Re: [Boost-users] [mpl] integral constant overflow

From: David Abrahams (dave_at_[hidden])
Date: 2008-03-11 19:35:19

on Sat Feb 16 2008, klaus triendl <klaus-AT-triendl.eu> wrote:

> 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_<std::numeric_limits<T>::digits>
> {};
>
> template<typename T>
> struct max_val_impl:
>      mpl::accumulate<
>          mpl::range_c<int, 1, significant_bits<T>::value>,
>          mpl::integral_c<T, 1>,
>          mpl::next<mpl::plus<mpl::_1, mpl::_1> >
>      >
> {};
>
> 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<char, 127>
> where
> mpl::integral_c<char, 127>::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.

Hi Klaus,

It's undefined behavior to overflow a signed integral type in C++.  The
only solution is to "not do that."  

HTH,

-- 
Dave Abrahams
Boost Consulting
http://boost-consulting.com