https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69261
Bug ID: 69261 Summary: Copying char arrays during constexpr evaluation does not work reliably Product: gcc Version: 5.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: jens.auer at cgi dot com Target Milestone: --- Created attachment 37331 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=37331&action=edit Preprocessed source code I was playing around with string processing for constexpr values, and wrote the following program to concatenate two strings at compile time: #include <cstdint> #include <tuple> #include <iostream> using std::size_t; template<size_t N> struct string_constexpr { constexpr string_constexpr() = default; template<size_t M> constexpr string_constexpr( char const (&d)[M] ): data{0} { static_assert( M <= N, "size!" ); for(size_t i=0; i != M; i++) { data[i] = d[i]; } } char data[N]; }; template<typename T, size_t N> T& operator<<(T& stream, string_constexpr<N> const& str) { return (stream << str.data); } template<int N> constexpr string_constexpr<N> s( char const (&d)[N] ) { string_constexpr<N> c{}; for(size_t i=0; i != N; i++) { c.data[i] = d[i]; } return c; } template<size_t N, size_t M> constexpr auto concat(string_constexpr<N> const& s1, string_constexpr<M> const& s2) { string_constexpr<N+M-1> s( s1.data ); for(size_t i=0; i != M; i++) { s.data[N+i-1] = s2.data[i]; } return s; } template<size_t N, size_t M> constexpr auto concat(char const (&x)[N], char const (&y)[M]) { string_constexpr<N+M-1> tmp{x}; for(size_t i=0; i != M; i++) { tmp.data[N+i-1] = y[i]; } return tmp; } void foo() { auto constexpr s1 = s( "bla" ); auto constexpr s2 = s( "blub" ); string_constexpr<8> constexpr s1s2 = concat(s1,s2); auto constexpr c = concat("bla", "blub"); std::cout << s1.data << std::endl << s2.data << std::endl << s1s2.data << std::endl << c << std::endl; } int main() { foo(); return 0; } Executables compiled with GCC and clang produce different outputs, and I think clang is right: $ g++ -std=c++1y static_strings.cpp $ ./a.out bla blub blablub $ clang++ -std=c++1y static_strings.cpp $ ./a.out bla blub blablub blablub