I have a class named Registry which correlates an ID with some data. I would like to make it so the underlying structure which stores these pairs can be any std::mapish type, of which the user can define (e.g.: std::map, std::unordered_map).

My initial thought was to do something like this:

template<typename Value, typename Container, typename ID = size_t>
class Registry{
    using Storage = Container<ID, value>;
    static_assert(std::is_same<Storage, std::map> || std::is_same<Storage, std::map>, "Underlying storage type must be a std::map-ish.");
    public:
    Storage<ID, Value> store;
    ...

However, trying to use the class results in an error:

Registry<bool, std::map> testRegistry;
err) argument list for class template std::map is missing

I understand the compiler's complaint, but is there any way to work around it so that this syntax (or something similar) might work?

Thanks for the advice.

CodePudding user response：

You need to declare Container as a template template parameter. E.g.

template<typename Value, template <typename...> class Container, typename ID = size_t>
class Registry{

    using Storage = Container<ID, Value>;
    static_assert(std::is_same_v<Storage, std::map<ID, Value>> || std::is_same_v<Storage, std::unordered_map<ID, Value>>, "Underlying storage type must be a std::map-ish.");
    public:
    Storage store;
    ...

Other issues:

• Storage is an instantiation, so don't specify template arguments for it.
• Specify template arguments for std::map.
• As the condition of static_assert you should use std::is_same_v (or std::is_same<...>::value instead.

CodePudding user response：

As alternative, you might provide only container, instead of key, value, and template container:

template <typename Container>
class Registry
{
    using ID = typename Container::key_type;
    using Value = typename Container::mapped_type;
    static_assert(std::is_same_v<Container, std::map<ID, Value>>
               || std::is_same_v<Container, std::unordered_map<ID, Value>>,
                  "Underlying storage type must be a std::map-ish.");
    // ...
};

With usage

Registry<std::map<std::size_t, bool>> testRegistry;
Registry<std::unordered_map<std::size_t, float>> testRegistry2;