Created attachment 26090 [details]
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.