基于基础元组与可变索引序列定义子元组及重复类型适配问题

原代码的toBaseTuple方法错误地使用类型作为std::get的模板参数，当BaseTuple存在重复类型时，std::get<类型>会因为歧义无法编译。核心问题是没有正确关联CustomTuple的元素位置与BaseTuple的原始索引I...。

修改toBaseTuple的实现，通过索引而非类型完成赋值，同时利用std::index_sequence关联CustomTuple的元素位置和BaseTuple的原始索引：

#include <tuple>
#include <string>
#include <utility>

using BaseTuple = std::tuple<int, double, std::string>;

template <size_t... I>
struct CustomTuple : public std::tuple<std::tuple_element_t<I, BaseTuple>...> {
    // 继承基类tuple的构造函数
    using std::tuple<std::tuple_element_t<I, BaseTuple>...>::tuple;

    // 从BaseTuple转换而来
    static CustomTuple fromBaseTuple(const BaseTuple& key) {
        return {std::get<I>(key)...};
    }

private:
    // 辅助函数：通过双索引序列完成赋值
    template <size_t... J>
    void fillBaseTuple(BaseTuple& t, std::index_sequence<J...>) {
        ((std::get<I>(t) = std::get<J>(*this)), ...);
    }

public:
    // 转换回BaseTuple
    BaseTuple toBaseTuple() {
        BaseTuple t;
        // 生成CustomTuple的元素索引序列J...，关联I...完成赋值
        fillBaseTuple(t, std::index_sequence_for<I...>{});
        return t;
    }
};

• 索引关联：std::index_sequence_for<I...>生成与I...长度相同的索引序列0,1,...,sizeof...(I)-1，对应CustomTuple的元素位置。
• 折叠表达式赋值：((std::get<I>(t) = std::get<J>(*this)), ...)遍历每一对(I,J)，将CustomTuple的第J个元素赋值给BaseTuple的第I个元素，完全基于索引操作，避免类型歧义。
• 兼容性：无论BaseTuple是否有重复类型，该实现都能正确工作，同时保留原有的fromBaseTuple和构造函数逻辑。

#include <iostream>

int main() {
    // 测试含重复类型的BaseTuple
    using BaseTupleWithDuplicate = std::tuple<int, int, std::string>;

    template <size_t... I>
    struct CustomTupleDuplicate : public std::tuple<std::tuple_element_t<I, BaseTupleWithDuplicate>...> {
        using std::tuple<std::tuple_element_t<I, BaseTupleWithDuplicate>...>::tuple;

        static CustomTupleDuplicate fromBaseTuple(const BaseTupleWithDuplicate& key) {
            return {std::get<I>(key)...};
        }

    private:
        template <size_t... J>
        void fillBaseTuple(BaseTupleWithDuplicate& t, std::index_sequence<J...>) {
            ((std::get<I>(t) = std::get<J>(*this)), ...);
        }

    public:
        BaseTupleWithDuplicate toBaseTuple() {
            BaseTupleWithDuplicate t;
            fillBaseTuple(t, std::index_sequence_for<I...>{});
            return t;
        }
    };

    BaseTupleWithDuplicate base{10, 20, "test"};
    auto custom = CustomTupleDuplicate<0,2>::fromBaseTuple(base);
    auto restored = custom.toBaseTuple();

    std::cout << std::get<0>(restored) << ", " << std::get<1>(restored) << ", " << std::get<2>(restored) << std::endl;
    // 输出：10, 20, test
    return 0;
}

内容的提问来源于stack exchange，提问作者efzzz