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?
标题是这样写的:给定一个std::tuple,我想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?,c++11,C++11,标题是这样写的: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?还是一种解决方法?有人能完成我的代码吗 #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::ge
#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;
}
按类型(而不是索引)从元组中获取值
从C++11开始,没有STL方法获取类型为T
的元组的第一个元素在C++14中,应该有一种方法可以使用新的重载
std::get
来执行您想要的操作。ISO纸张位于此处和此处
您可以在C++11中通过以下操作完成此操作:
#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);
}
获取元组中第n个元素的类型
有一种方法可以获得第n个元素的类型
std::tuple\u element::type
(谢谢@syam)是tuple的第n个元素的类型。只是为了搞笑,定义的不是你想要的,并且在某种意义上是非常有限的,你必须添加新的类型,它可以反向工作,但是这里有一个getnthtyperverse
,它只支持'int'和'char'类型哈哈
谢谢,您可以提供另一种使用boost或自行编写的解决方法吗?
decltype(std::get(tuple))
将产生T&
或const T&
,具体取决于选择的std::get
版本(这又取决于tuple
constness——我认为我们可以安全地忽略T&&
的情况,因为它需要抛出std::move
)。最好使用std::tuple\u元素::type
来获得“裸的”键入IMHO。@syam我在您的第n种类型的更改中添加了内容。例如:--哦,我们可能需要在get
函数上分别使用const&
、&
和&
重载,这样我们就可以分别通过const T&
、&
和std::move
返回。@Johannes我编辑了这篇文章的第二部分ost将包含您想要的三个重载--tuple&
、tuple const&
和tuple&
。我真的很好奇您为什么要使用这样的tuple。我的直觉告诉我,您应该重新考虑您试图解决的问题的方法。
#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);
}
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";
}
//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));
}
#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;
}
lastInt = 5
SecondLast = 10
ThirdLast = -1
LastChar = a