使用强类型枚举对类型和子类型进行建模

我想将ICMP标头定义为pod类型：

struct ICMPHeader
{
    uint8_t   Type;         // ICMP type
    uint8_t   Code;         // Subtype, value is dependent on ICMP type.
    uint16_t  Checksum;     // Error checking data. See RFC 1071
    uint32_t  RestOfHeader; // Varies based on ICMP type and code.
};

对于ICMPType字段，我可以使用强类型枚举来使其更好一点：

enum class ICMPType : uint8_t
{
    EchoReply              = 0,
    Reserved1              = 1,
    Reserved2              = 2,
    DestinationUnreachable = 3,
    SourceQuench           = 4
    // etc...
};
struct ICMPHeader
{
    ICMPType  Type;         // ICMP type
    uint8_t   Code;         // Subtype, value is dependent on ICMP type.
    uint16_t  Checksum;     // Error checking data. See RFC 1071
    uint32_t  RestOfHeader; // Varies based on ICMP type and code.
};

现在，我自然也想将Code字段指定为枚举。如果我可以使用模板专用化语法，那就太好了，但快速测试显示它不起作用：

// Compiler error
template<ICMPType>
enum class ICMPCode;    
template<>
enum class ICMPCode<ICMPType::DestinationUnreachable>
{
    DestinationNetworkUnreachable  = 0,
    DestinationHostUnreachable     = 1,
    DestinationProtocolUnreachable = 2
};

一种选择是将它们包装在结构中：

// Meaning of ICMP code is dependent on ICMP type.
template<ICMPType>
struct ICMPCode;
// Subcodes for DestinationUnreachable
template<> struct ICMPCode<ICMPType::DestinationUnreachable>
{
    enum class Code : uint8_t
    {
        DestinationNetworkUnreachable  = 0,
        DestinationHostUnreachable     = 1,
        DestinationProtocolUnreachable = 2
        // etc...
    };
};
// Access: ICMPCode<ICMPType::DestinationUnreachable>::Code::DestinationHostUnreachable

但是这样做让我觉得我只是在胡闹，把事情弄得太复杂了。

我想这是一个更普遍的问题的具体例子：如何设置类型和子类型的系统？有什么建议吗？

附言：

• 另请参阅：http://en.wikipedia.org/wiki/Internet_Control_Message_Protocol#Header

示例代码：

#include <iostream>
// Trying to model ICMP types and codes with strongly typed enums
// See also http://en.wikipedia.org/wiki/Internet_Control_Message_Protocol#Header

enum class ICMPType : uint8_t
{
    EchoReply              = 0,
    Reserved1              = 1,
    Reserved2              = 2,
    DestinationUnreachable = 3,
    SourceQuench           = 4
    // etc...
};

// Meaning of ICMP code is dependent on ICMP type.
template<ICMPType>
struct ICMPCode;

// Subcodes for DestinationUnreachable
template<> struct ICMPCode<ICMPType::DestinationUnreachable>
{
    enum class Code : uint8_t
    {
        DestinationNetworkUnreachable  = 0,
        DestinationHostUnreachable     = 1,
        DestinationProtocolUnreachable = 2
        // etc...
    };
};

ICMPCode<ICMPType::DestinationUnreachable>::Code GetReasonWhyDestinationIsUnreachable()
{
    return ICMPCode<ICMPType::DestinationUnreachable>::Code::DestinationHostUnreachable;
}

int main()
{
    std::cout << static_cast<int>(GetReasonWhyDestinationIsUnreachable()) << std::endl;
}