检查平等到多种可能性的速度 [英] Shorthand for checking for equality to multiple possibilities

问题描述

可能重复：

C ++将值与给定值进行比较

我需要检查在C的for循环中的相等，但是循环需要为x等于多个可能性。

I'm needing to check for equality in a for loop in C++, however the loop needs to work for x equaling multiple possibilities.

例如，现在我有类似的东西：

For example, right now I have something similar to:

if(x==a || x==b || x==c || x==d || x==e || x==f || x==g || x==h)
{
    //loop body
}

我有，它看起来混乱，我想知道是否有一个简写的方式说if（x ==（任何这些）或如果写出所有的是唯一的选择。

But with the number I have, it looks messy and I was wondering if there was a shorthand way of saying "if (x == (any of these))" or if writing them all out was the only option.

谢谢！

推荐答案

谢谢您的问题，现在我找到了一个解决方案一个我敢说），我会用它自己。

Thank you for your question, now as I found a solution (and an elegant one I dare say), I'll use it myself.

与使用std :: find的解决方案不同：a）在编译时被展开为N个比较b）使用任何类型的X可以比较

Unlike solutions with std::find : Will a ) be unrolled to N comparisons in compile time b) work with any types which X can be compared to

struct TagAnyOf {};

template <typename... Args>
std::tuple <TagAnyOf, Args...> AnyOf (Args&&... args)
{
   return std::tuple <TagAnyOf, Args...> (TagAnyOf(), std::forward<Args>(args)...);
}

template <class X, class Tuple, size_t Index, size_t ReverseIndex>
struct CompareToTuple
{
    static bool compare (const X& x, const Tuple& tuple)
    {
        return x == std::get<Index> (tuple) || CompareToTuple<X, Tuple, Index+1, ReverseIndex-1>::compare (x, tuple);
    }
};

template <class X, class Tuple, size_t Index>
struct CompareToTuple <X, Tuple, Index, 0>
{
    static bool compare (const X& x, const Tuple& tuple)
    {
        return false;
    }
};

template <typename X, typename... Args>
bool operator == (const X& x, const std::tuple<TagAnyOf, Args...>& any)
{
    typedef std::tuple <TagAnyOf, Args...> any_of_type;
    return CompareToTuple <X, any_of_type, 1, std::tuple_size<any_of_type>::value-1>::compare (x, any);
}

使用

int main()
{
    int x = 1;
    if (x == AnyOf (1, 2, 3, 4))
    {
        std::cout << "Yes!" << std::endl;
    }
    else
    {
        std::cout << "No!" << std::endl;
    }

    if (x == AnyOf (4, 3, 2, 1))
    {
        std::cout << "Yes!" << std::endl;
    }
    else
    {
        std::cout << "No!" << std::endl;
    }

    if (x == AnyOf (2, 3, 4, 5))
    {
        std::cout << "Yes!" << std::endl;
    }
    else
    {
        std::cout << "No!" << std::endl;
    }

    return 0;
}

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