枚举的元编程问题

发布于 2024-12-02 04:44:17 字数 386 浏览 0 评论 0原文

Atm 我有这样的东西：

template<int n>
struct Pow
{
  enum{val= Pow<n-1>::val<<1};
};
template<>
struct Pow<0>{
    enum{val =1};
};

我可以访问像 Pow<30>::val 这样的数据。很好，但我想这样做

   int main()
    {
    Pow<30>::val;

，然后使用变量访问所有值<0,30>；我知道我可以使用数组和动态编程，但是我可以用这种方式做到这一点吗？对不起英语。

原文

Atm i have sth like that:

template<int n>
struct Pow
{
  enum{val= Pow<n-1>::val<<1};
};
template<>
struct Pow<0>{
    enum{val =1};
};

I can acess data like Pow<30>::val. It's good but i want do like this

   int main()
    {
    Pow<30>::val;

and then use variable to
access all of value <0,30>
I knew that i can use array and dynamic programming but can i do that in this way?
Sorry for English.

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星軌x 2024-12-09 04:44:17

使用 C++0x 可变参数模板：

template<int... Indices>
struct powers {
    static const int value[sizeof...(Indices)];

    typedef powers<Indices..., sizeof...(Indices)> next;
};

template<int... Indices>
const int powers<Indices...>::value[sizeof...(Indices)] = { Pow<Indices>::val... };

template<int N>
struct build_powers {
    typedef typename build_powers<N - 1>::type::next type;
};

template<>
struct build_powers<1> {
    typedef powers<0> type;
};

然后：

int
main()
{
    // we want [0..30] inclusive so pass 31 as exclusive upper limit
    typedef build_powers<31>::type power_type;
    // 0..30 is 31 powers in all
    typedef const int array_type[31];

    array_type& ref = power_type::value;
    // ref[0] .. ref[30] are the values of Pow<0>::val .. Pow<30>::val
}

这就是使用数组但没有动态初始化。由于您希望结果作为变量而不是 TMP，我认为这已经足够了。

Using C++0x variadic templates:

template<int... Indices>
struct powers {
    static const int value[sizeof...(Indices)];

    typedef powers<Indices..., sizeof...(Indices)> next;
};

template<int... Indices>
const int powers<Indices...>::value[sizeof...(Indices)] = { Pow<Indices>::val... };

template<int N>
struct build_powers {
    typedef typename build_powers<N - 1>::type::next type;
};

template<>
struct build_powers<1> {
    typedef powers<0> type;
};

and then:

int
main()
{
    // we want [0..30] inclusive so pass 31 as exclusive upper limit
    typedef build_powers<31>::type power_type;
    // 0..30 is 31 powers in all
    typedef const int array_type[31];

    array_type& ref = power_type::value;
    // ref[0] .. ref[30] are the values of Pow<0>::val .. Pow<30>::val
}

So that's with using an array but without dynamic initialization. Since you want the result as a variable and not for TMP I feel this is adequate.

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