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    copied!<p><code>std::numeric_limits&lt;int&gt;::is_modulo</code> is a compile time constant. so you can use it for template specialization. problem solved, at least if compiler plays along with inlining.</p> <pre><code>#include &lt;limits&gt; #include &lt;stdexcept&gt; #include &lt;string&gt; #ifdef TESTING_SF bool const testing_sf = true; #else bool const testing_sf = false; #endif // C++ "extensions" namespace cppx { using std::runtime_error; using std::string; inline bool hopefully( bool const c ) { return c; } inline bool throw_x( string const&amp; s ) { throw runtime_error( s ); } } // namespace cppx // C++ "portability perversions" namespace cppp { using cppx::hopefully; using cppx::throw_x; using std::numeric_limits; namespace detail { template&lt; bool isTwosComplement &gt; int signed_from( unsigned const n ) { if( n &lt;= unsigned( numeric_limits&lt;int&gt;::max() ) ) { return static_cast&lt;int&gt;( n ); } unsigned const u_max = unsigned( -1 ); unsigned const u_half = u_max/2 + 1; if( n == u_half ) { throw_x( "signed_from: unsupported value (negative max)" ); } int const i_quarter = static_cast&lt;int&gt;( u_half/2 ); int const int_n1 = static_cast&lt;int&gt;( n - u_half ); int const int_n2 = int_n1 - i_quarter; int const int_n3 = int_n2 - i_quarter; hopefully( n == static_cast&lt;unsigned&gt;( int_n3 ) ) || throw_x( "signed_from: range error" ); return int_n3; } template&lt;&gt; inline int signed_from&lt;true&gt;( unsigned const n ) { return static_cast&lt;int&gt;( n ); } } // namespace detail inline int signed_from( unsigned const n ) { bool const is_modulo = numeric_limits&lt; int &gt;::is_modulo; return detail::signed_from&lt; is_modulo &amp;&amp; !testing_sf &gt;( n ); } } // namespace cppp #include &lt;iostream&gt; using namespace std; int main() { int const x = cppp::signed_from( -42u ); wcout &lt;&lt; x &lt;&lt; endl; } </code></pre> <p><hr> <strong>EDIT</strong>: Fixed up code to avoid possible trap on non-modular-int machines (only one is known to exist, namely the archaically configured versions of the Unisys Clearpath). For simplicity this is done by not supporting the value -2<sup><i>n</i>-1</sup> where <i>n</i> is the number of <code>int</code> value bits, on such machine (i.e., on the Clearpath). in practice this value will not be supported by the machine either (i.e., with sign-and-magnitude or 1&rsquo;s complement representation).</p>
 

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