How can I make a backwards-compatible version of declval?

Question

I have the following line of code that works on C++11 Red Hat Linux:

struct ConvertIfHasBrief< Obj, ObjResult, void_t< decltype( declval<ObjResult>().xData.xBrief ) > >

But I need this to also work on older version of C++ on an older version of Solaris. I'm having trouble figuring out what version of cc it has, but it's SunOS 5.9.

I've tried looking for alternative ways of writing this, but haven't found anything that even seems similar. Below is the whole file if it helps:

template< typename Ts >
struct make_void
{
    typedef void type;
};

template< typename Ts >
struct void_t
{
    using make_void<Ts>::type;
};

template< class Obj, class ObjResult, class = void >
struct ConvertIfHasBrief
{
    static auto Convert( Obj const &, ObjResult & ) -> CLStatus
    {
        return {}; // dummy value, not used
    }
};

template< class Obj, class ObjResult >
struct ConvertIfHasBrief< Obj, ObjResult, void_t< decltype( declval<ObjResult>().xData.xBrief ) > >
{
    static auto Convert( Obj const &xFrom, ObjResult &xTo ) -> CLStatus
    {
        return ::Convert( xFrom, xTo.xData.xBrief );
    }
};

template< class Obj, class ObjResult >
CLStatus convertObjToResult( const Obj & xFrom, ObjResult & xTo )
{
    CLStatus eStatus = CLSTATUS_SUCCESS;
    switch ( xTo.eType )
    {
        case CEPTFull:
            xTo.xData.xFull = xFrom;
            break;
        case CEPTBrief:
            eStatus = ConvertIfHasBrief< Obj, ObjResult >::Convert( xFrom, xTo );
            break;
        default:
            eStatus = CLSTATUS_INVALIDPROJECTIONTYPE;
    }
    return eStatus;
}

And for further reference, this is the question that this code resulted from: Can I have a single template with a variable object?

Can you use [`boost::declval`](http://www.boost.org/doc/libs/1_55_0/boost/utility/declval.hpp)? — NathanOliver, Jan 20 '16 at 20:05
If you don't have `std::declval`, you probably don't have rvalue references either. So all you need would be a function template declaration, no? — 5gon12eder, Jan 20 '16 at 20:07
[Check here](https://gcc.gnu.org/projects/cxx0x.html) for information that you will probably find quite relevant. Just check when each feature was added in GCC to determine if this is possible. — SirGuy, Jan 20 '16 at 20:10
@NathanOliver: No. `fatal error: boost/utility/declval.hpp: No such file or directory` — Jesse Jashinsky, Jan 20 '16 at 23:20
@5gon12eder: How would a function template declaration help? declval is being used to declare a variable, not a function. — Jesse Jashinsky, Jan 20 '16 at 23:56
@NathanOliver: No, that is not something I have authority over. — Jesse Jashinsky, Jan 21 '16 at 00:05
@JesseJ `std::declval` *is* a function. Or, more precisely, an undefined function template. — 5gon12eder, Jan 21 '16 at 13:59

How can I make a backwards-compatible version of declval?

0 Answers0