diff -wbBdu -Nrp -U 7 boost/mpl/string.hpp new/mpl/string.hpp
--- boost/mpl/string.hpp	2014-05-01 04:07:13.047597593 -0700
+++ new/mpl/string.hpp	2014-05-01 04:09:07.288597424 -0700
@@ -585,23 +585,36 @@ namespace boost { namespace mpl
         typedef                                                                                     \
             typename mpl::aux_::next_unless<BOOST_PP_CAT(i, n), iend>::type                         \
         BOOST_PP_CAT(i, BOOST_PP_INC(n));
         BOOST_PP_REPEAT(BOOST_MPL_LIMIT_STRING_SIZE, M0, ~)
         #undef M0
 
         typedef c_str type;
+#if defined(BOOST_NO_CXX11_CONSTEXPR)
         static typename Sequence::value_type const value[BOOST_MPL_LIMIT_STRING_SIZE+1];
+#else
+        constexpr static typename Sequence::value_type const value[BOOST_MPL_LIMIT_STRING_SIZE+1] = 
+        {
+            #define M0(z, n, data)                                                                      \
+            mpl::aux_::deref_unless<BOOST_PP_CAT(i, n), iend>::type::value,
+            BOOST_PP_REPEAT(BOOST_MPL_LIMIT_STRING_SIZE, M0, ~)
+            #undef M0
+            '\0'
+        };
+#endif
     };
 
+#if defined(BOOST_NO_CXX11_CONSTEXPR)
     template<typename Sequence>
     typename Sequence::value_type const c_str<Sequence>::value[BOOST_MPL_LIMIT_STRING_SIZE+1] =
     {
         #define M0(z, n, data)                                                                      \
         mpl::aux_::deref_unless<BOOST_PP_CAT(i, n), iend>::type::value,
         BOOST_PP_REPEAT(BOOST_MPL_LIMIT_STRING_SIZE, M0, ~)
         #undef M0
         '\0'
     };
+#endif
 
 }} // namespace boost
 
 #endif // BOOST_MPL_STRING_HPP_INCLUDED