5

标题说：给定一个 std::tuple，我想

  1. 获取给定类型的第一个元素
  2. 获取第 i 个元素的类型

STL 是否提供了解决方案？还是一种解决方法？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;
}

编译如下：

g++ -std=c++11 -Wall main.cpp -o main
4

2 回答 2

10

按类型从元组中获取值（而不是索引）

从 C++11 开始，没有 STL 方法可以获取 type 元组的第一个元素T。
在 C++14 中，应该有一种方法使用新的重载std::get来做你想做的事。ISO 文件位于此处N3404和此处N3670。

您可以在 C++11 中使用以下命令执行此操作：

#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);
}

我把它托管在 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 扩展它以支持类型的多个实例以及在元组中测试的谓词的想法，他提供了以下代码（也托管在 Ideone here上）
被警告：在 C++14 中，新的重载ofstd::get不允许元组中有多个相同类型的实例。相反，它会发出编译错误。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

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

于 2013-05-16T17:50:33.473 回答
0

只是为了咯咯笑，def不是你想要的，并且在某种意义上非常有限，你必须添加新类型并且它反向工作但这里getNthTypeReverse只支持'int'和'char'类型哈哈http://ideone.com /英国2JTC

#include <tuple>
#include <iostream>
using namespace std;

typedef std::tuple<int,char,int> TupleType;

template<typename TupleT,typename OnElementHandler, size_t N>
struct TupleIterator{    
    static void call(const TupleT& tuple, OnElementHandler& OnElement){ 
        auto nthElem = std::get<N>(tuple);
        OnElement(nthElem);        
        TupleIterator<TupleT,OnElementHandler,N-1>::call(tuple,OnElement);
    }
};
template<typename TupleT,typename OnElementHandler>
struct TupleIterator<TupleT,OnElementHandler,0>{  
     static void call(const TupleT& tuple, OnElementHandler& OnElement){ 
        auto firstElem = std::get<0>(tuple);        
        OnElement(firstElem);
    }
};
template<typename T1,typename T2>
struct IsSame{enum{result = 0};};

template<typename T>
struct IsSame<T,T>{ enum{result = 1}; };

template<typename T,size_t TargetCount, size_t BeginIndex, size_t EndIndex, typename Tuple>
T getNthTypeReverse(const Tuple& t, const T& defaultValue = T()){
    //assert 0 <= N <= tuple.size
    T result = defaultValue;    
    struct NthGrabber{
       T& result;
       const size_t n;
       size_t count;
       NthGrabber(T& r, const size_t n): result(r),n(n),count(0){}
       void operator()(const int i){
           if(IsSame<T,int>::result){
               ++count;
               if(count == n) result = i;
           }

       }
       void operator()(const char c){
           if(IsSame<T,char>::result){
               ++count;
               if(count == n) result = c;
           }
       }
       //overload for other version too...
    }OnElement(result,TargetCount+1); //WILL update result if condition meet
    const size_t tupleSize = EndIndex - BeginIndex;
    TupleIterator<TupleType,NthGrabber,tupleSize>::call(t,OnElement); 
    return result;
}
int main(){
    TupleType t(10,'a',5);    
    const size_t tupleSize = std::tuple_size<decltype(t)>::value - 1;
    int lastInt = getNthTypeReverse<int,0,0,tupleSize,TupleType>(t,-1); 
    int secondLastInt = getNthTypeReverse<int,1,0,tupleSize,TupleType>(t,-1); 
    int thirdLastInt = getNthTypeReverse<int,2,0,tupleSize,TupleType>(t,-1); 
    cout << "lastInt = " << lastInt << endl;    
    cout << "SecondLast = " << secondLastInt << endl;
    cout << "ThirdLast = " << thirdLastInt << endl;

    char lastChar = getNthTypeReverse<char,0,0,tupleSize,TupleType>(t,'\0'); 
    cout << "LastChar = "  << lastChar << endl;

    return 0;
}

输出：

lastInt = 5
SecondLast = 10
ThirdLast = -1
LastChar = a
于 2013-05-16T19:41:37.770 回答