Is it possible to have a std::unordered_map<Class, Class> as a member of Class?

Question

Can the struct var have a member std::unordered_map<var, var> ?

Here is a code example that is not compiling because hash<json::var> is declared after the use of dict json_dict = dict(); :

namespace json
{
    struct var;
    using dict = std::unordered_map<var, var>;

    struct var
    {
        int integer = 0;
        std::string string = "";
        dict json_dict = dict();
    }
}

namespace std
{
    template<> struct hash<json::var>
    {
        std::size_t operator()(const json::var& k) const
        {
            return hash<string>()(k.string);
        }
    };
}

Is there a solution preferably without using pointers?

Answer 1

You can define hash<json::var>::operator() after you define json::var itself:

namespace json
{
    struct var;
}

namespace std
{
    template<> struct hash<json::var>
    {
        // just a declaration:
        std::size_t operator()(const json::var& k) const;
    };
}

namespace json
{
    using dict = std::unordered_map<var, var>;

    struct var
    {
        int integer = 0;
        std::string string = "";
        dict json_dict = dict();
    };
}

// definition: can access `json::var`'s members
std::size_t std::hash<json::var>::operator()(const json::var& k) const 
{ 
    return k.integer; 
}

live wandbox example

---

Unrelated, but

string string = "";
dict json_dict = dict();

is equivalent to

string string;
dict json_dict;