Recursive unordered_map

Question

I have a tree structure that internally uses unordered map

#include <unordered_map>

struct Node {
        std::unordered_map<int, Node> children;
};

int main() {
        Node a;
}

It works just fine on Apple clang 11.0.3 and MSVC v19.24, but it fails to compile on clang 10.0.0 and gcc 10.1

While regular std::map works just fine on all compilers. I failed to find the reason for this discrepancy. Is there any way to use std::unordered_map as a value for itself? Or pointers is the only solution here?

Here's the compiler explorer link https://godbolt.org/z/6eYch9

Here's an error from gcc:

    #3 with x86-64 gcc 10.1
    In file included from /opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/unordered_map:43,
                    from <source>:1:
    /opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/stl_pair.h:
In instantiation of 'struct std::pair<const int, Node>':
    
    /opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/ext/aligned_buffer.h:91:28:
required from 'struct __gnu_cxx::__aligned_buffer<std::pair<const int,
Node> >'
    
    /opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/hashtable_policy.h:233:43:
required from 'struct
std::__detail::_Hash_node_value_base<std::pair<const int, Node> >'
    
    /opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/hashtable_policy.h:279:12:
required from 'struct std::__detail::_Hash_node<std::pair<const int,
Node>, false>'
    
    /opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/hashtable_policy.h:1973:13:
required from 'struct
std::__detail::_Hashtable_alloc<std::allocator<std::__detail::_Hash_node<std::pair<const
int, Node>, false> > >'
    
    /opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/hashtable.h:173:11:
required from 'class std::_Hashtable<int, std::pair<const int, Node>,
std::allocator<std::pair<const int, Node> >,
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> >'
    
    /opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/unordered_map.h:105:18:
required from 'class std::unordered_map<int, Node>'

    <source>:4:39:   required from here
    /opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/stl_pair.h:218:11:
error: 'std::pair<_T1, _T2>::second' has incomplete type

      218 |       _T2 second;                ///< The second member
          |           ^~~~~~
    <source>:3:8: note: forward declaration of 'struct Node'
        3 | struct Node {
          |        ^~~~
    Compiler returned: 1

Answer 1

STL containers are not required to work with incomplete types. If you don't mind extra indirection, then the workaround is std::map<int, std::unique_ptr<Node>>

Answer 2

It's the same problem as doing e.g.

struct Node
{
    Node child;  // An instance of the full structure
};

You can't use a structure (or class) before it's fully defined, which it is at the closing }.

You can however define pointers to the structure, because then the compiler don't need the full structure definition, only know the name of the structure:

struct Node
{
    Node* child;  // Pointer to the structure
};

So to solve your problem, you need a map of pointers:

std::unordered_map<int, Node*> children;