c++ - 依赖于 char 符号的专用结构模板

Question

我有一个函数 template<typename T> void frobnicate()做的东西。我需要 T 成为少数几个选择类型之一，并且我需要一些关于这些类型的信息。我通过提供特征来做到这一点:

template<typename T_Raw>
struct FrobnicationTraits
{
        static constexpr bool isValidFrobnicationType = false;
};

template<>
struct FrobnicationTraits<unsigned char>
{
        static constexpr bool isValidFrobnicationType = true;
        static constexpr bool isUnsigned = true;
        static constexpr unsigned char minValue = 0;
        static constexpr unsigned char maxValue = 255;
        //Elided in each specialization: some other values that are needed by frobnicate()
};

template<>
struct FrobnicationTraits<signed char>
{
        static constexpr bool isValidFrobnicationType = true;
        static constexpr bool isUnsigned = false;
        static constexpr signed char minValue = -128;
        static constexpr signed char maxValue =  127;
};

//Elided: versions for unsigned short and signed short

然后:

template<typename T, typename = typename std::enable_if<FrobnicationTraits<T>::isValidFrobnicationType>::type>
void frobnicate()
{
    using Traits = FrobnicationTraits<T>;
    auto promotedMinValue = +Traits::minValue;
    auto promotedMaxValue = +Traits::maxValue;
    std::cout << "Frobnicating with " << (Traits::isUnsigned ? "unsigned" : "  signed") << " type of size " << sizeof(T) <<" between " << promotedMinValue << " and " << promotedMaxValue << std::endl;
    //...
}

这适用于这些调用:

int main()
{
    frobnicate<unsigned char>();  // Prints "Frobnicating with unsigned type of size 1 between 0 and 255"
    frobnicate<  signed char>();  // Prints "Frobnicating with   signed type of size 1 between -128 and 127"
    frobnicate<unsigned short>(); // Prints "Frobnicating with unsigned type of size 2 between 0 and 65535"
    frobnicate<  signed short>(); // Prints "Frobnicating with   signed type of size 2 between -32768 and 32767"
    //frobnicate<unsigned int>(); // Correctly fails to compile, because the enable_if is not satisfied, since FrobnicationTraits<unsigned int> uses the default FrobnicationTraits where isValidFrobnicationType is false
    return 0;
}

但现在我希望能够正确 frobnicate<char>() .
char被实现定义为有符号或无符号，但它总是被认为是与 unsigned char 不同的类型和 signed char .所以只需调用 frobnicate<char>()无法编译，因为它不会选择任何专门的 FrobnicationTraits。
我可以手动添加一个专业:

template<>
struct FrobnicationTraits<char>
{
        static constexpr bool isValidFrobnicationType = true;
        static constexpr bool isUnsigned = true;
        static constexpr char minValue = 0;
        static constexpr char maxValue = 255;
};

但我无法提前知道这是否正确。事实上，我碰巧选择了错误的值，所以这会打印“Frobnicating with unsigned type of size 1 between 0 and -1”。

我想有效地做到这一点:

template<> // I don't know where to put the `std::enable_if<>` here, but use this if char is unsigned
struct FrobnicationTraits<char>
{
        static constexpr bool isValidFrobnicationType = true;
        static constexpr bool isUnsigned = true;
        static constexpr char minValue = 0;
        static constexpr char maxValue = 255;
};

template<> // I don't know where to put the `std::enable_if<>` here, but use this if char is signed
struct FrobnicationTraits<char>
{
        static constexpr bool isValidFrobnicationType = true;
        static constexpr bool isUnsigned = false;
        static constexpr char minValue = -128;
        static constexpr char maxValue =  127;
};

理想情况下，解决方案不应使用预处理器。

Answer 1

有 std::is_signed 可以告诉你char签名与否。如果不需要区分char和 signed char/unsigned char您可以通过 std::conditional 从其中任何一个继承来重用这些特征:

template<>
struct FrobnicationTraits<char> : FrobnicationTraits<
                                      std::conditional_t<         
                                          std::is_signed_v<char>, 
                                          signed char,
                                          unsigned char
                                      >
                                  >
{};

也许使用帮助器:

using signed_or_unsigned_char = std::conditional_t<std::is_signed_v<char>,signed char, unsigned char>;

然后继承FrobnicationTraits<char>来自 FrobnicationTraits<signed_or_unsigned_char> .

PS:请注意， std::numerical_limits 已经涵盖了您的一些特征功能。 .如果您使用 std::is_signed直接来说，你在 trait 中真正需要的只是 isValidFrobnicationType .