创建一个C std :: tuple投影函数

我正在寻找std :: tuple的投影函数.即,该函数接收整数列表作为模板参数,并返回仅具有这些索引处的值的元组.

例如,考虑我有一个元组元组< int,String,char,float> t,然后调用项目< 1,3>(t)将导致元组类型元组< String,float>的元组.

当然,关键似乎是递归模板.但是,我无法让它工作.我尝试通过声明一个结构来计算投影操作的返回类型.即使这样也失败了这是我尝试过的：

template<typename T, size_t... indexes>
class Projection{
};

// Specialization for no index => Empty tuple
template<>
template<typename... T>
class Projection<std::tuple<T...>>{
   typedef std::tuple<T...> InputTuple;
   typedef std::tuple<> Tuple;
};

// Specialization for one or more parameters
template<size_t Index1, size_t... others>
template<typename... T>
class Projection<std::tuple<T...>,Index1,others...>{
    typedef std::tuple<T...> InputTuple;

    // The type of a projection with one column less
    typedef Projection<std::tuple<T...>,others...> TupleMinusOne;

    // The result type is the catenation of TupleMinusOne plus the column projected in this     step
    typedef decltype(std::tuple_cat(std::make_tuple(std::get<Index1>(InputTuple())),typename TupleMinusOne::Tuple())) Tuple;
};

这编译.带有空元组的基本案例也有效,即：

Projection<std::tuple<int,std::string>>::Tuple t;

结果是一个空元组.但是,递归情况不会编译：

Projection<std::tuple<int,std::string>,1>::Tuple t;

我收到以下错误：

Test.cpp:79:1: error: ‘Tuple’ is not a member of ‘Projection<std::tuple<int, float>, 1ul>’

所以似乎递归情况不被识别,但为什么呢？

解决方法:

很少需要递归模板; pack expansion通常更清晰,更简单.在这种情况下,只需使用tuple_element：

template<typename T, size_t... indexes>
class Projection{
public:
    using Tuple = std::tuple<typename std::tuple_element<indexes, T>::type...>;
};

同上项目：

template<size_t... indexes, typename T>
auto project(const T &t) -> typename Projection<T, indexes...>::Tuple {
    return typename Projection<T, indexes...>::Tuple(std::get<indexes>(t)...);
}

标签：c,templates,variadic-templates,stdtuple
来源： https://codeday.me/bug/20190830/1770848.html