C++可变参数模板:在索引处删除/替换类型

Question

有办法:

1)删除索引处的项目:

// Removes item at index N, e.g. Remove<2, a, b, c, d> results in <a, b, d>
template<std::size_t N, typename ...Args>
struct Remove {
    // ???
};

2)替换索引处的项目:

// Replaces item at index N with T, e.g. Replace<2, x, a, b, c, d> results in <a, b, x, d>
template<std::size_t N, typename T, typename ...Args>
struct Replace {
    // ???
};

3)更换范围内的物品

// Replaces items in range [N1, N2] with T, e.g. ReplaceRange<2, 3, x, a, b, c, d> results in <a, b, x>
template<std::size_t N1, std::size_t N2, typename T, typename ...Args>
struct ReplaceRange {
    // ???
};

我想让它像这样使用

class is_true {
public:
    bool operator()() const { return true; }
};


class is_false {
public:
    bool operator()() const { return false; }
};

class And {
};

class Or {
};

Filter f<is_true, And, is_true, Or, is_false>();

现在,我想将其折叠为:

< FilterOr < FilterAnd <is_true, is_true>, is_false > >

哪里

template<typename Lhs, typename Rhs>
class FilterAnd {
public:
    bool operator()() const { return Lhs() && Rhs(); }
};

template<typename Lhs, typename Rhs>
class FilterOr {
public:
    bool operator()() const { return Lhs() || Rhs(); }
};

所以,我正在尝试执行以下操作:

class Filter {
public:
    template <typename ...Args>
    void apply() {
        holder_ = FilterHolder<typename FoldOperator<Or, FilterOr, typename FoldOperator<And, FilterAnd, Args...>::type >::type >();
    }
}

FoldOperator基本上删除运算符参数,并用Filterclass 替换运算符,例如对于参数<is_true, And, is_true>,我想删除arguments(is_true)并And用Filter 替换operator():FilterAnd<is_true, is_true>其中arguments与从列表中删除的参数相同.所以,我需要替换/删除模板来执行它.

提前致谢.

Answer 1

这在C++ 14中更容易std::index_sequence,所以我已经使用过了.你可以在网上找到很多实现.

我std::tuple用来整理周围的一切.

因为Replace,我曾经为每个类型索引std::conditional选择一个T或正确的元组元素.

为Remove,我走decltype的tuple_cat廷一个空元组或含有用于非去除型索引正确元组元素的元组.

namespace detail{
template<std::size_t N, typename T, typename Tuple, std::size_t... Idx>
auto replace (std::index_sequence<Idx...>) ->
    std::tuple<std::conditional_t<N == Idx, 
                                  T, 
                                  std::tuple_element_t<N, Tuple>>...>;

template<std::size_t N, typename Tuple, std::size_t... Idx>
auto remove (std::index_sequence<Idx...>) ->
    decltype(std::tuple_cat(
      std::declval<
        std::conditional_t<(N == Idx), 
                           std::tuple<>, 
                           std::tuple<std::tuple_element_t<Idx, Tuple>>
      >
    >()...
    ));
}

template <std::size_t N, typename T, typename... Args>
using Replace = decltype(detail::replace<N,T,std::tuple<Args...>>
                         (std::index_sequence_for<Args...>{}));

template <std::size_t N, typename... Args>
using Remove = decltype(detail::remove<N,std::tuple<Args...>>
                        (std::index_sequence_for<Args...>{})); 

ReplaceRange 留给读者的练习.

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