Template type argument `T` where `T*` expands to `nullptr

What I'm looking for is basically this:

template <typename T> 
struct a { 
    using pointer_type = T*;
};

What I want is such X so that a<X>::pointer_type evaluates to nullptr_t. Is this possible?

Edit: This is what I actually need (the pointer_type is hidden in the signatures of the ENCODER and DECODER template arguments as MSG *)

template <int MSGNUM,
    typename MSG,
    int(&ENCODER)(const MsgHeader* pHeader, const MSG* pMessage, unsigned char* destBuf, int destBufSize),
    int(&DECODER)(const MsgHeader* pHeader, const unsigned char* msgBody, int msgBodyLen, MSG* decodedMsg)>
struct msgid {
    using msg_type = MSG;
    static constexpr int msgnum = MSGNUM;
    static constexpr auto encoder = ENCODER;
    static constexpr auto decoder = DECODER;
};

using MSG1 = msgid<1,msg1struct,encodeMsg1,decodeMsg1>;
using MSG2 = msgid<2,msg2struct,encodeMsg2,decodeMsg2>;
using HDRONLY= msgid<-1,X,encodeHdr,decodeHdr>;

HDRONLY would have to accept a nullptr where the decoded msg structure is used.

CodePudding user response：

std::nullptr_t is not a pointer type. It's a type with an implicit conversion to any pointer type.

You could have a specialisation:

template <> struct a<std::nullptr_t> { using pointer_type = std::nullptr_t; };

CodePudding user response：

You could also use std::conditional from the <type_traits> header:

#include <type_traits>

// ...

template <typename T> 
struct a { 
    using pointer_type = typename std::conditional<std::is_same<T, std::nullptr_t>::value,
                             std::nullptr_t,
                             T*
                         >::type;
};

CodePudding user response：

Thanks to @Ruks this is what I came up with:

template <int MSGNUM,
    typename MSG,
    int(&ENCODER)(const MsgHeader* pHeader, typename std::conditional<std::is_same<MSG, std::nullptr_t>::value, std::nullptr_t, const MSG *>::type pMessage, unsigned char* destBuf, int destBufSize),
    int(&DECODER)(const MsgHeader* pHeader, const unsigned char* msgBody, int msgBodyLen, typename std::conditional<std::is_same<MSG, std::nullptr_t>::value, std::nullptr_t, MSG *>::type decodedMsg)>
struct msgid {
    using msg_type = MSG;
    static constexpr int msgnum = MSGNUM;
    static constexpr auto encoder = ENCODER;
    static constexpr auto decoder = DECODER;
};