Re: Compile time error testing

From:

Carl Barron <cbarron413@adelphia.net>

Newsgroups:

comp.lang.c++.moderated

Date:

5 Jun 2006 19:29:58 -0400

Message-ID:

<050620061615301709%cbarron413@adelphia.net>

In article <1149463680.429963.146110@i39g2000cwa.googlegroups.com>,
<glaird@pacifier.com> wrote:

I seem to remember that some sort of C++ language compile time testing
facility exists--though I can't seem to find anything about it in the
C++ docs or my compiler docs (Borland C++ Builder).

Given the following;

struct ST{
   char ch1[2];
   char ch2[100];
   int ii;
};

I seem to recall that one could somehow write something like:

if(sizeof(ST) != 106)error("Message");

and have the expression evaluated at compile time and flag an error if
needed.

Am I recalling something from another lifetime or maybe a future
lifetime--or does this exist now?

  boost's mpl library has compile time asserts
  #include <boost/mpl/assert.hpp>
  struct ST { /* as above */};

  BOOST_MPL_ASSERT_RELATION(sizeof(ST),==,106);

  btw its false on my machine, [needs to pad between the ch2 and ii]

   a simple approach is:
    template <bool B> struct ST_size_error;
    template <> struct ST_size_error<true> {typedef char type;};

    ST_size_error<sizeof(ST)==106>::type error_check;
    should produce a fairly readable error message if sizeof(ST) != 106.

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