#include <iostream>#include <unordered_map>#include <string>struct tree_node { // tree_node() : attrib_val{"null"} {} std::unordered_map<std::string, tree_node> child;};int main(int argc, char const *argv[]){ return 0;}This code compiles just fine on my mac with clang:
$ g++ --versionConfigured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include/c++/4.2.1Apple LLVM version 10.0.1 (clang-1001.0.46.4)Target: x86_64-apple-darwin18.7.0Thread model: posixInstalledDir: /Library/Developer/CommandLineTools/usr/bin$ g++ -std=c++11 test.cpp$ On my linux machine, with gcc 9.1.0, I get the following error:
In file included from /usr/um/gcc-9.1.0/include/c++/9.1.0/bits/stl_algobase.h:64, from /usr/um/gcc-9.1.0/include/c++/9.1.0/bits/char_traits.h:39, from /usr/um/gcc-9.1.0/include/c++/9.1.0/ios:40, from /usr/um/gcc-9.1.0/include/c++/9.1.0/ostream:38, from /usr/um/gcc-9.1.0/include/c++/9.1.0/iostream:39, from test.cpp:1:/usr/um/gcc-9.1.0/include/c++/9.1.0/bits/stl_pair.h: In instantiation of ‘struct std::pair<const std::__cxx11::basic_string<char>, tree_node>’:/usr/um/gcc-9.1.0/include/c++/9.1.0/ext/aligned_buffer.h:91:28: required from ‘struct __gnu_cxx::__aligned_buffer<std::pair<const std::__cxx11::basic_string<char>, tree_node> >’/usr/um/gcc-9.1.0/include/c++/9.1.0/bits/hashtable_policy.h:233:43: required from ‘struct std::__detail::_Hash_node_value_base<std::pair<const std::__cxx11::basic_string<char>, tree_node> >’/usr/um/gcc-9.1.0/include/c++/9.1.0/bits/hashtable_policy.h:264:12: required from ‘struct std::__detail::_Hash_node<std::pair<const std::__cxx11::basic_string<char>, tree_node>, true>’/usr/um/gcc-9.1.0/include/c++/9.1.0/bits/hashtable_policy.h:2016:13: required from ‘struct std::__detail::_Hashtable_alloc<std::allocator<std::__detail::_Hash_node<std::pair<const std::__cxx11::basic_string<char>, tree_node>, true> > >’/usr/um/gcc-9.1.0/include/c++/9.1.0/bits/hashtable.h:173:11: required from ‘class std::_Hashtable<std::__cxx11::basic_string<char>, std::pair<const std::__cxx11::basic_string<char>, tree_node>, std::allocator<std::pair<const std::__cxx11::basic_string<char>, tree_node> >, std::__detail::_Select1st, std::equal_to<std::__cxx11::basic_string<char> >, std::hash<std::__cxx11::basic_string<char> >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, false, true> >’/usr/um/gcc-9.1.0/include/c++/9.1.0/bits/unordered_map.h:105:18: required from ‘class std::unordered_map<std::__cxx11::basic_string<char>, tree_node>’test.cpp:7:46: required from here/usr/um/gcc-9.1.0/include/c++/9.1.0/bits/stl_pair.h:215:11: error: ‘std::pair<_T1, _T2>::second’ has incomplete type 215 | _T2 second; /// @c second is a copy of the second object | ^~~~~~test.cpp:5:8: note: forward declaration of ‘struct tree_node’ 5 | struct tree_node {It doesn't like the tree_node as a value in unordered_map for some reason.