http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51558
Bug #: 51558 Summary: Declaration of unspecialized std::hash<_Tp>::operator()(_Tp) turns compile-time errors into link-time errors Classification: Unclassified Product: gcc Version: 4.7.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: libstdc++ AssignedTo: unassig...@gcc.gnu.org ReportedBy: jyass...@gcc.gnu.org Created attachment 26090 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=26090 Declare std::hash<T> instead of defining it. libstdc++'s current definition of the unspecialized std::hash template gives bad error messages when a user forgets to define a hash function for their type. Specifically, providing a declaration but no definition of operator() moves the error from compile to link time: $ cat test.cc #include <unordered_set> struct MyStruct { int i; }; bool operator==(const MyStruct& lhs, const MyStruct& rhs) { return lhs.i == rhs.i; } int main() { std::unordered_set<MyStruct> s; s.insert(MyStruct{3}); } $ g++ -g -std=c++11 test.cc -o test /tmp/cclzhwaU.o: In function `std::__detail::_Hash_code_base<MyStruct, MyStruct, std::_Identity<MyStruct>, std::equal_to<MyStruct>, std::hash<MyStruct>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, true>::_M_hash_code(MyStruct const&) const': .../include/c++/4.7.0/bits/hashtable_policy.h:702: undefined reference to `std::hash<MyStruct>::operator()(MyStruct) const' collect2: error: ld returned 1 exit status With the attached patch, the error is much more informative, if not exactly concise: $ g++ -g -std=c++11 test.cc -o test In file included from .../include/c++/4.7.0/bits/hashtable.h:35:0, from .../include/c++/4.7.0/unordered_set:45, from test.cc:1: .../include/c++/4.7.0/bits/hashtable_policy.h: In instantiation of ‘struct std::__detail::_Hash_code_base<MyStruct, MyStruct, std::_Identity<MyStruct>, std::equal_to<MyStruct>, std::hash<MyStruct>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, true>’: .../include/c++/4.7.0/bits/hashtable.h:149:11: required from ‘class std::_Hashtable<MyStruct, MyStruct, std::allocator<MyStruct>, std::_Identity<MyStruct>, std::equal_to<MyStruct>, std::hash<MyStruct>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, true, true, true>’ .../include/c++/4.7.0/bits/unordered_set.h:46:11: required from ‘class std::__unordered_set<MyStruct, std::hash<MyStruct>, std::equal_to<MyStruct>, std::allocator<MyStruct>, true>’ .../include/c++/4.7.0/bits/unordered_set.h:277:11: required from ‘class std::unordered_set<MyStruct>’ test.cc:9:32: required from here .../include/c++/4.7.0/bits/hashtable_policy.h:740:20: error: ‘std::__detail::_Hash_code_base<_Key, _Value, _ExtractKey, _Equal, _H1, _H2, std::__detail::_Default_ranged_hash, true>::_M_h1’ has incomplete type ... In particular, the "required from here" line points at the actual source location that needs to be able to find the definition. I believe the patch is allowed by C++11 since I can't find a specification of the contents of the unspecialized template, and libc++ uses basically the same technique: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/__functional_base?view=markup. Another way to accomplish something similar would be to delete the operator() declaration from the std::hash<T> definition. I believe that's not as good because it produces error messages saying that std::hash lacks an operator() rather than that the template has incomplete type. Better yet would be finding a way to include "please specialize me" in the error message.