Re: [boost] Compile-time assert

From: John Maddock (john_at_[hidden])
Date: 2006-11-08 04:53:39

Johan Paulsson wrote:
> I wrote:
>> Below is the compile-time assert I use.
>> Would it be a useful addition to Boost?
>
> To make it easier to digest, I put together a minimal web page with
> - Source code
> - A test
> - Compiler errors from the test, using different compilers (It's
> supposed to make compiler errors). Compiler errors from the same test
> using BOOST_STATIC_ASSERT, BOOST_MPL_ASSERT and BOOST_MPL_ASSERT_MSG
> is
> also included, to make it easy to compare.
>
> I think the benefits of my compile time assert is:
> - No macros
> - No compiler-specific implementation
> - It's easy to make useful error-messages
>
> Please have a look at <http://www.kuodo.com/code/assert/readme.html>
>
> Or download it (12kb) <http://www.kuodo.com/code/assert/assert.zip>

Frankly it's hard to choose between them on error message quality (which is 
to say none are all that good).

One of the requirements that came up for static asserts when 
BOOST_STATIC_ASSERT was talked about was a declaration like syntax, in fact 
both:

static_assert<condition> x;

and

typedef static_assert<condition> x;

were explicitly rejected at the time.

I also seem to remember that some compilers at the time evaluated typedefs 
in a lazy manner: if the typedef was unused then it was never evaluated and 
the static-assertion was never triggered.  That's the reason for some of the 
convolutions inside static_assert.hpp.  I suspect, however, that current 
compilers have moved away from such "lazyness".

And finally, I hope that native static assertions will make it into 
compilers soon (they're in the C++ std working draft), when that happens it 
would be trivial to redefine macro based solutions to use the native 
version: and that really will produce good error messages one hopes!

John.