`std::pair` `second` has incomplete type with `unordered_map` tree

Question

I was reviewing some older code of mine and I saw the code using pointers to implement a tree of Variant objects. It is a tree because each Variant can contain an unordered_map of Variant*.

I looked at the code and wondered why isn't it just using values, a std::vector<Variant>, and std::unordered_map<std::string, Variant>, instead of Variant*.

So I went ahead and changed it. It seemed okay except one thing, I got errors:

/usr/local/include/c++/6.1.0/bits/stl_pair.h:153:11: error: 'std::pair<_T1, _T2>::second' has incomplete type
       _T2 second;                /// @c second is a copy of the second object
           ^~~~~~ main.cpp:11:8: note: forward declaration of 'struct Variant'
 struct Variant
        ^~~~~~~

So I figured I could trick the compiler into delaying the need to know that type, which didn't work either.

Working Not Working! (MCVE)

I thought this worked earlier but it actually doesn't, I forgot ::type on the using HideMap...

#include <vector>
#include <unordered_map>
#include <iostream>

template<typename K, typename V>
struct HideMap
{
    using type = std::unordered_map<K, V>;
};

struct Variant
{
    using array_container = std::vector<Variant>;

    // Does not work either
    using object_container = typename HideMap<std::string, Variant>::type;

    // Fails
    //using object_container = std::unordered_map<std::string, Variant>;

private:
    union Union
    {
        std::int64_t vint;
        array_container varr;
        object_container vobj;

        // These are required when there are union
        // members that need construct/destruct
        Union() {}
        ~Union() {}
    };

    Union data;
    bool weak;
};

int main()
{
    Variant v;
    std::cout << "Works" << std::endl;
}

So, my question is, why does it work okay for vector and not unordered_map?

If the problem is the inability to use incomplete types, is there a way to delay the instantiation of the unordered_map? I really don't want every object property to be a separate new allocation.

Answer 1

This uses placement new to defer the initialization of the Union to the constructor where Variant is a complete type. You need to reinterpret_cast everywhere you need to use the Union. I made an effort to not have any strict-alignment violations.

#include <algorithm>
#include <iostream>
#include <unordered_map>
#include <vector>

struct Variant {
    Variant();
    ~Variant();

    private:
    std::aligned_union<0, std::vector<Variant>,
                         std::unordered_map<std::string, void *>,
                         std::int64_t>::type data;
};

namespace Variant_detail {
    using array_container = std::vector<Variant>;
    using object_container = std::unordered_map<std::string, Variant>;

    union Union {
        std::int64_t vint;
        array_container varr;
        object_container vobj;

        // These are required when there are union
        // members that need construct/destruct
        Union() {}
        ~Union() {}
    };
}

Variant::Variant() {
    //make sure that std::unordered_map<std::string, Variant> is not too large
    static_assert(sizeof(std::unordered_map<std::string, Variant>) <=
                      sizeof data, "Variant map too big");
    static_assert(alignof(std::unordered_map<std::string, Variant>) <=
                      alignof(decltype(data)), "Variant map has too high alignment");
    auto &my_union = *new (&data) Variant_detail::Union;
    my_union.vint = 42;
}

Variant::~Variant() {
    reinterpret_cast<Variant_detail::Union &>(data).~Union();
}

int main() {
    Variant v;
    std::cout << "Works" << std::endl;
}