Map that works with incomplete types.

hashmap.hpp

#ifndef TEST_HASHMAP_HPP
#define TEST_HASHMAP_HPP

#include <vector>

namespace test {

template <typename F, typename S>
struct pair;

template <typename K, typename V>
struct hashmap {
    typedef pair<K, V> value_type;

    hashmap();
    hashmap(hashmap const&);
    ~hashmap();

    void insert(K, V);
    V& find(K const&);
    
private:
    value_type* store;
};

template <typename F, typename S>
struct pair {
    F first;
    S second;
};

template <typename K, typename V>
inline hashmap<K, V>::hashmap()
    : store(new value_type[10])
{ }

template <typename K, typename V>
inline hashmap<K, V>::hashmap(hashmap const& other)
    : store(new value_type[10])
{
    for (unsigned i = 0; i < 10; ++i)
        store[i] = other.store[i];
}

template <typename K, typename V>
inline hashmap<K, V>::~hashmap()
{
    delete [] store;
}

} // namespace test

#endif // ndef TEST_HASHMAP_HPP

main.cpp

#include "hashmap.hpp"

struct state {
    int value;
    test::hashmap<int, state> transitions;
};

int main() {
    state automaton;
}