subject: Re: Constant expression (Was: Re: Loop invariance) Date: 1998/08/06 Message-ID: <35C9D918.63C5@smarts.com>#1/1 X-Deja-AN: 378580556 X-NNTP-Posting-Host: fine.smarts.com 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: System Management ARTS Mime-Version: 1.0 Newsgroups: comp.std.c++ Originator: clamage@taumet | > > const int days_in_Q1 = 31+28+31; | > | > 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? For what, exactly? That integer constant expressions must be evaluated at compile time, one can tell through templates: template class A { ... }; Then A<3> and A<1+2> must be the same type. Also, after const int three = 4 - 1; A must also be the same type. ('three' is an integral constant expression since it is a const variable of integral type that is initialized to an integral constant expression.) Since template parameters can't be floats, you can't make the same statement for float constant expressions. The definition of integral constant expression (5.9) allows floating point *literals* to appear if they are cast to an integral or enumeration type; but something like int(1.0 + 3.0) isn't required to be accepted; and if you do something odd like: const int Three = 1.0 + 2.0; then 'Three' is not an integral constant expression, since its initializing value is not (and hence A is illegal). Hence, a C++ compiler must be able to perform integer arithmetic on constants; and it must be able to convert a float literal to an integer; and it must do such arithmetic and conversions in template instantia- tion. In other contexts, I'll quite agree that no standard-conforming program can tell whether 'three' is evaluated at compile time, once at run- time; once every time the variable is used (other than in a template instantiation); or at some arbitrary combination of these. Since the evaluation of constant expressions can't have side-effects, a compiler could choose to recompute them between every pair of instructions it generated, and you couldn't tell. -- Jerry [ 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 ]