On Wed, Sep 26, 2012 at 12:00 PM, Lorenzo Caminiti <lorcaminiti@gmail.com> wrote:
However, N3351 says that axioms should not be checked by the compiler...
``Axioms express the semantics of a concept’s required syntax; they are assumed to be true and must not be checked by the compiler beyond conformance to the C++ syntax. Any additional checking is beyond the scope of the compiler’s translation requirements. The compiler must not generate code that evaluates axioms as preconditions, either. This could lead to program errors if the evaluated assertions have side effects. For example, asserting that distance(first, last) > 1 when the type of those iterators is istream_iterator will consume the first element of the range, causing the assertion to pass, but the algorithm to have undefined behavior.''
For now, I find this argument for not generating preconditions from axioms weak. How about you can only use constant expressions within axioms because axioms (as contracts and specifically preconditions) are not supposed to alter the program's stats? Then I can automatically generate preconditions from axioms and check them at run-time--why would this be a bad idea? Then you need to implement a constant-correct version of distance() in order to program the axiom but that's a good thing IMO... (it reflects the fact that axioms should not alter the program's state). In any case, I wanted to start a bit of a discussion about this topic in Boost. At the moment I'm still learning N3351 so too early for any design decision but I thing two things can potentially be done with axioms: 1. Check they syntactically compile. 2. Automatically generate preconditions form them that are checked at run-time. (Any more??) I'm keeping an open mind for now and considering both options. Thanks. --Lorenzo