• 作者: 雪山肥鱼
• 时间：2022027 12:42
• 目的: integer_sequence

integer_sequence 简介

integer_sequence 作用是产生一个整型数列。

 template <class T, T... Ints>
struct integer_sequece {
    using value_type = T;
    
    static constexpr size_t size() noexcept
    {
        return sizeof...(Ints);
    }
};

1. Ints 是一个非类型模板参数包， T: int, unsigned int, short等。一堆数字而已。

• std::make_integer_sequence 别名模板，可以理解为 就是 std::integer_sequence:

template <class T, T N>
using make_integer_sequence = integer_sequnce<T, 0,1,2,3,4,...,N-1序列>

int main(int argc, char ** argv)
{
    std::make_integer_sequence<int, 5> tmpobj;
    cout << "tmpobj类型：" << typeid(decltype(tmpobj)).name() << endl;
    return 0;
}

图片.png

得到 元素类型 + 个数 0 - 4

• std::make_index_sequence

//伪代码
template<size_t N>
using make_index_sequence = integer_sequence<std::size_t, 0,1,2...N-1>

    std::make_index_sequence<5> tmobj2;
    cout << "tmpobj2类型：" << typeid(decltype(tmobj2)).name() << endl;

图片.png

正向排列数字生成一个类型 integer_sequence

简单实现 make_integer_sequence

//向integer_sequence 末尾插入元素：
//泛化版本:
template<typename INSEQ, unsigned int NewElem>
struct IntSeq_PushBack;//不适用泛化版本，所以泛化版本 只声明不定义

//特化版本才插入元素
template<typename T, unsigned int... Elems, unsigned int NewElem>//特化版本int...可以放在中间，泛化版本必须放到最后
struct IntSeq_PushBack<std::integer_sequence<T, Elems...>, NewElem> {
    using type = integer_sequence<T, Elems..., NewElem>;
};

//泛化
template <typename T, unsigned int N>
struct Integer_Sequence {
    //依次遍历出 4 3 2 1，往末尾插入元素操作
    using type = typename InSeq_PushBack<typename Integer_Sequence<T, N-1>::type, N-1>::type;//
};

//特化
template<typename T>
struct Integer_Sequence<T, 1>//递归到1
{
    using type = std::integer_sequence<T, 0>
};

template <typename T, unsigned int N>
using Integer_Sequence_T = typename Integer_Sequence<T, N>::type;//功能上和 make_integer_sequence类似


int main(int argc, char ** argv)
{
    std::make_integer_sequence<int, 5> tmpobj;
    cout << "tmpobj类型：" << typeid(decltype(tmpobj)).name() << endl;
    
    std::make_index_sequence<5> tmobj2;
    cout << "tmpobj2类型：" << typeid(decltype(tmobj2)).name() << endl;
    
    Integer_Sequence_T<int, 4> tmpobj3;
    cout << "tmpobj3类型：" << typeid(decltype(tmpobj3)).name() << endl;

    return 0;
}

非常蒙蔽.png

逆向排列数字生成一个类型

template<typename INSEQ, unsigned int NewElem>
struct IntSeq_PushFront;

template<typename T, unsigned int ... Elems, unsigned int NewElem>
struct IntSeq_PushFront<integer_sequence<T, Elems...>, NewElem> {
    using type = integer_sequence<T, NewElem,Elems...>;
};

template <typename T, unsigned int N, unsigned Count =1>
struct Integer_Sequence_Revers{
    using type = typename IntSeq_PushFront<typename Integer_Sequence_Revers<T, N - 1>::type, N - Count>::type;
};

template <typename T, unsigned int N>
struct Integer_Sequence_Revers<T, N, N>
{
    using type = integer_sequence<T, N-1>;
};

template<typename T, unsigned int N>
using Integer_Sequence_Revers_T =  typename Integer_Sequence_Revers<T, N>::type;

int main(int argc, char ** argv) {
    Integer_Sequence_Revers_T<int, 4> tmpobj4;
    cout << typeid(decltype(tmpobj4)).name() << endl;
    return 0;
}

1. 注意上述代码中 count 并没有自增
2. 直到N == Count == 1 时，才会走特化版本。

Repeat Integer

有空再回过头来看。