此模板如何工作以查找元組索引？

Question

在我的項目中，通過模板中定義找到一個元組的索引功能，通過我還是不明白它是如何工作： 似乎有一個遞歸，但我不知道它是如何終止權指數的想法？

// retrieve the index (for std::get) of a tuple by type 
// usage: std::get<Analysis::type_index<0, Type, Types ...>::type::index>(tuple) 
// TODO: should make this tidier to use 
template<int Index, class Search, class First, class ... Types> 
struct type_index 
{ 
    typedef typename Analysis::type_index<Index + 1, Search, Types ...>::type type; 
    static constexpr int index = Index; 
}; 

template<int Index, class Search, class ... Types> 
struct type_index<Index, Search, Search, Types ...> 
{ 
    typedef type_index type; 
    static constexpr int index = Index; 
}; 

Answer 1

專業化是終止條件。請注意，它需要First等於Search：

type_index<Index, Search, Search, Types ...> 
        ^^^^^^ ^^^^^^ 

例如，如果您有

type_index<0, C, A, B, C, D>, 

這不符合專業化，所以通用模板將被使用，重定向啓動（通過其type部件）

type_index<0, C, A, B, C, D>::type = type_index<1, C, B, C, D>::type 

但是這不可求值或者直到達到鏈

type_index<0, C, A, B, C, D>::type = ... = type_index<2, C, C, D>::type 

此時部分特可以被使用，它說，

type_index<2, C, C, D>::type = type_index<2, C, C, D> 
type_index<2, C, C, D>::index = 2 

等

type_index<0, C, A, B, C, D>::type::index = 2 
       ^^^^ 
       0 1 2 3 

如預期。

---

請注意，您不需要攜帶Index周圍，確實可以放下整個::type事情：

template<typename, typename...> 
struct type_index; 

template<typename Search, typename Head, typename... Tail> 
struct type_index<Search, Head, Tail...> { 
    // Search ≠ Head: try with others, adding 1 to the result 
    static constexpr size_t index = 1 + type_index<Search, Tail...>::index; 
}; 

template<typename Search, typename... Others> 
struct type_index<Search, Search, Others...> { 
    // Search = Head: if we're called directly, the index is 0, 
    // otherwise the 1 + 1 + ... will do the trick 
    static constexpr size_t index = 0; 
}; 

template<typename Search> 
struct type_index<Search> { 
    // Not found: let the compiler conveniently say "there's no index". 
}; 

這個工作過程：

type_index<C, A, B, C, D>::index 
    = 1 + type_index<C, B, C, D>::index 
    = 1 + 1 + type_index<C, C, D>::index 
    = 1 + 1 + 0 
    = 2 

，如果類型不在該列表中，會說像（GCC 6.2.1）：

In instantiation of ‘constexpr const size_t type_index<X, C>::index’: 
    recursively required from ‘constexpr const size_t type_index<X, B, C>::index’ 
    required from ‘constexpr const size_t type_index<X, A, B, C>::index’ 
    required from [somewhere] 
error: ‘index’ is not a member of ‘type_index<X>’ 

我覺得很明顯。