How to obtain inner type of template type?

Question

I've got the following situation:

This is a wrapper type

template <typename wrapperInnerType>
struct wrapperT{ ... };
using wrapper = wrapperT<float>;

And it's used in this class

template <typename wrapperType>
class InData{
    wrapperInnerType var;   //<-- How to get inner type?
};

My question is what is the easiest way to get wrapperInnerType from wrapperType?

Ideally I would like it to be still possible to use InData<wrapper> myData; when using that class (instead of having multiple types in declaration like in InData<wrapper, float> myData; as an example).

Does this answer your question? [How can I get the innermost template parameter type?](https://stackoverflow.com/questions/25187323/how-can-i-get-the-innermost-template-parameter-type) — NutCracker, Jan 27 '20 at 09:53

super · Accepted Answer · 2020-01-27T09:57:35.353

4

The easiest way is to define a type alias in wrapperT.

template <typename wrapperInnerType>
struct wrapperT {
    using innerType = T;
    // ...
};
using wrapper = wrapperT<float>;

template <typename wrapperType>
class InData{
    typename wrapperType::innerType var;
};

Another approach could be to make a helper struct that extract it using a template template parameter and partial specialization. This would avoid having to modify wrapperT and could potentially give some re-usability. The template signature of wrap would however have to be known for this to work.

template <typename T>
struct getInnerType;

template <template <typename> typename wrap, typename Inner>
struct getInnerType<wrap<Inner>> {
    using type = Inner;
}

template <typename T> // For more convenient use as pointed out in comments
using getInnerType_t = typename getInnerType<T>::type

template <typename wrapperType>
class InData{
    getInnerType_t<wrapperType> var;   //<-- How to get inner type?
};

edited Jan 27 '20 at 09:57

answered Jan 27 '20 at 09:52

super

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And also `template using getInnerType_t = typename getInnerType::type;` to simplify usage in `InData`. – Jarod42 Jan 27 '20 at 09:54
Thanks, went with the first approach and it's exactly what I needed! – Shrimp Jan 27 '20 at 09:57
@Jarod42 Indeed, why not. Added. – super Jan 27 '20 at 09:57
Out of curiosity, what is `template typename wrap` called?(or if you had some reference to better understand how it works) – tangy Jan 27 '20 at 10:02
1

That's a template template parameter, cppref has a reasonable explanation. @tangy – n314159 Jan 27 '20 at 10:06

score 3 · Answer 2 · answered Jan 27 '20 at 09:51

You could declare the following class template, wrapper_inner_type:

template<typename>
struct wrapper_inner_type;

Then, specialize it for the wrapper, wrapperT<InnerType>, where InnerType is the type you want to find out:

template<typename InnerType>
struct wrapper_inner_type<wrapperT<InnerType>> {
   using type = InnerType;
};

You can also define this alias template for convenience:

template<typename T>
using wrapper_inner_type_t = typename wrapper_inner_type<T>::type;

Finally, in order to get the inner type inside InData:

template <typename wrapperType>
class InData{
    wrapper_inner_type_t<wrapperType> var;
};

How to obtain inner type of template type?

2 Answers2