Using RTTI hash with a template function

Question

I understand that templates are compile-time, and typeinfo-related are runtime, but I'm wondering if I can achieve my particular task.

I have a factory method using templates to create objects of a particular type; I also have a preloader (reading data from disk), which determines what type of object is to be created, but doesn't actually create it - that's the responsibility of the creator, and is executed on demand.

void Creator::Spawn(Preloader* pl)
{
    std::unordered_map<size_t, std::type_index>   hashmap;

    // assume ObjectType is simply a wrapper around a hash
    hashmap[ObjectType<Type1>::GetType().Hash()] = typeid(Type1);
    hashmap[ObjectType<Type2>::GetType().Hash()] = typeid(Type2);

    for ( auto& const i : pl->GetPreloadInfo() )
    {
        size_t  hash = i->type_hash.Hash();

        // similar-to-desired usage
        FactoryCreate<hashmap[hash]>();
    }
}

Is there any way to achieve this? Obviously I can do manual checks for each, like below, but it's nasty at best.

        // poor, manual implementation
        if ( hash == ObjectType<Type1>::GetType().Hash() )
            FactoryCreate<Type1>();
        else if ( hash == ObjectType<Type2>::GetType().Hash() )
            FactoryCreate<Type2>();

Everything I've tried so far has hit the runtime vs compile-time differences, though I'm definitely not aware of all the newest C++11 tricks that may assist (C++14 not usable).

Partially related question here: Use data type (class type) as key in a map

Barry · Accepted Answer · 2016-04-29 22:11:11Z

Assuming the hash part is set in stone, you can create a map from those type hashes to your factory functions directly:

using map_type = std::unordered_map<size_t, std::function<void()>>;

template <class... Ts>
map_type create_hash_map() {
    map_type map;

    // emplace (hash<T>, FactoryCreate<T>) for each T
    using swallow = int[];
    (void)swallow{0,
        (void(
            map.emplace(ObjectType<Ts>::GetType().Hash(),
                []{ FactoryCreate<Ts>(); }
                )
        ), 0)...
    };

    return map;
}

Then we can just use that map directly:

void Creator::Spawn(Preloader* pl)
{
    static auto hashmap = create_hash_map<Type1, Type2>();

    for ( auto& const i : pl->GetPreloadInfo() )
    {
        size_t  hash = i->type_hash.Hash();
        hashmap[hash]();
    }
}

That doesn't have error-checking, so if the hash isn't actually in the map, you'll get a bad_function_call exception from std::function. If you need error checking, you can instead do a lookup in the map first:

auto it = hashmap.find(hash);
if (it != hashmap.end()) {
    (it->second)(); 
}
else {
    // error!
}

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