Question: Hashmap as template parameter to another class

[lczech]

I am trying to write a class template that accepts a Hashmap as a template parameter, but in a way where the key and value types do not have to be defined, and instead are set by the class:

template<template<typename, typename, typename...> class Hashmap = std::unordered_map>
class MyClass {
public:
    using MapType = Hashmap<std::string, int>;
    MapType my_map;
};

in order to allow for easy usage (user does not have to provide std::string and int), and because the key/value types are meant to be internal to the class anyway.

In other words, I'd like the class to be usable like this:

MyClass<std::unordered_map> obj1;
MyClass<phmap::parallel_flat_hash_map> obj2;

This however fails to compile for parallel_flat_hash_map, as it contains a template parameter size_t N = 4, and non-type parameters are not allowed in variadic template template parameters...

Is there a way to get this to work, or do I have to change the template arguments of MyClass and have the user specify

MyClass<phmap::parallel_flat_hash_map<std::string, int>> obj3;

explicitly each time?

Thank you in advance!
Lucas

[greg7mdp]

The user of the class could do this for example:

template <typename K, typename V>
using parallel_flat_hash_map = phmap::parallel_flat_hash_map<K, V, phmap::priv::hash_default_hash<V>, phmap::priv::hash_default_eq<K>, phmap::priv::Allocator<phmap::priv::Pair<K, V>>, std::mutex>;

and then use parallel_flat_hash_map as the template argument for MyClass

[lczech]

Thanks! It didn't work with exactly that, as the N template param seems to be needed (and it isn't part of your suggested typedef), but as I am happy with the defaults for most template params anyway, I got it to work with this:

template <typename K, typename V>
using parallel_flat_hash_map = phmap::parallel_flat_hash_map<K, V>;

and plug that one into MyClass.

Cheers and thanks again!
Lucas