compile time asserts [preconditions]

From: John Maddock (John_Maddock_at_[hidden])
Date: 2000-07-30 05:24:11

Hey guys,

Thanks for all the comments on this - unfortunately I posted this mail to
the wrong address yesterday (sorry Steve), so this now may be a little out
of date.  If you want I'm happy to change the name to static_assert or
whatever; anyway here goes again:

Thanks, I obviously should have logged on yesterday, to pick up the debate,
instead I was also working on the compile time assert package - hey ho.

With regard to naming, I chose BOOST_PRECONDITION(x) - the intent is to say
"look guys if this isn't true then don't even bother compiling this code",
for the class which generates the error message I used:
COMPILE_TIME_PRECONDITION_FAILED, which should catch the eye: note that
these names are not set in stone, it's almost trivial to change them if
there is something clearer around (I don't like postulate though - sorry!).

The docs are preliminary, but should give the general idea, along with some
reasonably useful examples.

Someone asked about usage at namespace scope in a header - this can produce
conflicts and this is covered in the docs along with a recomendation to
insert the preconditions into their own namespace so that the scope of the
declarations are unique to that header.

Finally with regard to template instantiation - regardless of whether an
enum or typedef based approach is used, the declarations should be
instantiated at the same time as the template regardless of whether they
are used - both typedefs and enum's are descibed as declarations in the
standard (not definitions), and it's only definitions which are
instantiated on demand, all declarations should be instantiated with the
template (anyhow that's my reading of it).  However since Beman has
encountered problems with unused declarations not being instantiated, I've
put both versions in the header; the default is to use an enum, but
defining BOOST_USE_TYPEDEF_PRECONDITION will switch over to the typedef
version - it's there in case a particular compiler has a problem with the
enum version.

Anyway its all in the vault under "precondition".

- John.