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* [Bug libstdc++/51558] New: Declaration of unspecialized std::hash<_Tp>::operator()(_Tp) turns compile-time errors into link-time errors
@ 2011-12-14 22:33 jyasskin at gcc dot gnu.org
  2011-12-14 22:59 ` [Bug libstdc++/51558] " redi at gcc dot gnu.org
                   ` (8 more replies)
  0 siblings, 9 replies; 10+ messages in thread
From: jyasskin at gcc dot gnu.org @ 2011-12-14 22:33 UTC (permalink / raw)
  To: gcc-bugs

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: unassigned@gcc.gnu.org
        ReportedBy: jyasskin@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.


^ permalink raw reply	[flat|nested] 10+ messages in thread
end of thread, other threads:[~2011-12-15 22:16 UTC | newest]

Thread overview: 10+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2011-12-14 22:33 [Bug libstdc++/51558] New: Declaration of unspecialized std::hash<_Tp>::operator()(_Tp) turns compile-time errors into link-time errors jyasskin at gcc dot gnu.org
2011-12-14 22:59 ` [Bug libstdc++/51558] " redi at gcc dot gnu.org
2011-12-15  1:42 ` paolo.carlini at oracle dot com
2011-12-15  1:47 ` paolo.carlini at oracle dot com
2011-12-15  2:04 ` redi at gcc dot gnu.org
2011-12-15  2:25 ` paolo.carlini at oracle dot com
2011-12-15  4:49 ` paolo.carlini at oracle dot com
2011-12-15 10:54 ` paolo.carlini at oracle dot com
2011-12-15 22:16 ` paolo at gcc dot gnu.org
2011-12-15 22:30 ` paolo.carlini at oracle dot com

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