Self-referencing using unordered_map causes problems for gcc 5.3 but not clang

Question

The following code does not compile under gcc 5.3 (this is a smaller version taken from most of the code):

#include <unordered_map> #include <string> class Foo { std::unordered_map<std::string, Foo> m; //"self-referential" }; int main() { Foo f; return 0; } 

with the following errors:

 g++ --std=c++1y -c rh.cpp In file included from /usr/local/include/c++/5.3.0/utility:70:0, from /usr/local/include/c++/5.3.0/unordered_map:38, from rh.cpp:1: /usr/local/include/c++/5.3.0/bits/stl_pair.h: In instantiation of 'struct std::pair<const int, Foo>': /usr/local/include/c++/5.3.0/ext/aligned_buffer.h:85:34: required from 'struct __gnu_cxx::__aligned_buffer<std::pair<const int, Foo> >' /usr/local/include/c++/5.3.0/bits/hashtable_policy.h:246:43: required from 'struct std::__detail::_Hash_node_value_base<std::pair<const int, Foo> >' /usr/local/include/c++/5.3.0/bits/hashtable_policy.h:292:12: required from 'struct std::__detail::_Hash_node<std::pair<const int, Foo>, false>' /usr/local/include/c++/5.3.0/bits/hashtable_policy.h:1896:60: required from 'struct std::__detail::_Hashtable_alloc<std::allocator<std::__detail::_Hash_node<std::pair<const int, Foo>, false> > >' /usr/local/include/c++/5.3.0/bits/hashtable.h:170:11: required from 'class std::_Hashtable<int, std::pair<const int, Foo>, std::allocator<std::pair<const int, Foo> >, std::__detail::_Select1st, std::equal_to<int>, std::hash<int>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<false, false, true> >' /usr/local/include/c++/5.3.0/bits/unordered_map.h:101:18: required from 'class std::unordered_map<int, Foo>' rh.cpp:4:32: required from here /usr/local/include/c++/5.3.0/bits/stl_pair.h:102:11: error: 'std::pair<_T1, _T2>::second' has incomplete type _T2 second; /// @c second is a copy of the second object ^ rh.cpp:3:7: note: forward declaration of 'class Foo' class Foo { 

There is no problem with code using clang (I tested 3.8 on Linux and 3.9 on OSX):

clang++ --std=c++1y --stdlib=libc++ -c rh.cpp

Linux also uses clang + libstdC ++.

The problem seems to boil down to libstdc ++, using __gnu_cxx::__aligned_buffer in its hash map implementation, which needs a full type.

Both standard libraries work fine when using std :: map, but this is not a solution I can make. Also, the value type of the map is not a pointer to Foo.

Are there any other changes I can make for code to work in gcc / libstdc ++?

Thanks!

Answer 1

I suggest you change the Foo class as follows:

 class Foo { public: Foo(); private: using MapType = std::unordered_map<std::string, Foo>; std::shared_ptr<MapType> m_ptr; }; // Here Foo is already defined. Foo::Foo() { m_ptr = std::make_shared<MapType>(); } 

This code is compiled using clang 3.9 and gcc 6.2 .