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c++templatesvariadic-templates

Understanding Variadic Templates

Trying to understand this bit of code using variadic templates:

template <typename... T>
struct FooGroup;

template <typename... FooTypes, typename... BarTypes>
struct TEST<FooGroup<FooTypes...>, BarGroup<BarTypes...>>

I know variadic templates allows you to pass in unknown type of unknown amount. However, this is confusing to me. Can anyone shed some light?

Solution

  • This code

    template <typename... T>
struct FooGroup;

    declare FooGroup as a struct that receive a variadic list of type template parameter.

    template <typename... FooTypes, typename... BarTypes>
struct TEST<FooGroup<FooTypes...>, BarGroup<BarTypes...>>

    is part of template specialization.

    I suppose that TEST is declared as follows

    template <typename, typename>
struct TEST;

    so receiving two template types parameter.

    With 

    template <typename... FooTypes, typename... BarTypes>
struct TEST<FooGroup<FooTypes...>, BarGroup<BarTypes...>>

    you declare a partial specialization of TEST in case the first template parameter is in the form FooGroup<FooTypes...> (where FooTypes... is a variadic list of template parameters) and the second template parameter is in the form BarGroup<BarTypes...> (where I suppose BarGroup is defined almost as FooGroup and BarTypes... is another variadic list of template parameters)