是否可以单线获取第一类参数包？ [英] Is it possible to get the first type of a parameter pack in a one-liner?

问题描述

我在可变参数模板类中给出了一个参数包，并且想要提取第一种类型。

I have a parameter pack given in a variadic template class and want to extract the first type.

目前，我这样做是可行的，但是有点麻烦。是否可以使同一件事更简单？ FirstEntityType 应该定义为具有 EntityTs 中第一类型的类型。注意，我要保留类模板的签名。我知道可以写 template< typename FirstEntityType，typename ... OtherEntityTypes> ，但这是我不想做的事。

Currently I do this, which works fine but is somehow cumbersome. Is it possible to do the same thing simpler? FirstEntityTypeshould be defined to have the type of the first type in EntityTs. Note, I want to keep the signature of the class template. I know that it would be possible to write template<typename FirstEntityType, typename... OtherEntityTypes>, it is however something that I don't want to do.

template<typename... EntityTs>
class EntityContext
{
    template<typename T, typename ... Ts>
    struct K {
        using type = T;
    };

    using FirstEntityType = typename K<EntityTs...>::type;

   // ...
}

推荐答案

您可以使用FirstEntityType = std :: tuple_element_t< 0，std :: tuple<来编写：

You could write:

using FirstEntityType = std::tuple_element_t<0, std::tuple<EntityTs...>>;

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