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Yakov Gerlovin February 2016

How to switch to typedef from template aliasing?

I have a template class that basically implements registry design pattern. Values are registered with Keys and are stored in some container:

template <typename Key, typename Value, 
    template <typename...> class Container >
class registry
    typedef Key   key_type;
    typedef Value value_type;

    typedef std::pair<key_type, value_type> element_type;
    typedef Container<element_type> container_type;

Then, I can use it with sequence containers like this:

registry<const char*,void*, std::list> r1;
registry<const char*,void*, std::vector> r2;

I can even use it with alias:

template <typename T>
using alias_container = std::array<T, 10>;

registry<const char*,void*, alias_container > r4;

But I can't figure out how to use it with typedef like this:

template <typename T>
class my_container2 
    typedef std::array<T,3> type;

I basically want something like this:

registry<const char*,void*, my_container2::type > r5;

Thanks a lot for your help.


NathanOliver February 2016

type is dependent on the template type provided to my_container2. In order to use it you need to specify the template parameter like

registry<const char*,void*, my_container2<some_type>::type > r4;
                                          ^^^^^^^^^ template type here

This is the same concept as iterators to standard types. You can't use container_type::iterator. You have to use container_type<some_type>::iterator.

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Asked in February 2016
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