The following code fails to compile under gcc 5.3 (it's a reduced version taken from a larger piece of 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 issue with the code using clang (I tested 3.8 on Linux, and 3.9 on OSX):
clang++ --std=c++1y --stdlib=libc++ -c rh.cpp
On linux, using clang + libstdc++ also fails.
The issue seems to come down to libstdc++ using an __gnu_cxx::__aligned_buffer
in its hash map implementation, which needs a complete type.
Both standard libraries work fine when a std::map is used, but that is not a solution I can accept. Nor is having the value type of the map be a pointer to Foo .
Is there any other change that I can make to have the code work on gcc/libstdc++ ?
Thanks!
I suggest you to modify Foo class in the next manner:
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>();
}
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