Hi,
I've recently started studying the book "c++ template meta programming"
(I like it a lot!) and one of my own exercises was to get an integral
type's max/min value.
To get the type's maximal value I've created a metafunction max_val;
Compiling the code below with vc++2008 warning level 2+ yields the
warning "truncation of constant value" or "integral constant overflow"
if I use an int instead of a char.
<code>
namespace mpl = boost::mpl;
template<typename T>
struct significant_bits: mpl::int_
{};
template<typename T>
struct max_val_impl:
mpl::accumulate<
mpl::range_c,
mpl::integral_c,
mpl::next >
>
{};
template<typename T>
struct max_val: max_val_impl<T>::type
{
BOOST_MPL_ASSERT((boost::is_integral<T>));
};
int main()
{
const char maxval = max_val<char>::value;
return 0;
}
</code>
The problematic metafunction instantiation boils down to
mpl::integral_c
where
mpl::integral_c::next
results in 128 which is then truncated.
Of course I can turn off the compiler warnings but since the warning
happens in integral_wrapper.hpp and the integral constant stuff is
included in many other files including the boost type traits it would be
tedious to always remember to include integral_c.hpp explicitly before
any other headers with the warnings turned off.
I don't know whether there is a really satisfying option, just wanted to
discuss this point.
Klaus Triendl
