使用SFINAE在C ++中强制类型转换

Question

I am writing a class (BufferInserter) that transparently translates user defined messages to their network endian format and packs the result into a user-supplied buffer. Here is a simple example of a Message and its byte-swapped counterpart:

//Native message (not in network endian format)
struct Message
{
    short val;
    Message(short v): val(v){}
};

//Network endian format of Message
struct OtaMessage
{
    typedef Message NativeType;
    short val;

    operator Message() const
    {
        return Message(val >> 8 | val << 8); 
    }

    OtaMessage(const Message& m)
        : val(val >> 8 | val << 8)
    {}
};

Here is a highly simplified version of BufferInserter:

class BufferInserter
{
public:
    BufferInserter(char* buffer)
        :buf(buffer)
    {}

    template<typename T>
    char* insertStruct(T s, typename T::NativeType = 0)
    {
        const std::size_t size = sizeof(T);
        *reinterpret_cast<T*>(buf) = s;
        buf += size;
        return buf;
    }
private:
    char* buf;
};

The hope was a user could do something like:

Message m(1);
char buf[256];
BufferInserter ins(buf);
ins.insertStruct(m);

And the C++ type deduction machinery would skip passing the native Message to insertStruct because Message does not have a NativeType typedef, and instead would convert the Message to a OtaMessage. That's not what happens, instead I get the compiler error (g++4.7)

test.cpp:55:23: error: no matching function for call to ‘BufferInserter::insertStruct(Message&)’
     ins.insertStruct(m);
                       ^
test.cpp:55:23: note: candidate is:
test.cpp:34:11: note: template<class T> char* BufferInserter::insertStruct(T, typename T::NativeType)
     char* insertStruct(T s, typename T::NativeType = 0)
           ^
test.cpp:34:11: note:   template argument deduction/substitution failed:
test.cpp: In substitution of ‘template<class T> char* BufferInserter::insertStruct(T, typename T::NativeType) [with T = Message]’:
test.cpp:55:23:   required from here
test.cpp:34:11: error: no type named ‘NativeType’ in ‘struct Message’

There is a similar error for MSVC 2013 so its probably not a compiler bug.

Obviously this works:

Message m(1);
char buf[256];
BufferInserter ins(buf);
ins.insertStruct(OtaMessage(m));

But I wanted to avoid the user having to know about the byte swapping. I also could add a conversion operator to OtaMessage in the Message struct:

struct Message
{
    short val;
    Message(short v): val(v){}
    operator OtaMessage()
    {
       val = v<<8 | v>>8;
    }
};

There are 2 issues with this:

1. Again, I don't want the user to be aware of OtaMessage
2. OtaMessage is defined after Message so a Message-->OtaMessage is not possible. Interestingly after trying this in MSVC 2013, I crashed the compiler:

1>Source.cpp(74): fatal error C1001: An internal error has occurred in the compiler. 1> (compiler file 'msc1.cpp', line 1325) 1> To work around this problem, try simplifying or changing the program near the locations listed above.

Any help?

Answer 1

That's not how type deduction works. Perhaps you could implement a traits-like approach, letting BufferInserter::insertStruct<Message> be deduced, and then convert with your traits class. Some key pieces of it might look like this:

template <typename T>
struct OtaConverter {
  // using ota_type = T; -- don't provide a base ota_type
};

: : :

template <> struct OtaConverter<Message> {
  using ota_type = OtaMessage;
};

: : :

template<typename T>
char* BufferInserter::insertStruct(T s, typename OtaConverter<T>::ota_type* = 0)
{
    using OT = typename OtaConverter<T>::ota_type;
    const std::size_t size = sizeof(OT);
    *reinterpret_cast<OT*>(buf) = OT(s);
    buf += size;
    return buf;
}

You'll also need to fix your OtaMessage constructor as it currently does not reference m.val . For safety you should also mark your constructors explicit.

See this live example . Note that removing the specialization of OtaConverter currently results in intentional compilation errors.

Answer 2

I encounterd the same problem (in other circumstances) and I fixed it by updating my MSVC2013 Tools -> Extensions and Updates -> Updates -> Product Updates (error fixed since update 2 https://support.microsoft.com/fr-fr/kb/2927432 ).

Cheers,