标题说:给定一个std::tuple,我想

  • 获取给定类型
  • 的第一个元素
  • 获取第i个元素的类型

  • STL 是否提供解决方案?还是解决方法?有人可以尝试完成我的代码吗?
    #include <tuple>
    
    int main ()
    {
        std::tuple<int,char,int> mytuple (10,'a', 5);
    
        // how to get the first int element here? (10)
        // int x = std::get_me_the_first<int>(mytuple);
    
        // how to get the type of the second element here?
        // std::get_me_type_of<1> ch = 'x';
    
        return 0;
    }
    

    像这样编译:
    g++ -std=c++11 -Wall main.cpp -o main
    

    最佳答案

    按类型(而不是索引)从元组获取值

    从C++ 11开始,没有STL方法可以获取T类型的元组的第一个元素。
    在C++ 14中,应该有一种方法使用新的std::get重载来执行所需的操作。 ISO纸位于N3404N3670

    您可以在C++ 11中执行以下操作:

    #include<tuple>
    #include<type_traits>
    #include<string>
    #include<iostream>
    
    template<int Index, class Search, class First, class... Types>
    struct get_internal
    {
        typedef typename get_internal<Index + 1, Search, Types...>::type type;
            static constexpr int index = Index;
    };
    
    template<int Index, class Search, class... Types>
    struct get_internal<Index, Search, Search, Types...>
    {
        typedef get_internal type;
        static constexpr int index = Index;
    };
    
    template<class T, class... Types>
    T get(std::tuple<Types...> tuple)
    {
        return std::get<get_internal<0,T,Types...>::type::index>(tuple);
    }
    

    我将其托管在Ideone here上,但这是我的后代测试功能
    int main()
    {
        std::tuple<int, double, std::string> test{1, 1.7, "test"};
        std::cout<<"get<0> == get<int> :"<< (std::get<0>(test) == get<int>(test))<< "\n";
        std::cout<<"get<1> == get<double> :"<<(std::get<1>(test) == get<double>(test))<< "\n";
        std::cout<<"get<2> == get<std::string> :"<<(std::get<2>(test) == get<std::string>(test))<< "\n";
    }
    

    基于@Yakk的扩展想法,以支持类型的多个实例以及要在元组中进行测试的谓词,他提供了以下代码(也托管在Ideone here上)
    ,请注意:在C++ 14中,std::get的新重载,不允许在元组中使用多个相同类型的实例。相反,它将发出编译错误。此外,C++ 14版本也不支持谓词。
    //Include same headers as before
    template<bool b, typename T=void>
    using EnableIf = typename std::enable_if<b,T>::type;
    
    template<int Index, template<typename T>class Search, int Which, typename, class First, class... Types>
    struct get_internal:
        get_internal<Index + 1, Search, Which, void, Types...>
    {};
    
    template<int Index, template<typename T>class Search, int Which, class First, class... Types>
    struct get_internal<Index, Search, Which, EnableIf<!Search<First>::value>, First, Types...>:
        get_internal<Index + 1, Search, Which, void, Types...>
    {};
    template<int Index, template<typename T>class Search, int Which, class First, class... Types>
    struct get_internal<Index, Search, Which, EnableIf<Search<First>::value>, First, Types...>:
        get_internal<Index + 1, Search, Which-1, void, Types...>
    {};
    template<int Index, template<typename T>class Search, class First, class... Types>
    struct get_internal<Index, Search, 0, EnableIf<Search<First>::value>, First, Types...>:
        std::integral_constant<int, Index>
    {};
    
    template<template<typename>class Test, int Which=0, class... Types>
    auto get(std::tuple<Types...>& tuple)->
      decltype(std::get<get_internal<0,Test,Which,void,Types...>::value>(tuple))
    {
        return std::get<get_internal<0,Test,Which,void,Types...>::value>(tuple);
    }
    template<template<typename>class Test, int Which=0, class... Types>
    auto get(std::tuple<Types...> const& tuple)->
      decltype(std::get<get_internal<0,Test,Which,void,Types...>::value>(tuple))
    {
        return std::get<get_internal<0,Test,Which,void,Types...>::value>(tuple);
    }
    template<template<typename>class Test, int Which=0, class... Types>
    auto get(std::tuple<Types...>&& tuple)->
      decltype(std::move(std::get<get_internal<0,Test,Which,void,Types...>::value>(tuple)))
    {
        return std::move(std::get<get_internal<0,Test,Which,void,Types...>::value>(tuple));
    }
    
    template<typename T>
    struct is_type {
      template<typename U>
      using test = std::is_same<T,U>;
    };
    
    template<class T, int Which=0, class... Types>
    T& get(std::tuple<Types...>& tuple)
    {
        return get<is_type<T>::template test,Which>(tuple);
    }
    template<class T, int Which=0, class... Types>
    T const& get(std::tuple<Types...> const& tuple)
    {
        return get<is_type<T>::template test,Which>(tuple);
    }
    template<class T, int Which=0, class... Types>
    T&& get(std::tuple<Types...>&& tuple)
    {
        return std::move(get<is_type<T>::template test,Which>(tuple));
    }
    

    获取元组中第n个元素的类型

    有一种获取第n个元素的类型的方法。 std::tuple_element<n, decltype(tuple)>::type(感谢@syam)是元组的第n个元素的类型。

    关于c++11 - 对于std::tuple,如何按类型获取数据,以及如何按索引获取类型?,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/16594002/

    10-11 22:47
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