It's a long story, but I happen to be trying my hand at writing a
"safe bit flags" class for C++11 and it would be extremely helpful to
be able to do compile-time value checks on constructor arguments. It
seems like this should be possible via 'constexpr' constructors that
use static_assert inside the constructor body, but it doesn't work
with either g++ 4.6 or 4.7. Here's an example:
$ cat test.cc
// intent is for K instances to be initializable only with an integer
// less than or equal to std::numeric_limits<int>::digits
#include <limits>
struct K
{
K() = delete;
K(const K&) = default;
constexpr K(int s) : value(s == 0 ? 0 : 1 << (s-1))
{ static_assert(s <= std::numeric_limits<unsigned int>::digits, "overflow"); }
const unsigned int value;
};
const K good(1); // should compile
const K bad(999); // should not compile
$ g++-4.6 -std=c++0x -fsyntax-only test.cc
test.cc: In constructor âconstexpr K::K(int)â:
test.cc:10:5: error: non-constant condition for static assertion
test.cc:10:5: error: âsâ is not a constant expression
Same errors with g++ 4.7. If I stick 'constexpr' on the constructor
argument as well, I get either a parse error (if 'constexpr' is before
'int') or 'error: a parameter cannot be declared "constexpr"' (if
'constexpr' is after 'int'). (Has C++11 broken the syntax invariant
that specifiers and qualifiers can go either before or after the base
type? Feh.)