Can I have a single template with a variable object? - c++

Question

I currently have the following code:
template< class Obj, class ObjResult >
CLStatus convertObjToResult2( const Obj & xFrom, ObjResult & xTo )
{
CLStatus eStatus = CLSTATUS_SUCCESS;
switch ( xTo.eType )
{
case CEPTFull:
xTo.xData.xFull = xFrom;
break;
case CEPTBrief:
eStatus = Convert( xFrom, xTo.xData.xBrief );
break;
default:
eStatus = CLSTATUS_INVALIDPROJECTIONTYPE;
}
return eStatus;
}
template< class Obj, class ObjResult >
CLStatus convertObjToResult1( const Obj & xFrom, ObjResult & xTo )
{
CLStatus eStatus = CLSTATUS_SUCCESS;
switch ( xTo.eType )
{
case CEPTFull:
xTo.xData.xFull = xFrom;
break;
default:
eStatus = CLSTATUS_INVALIDPROJECTIONTYPE;
}
return eStatus;
}
All ObjResults have an xFull, but only some have an xBrief, where xData is a union. This resulted in me writing the two different templates above, but it would be great if I could somehow have just one template.
I can't simply use convertObjToResult2, since it will fail to compile with object types that do not have an xBrief. I looked at this answer to see if it would help, but I don't understand at all what it's doing.

Answer 1

Since C++ does not and won't have a static if feature, you need a work around.
If you can't overload Convert as you said in comments, I thought of a layer above it which can be specialized depending on if ObjResult has the member or not.
template <class Obj, class ObjResult, class = void>
struct ConvertIfHasBrief {
static auto Convert(Obj const &, ObjResult &) -> CLStatus {
return {};// dymmy value, not used
}
};
template <class Obj, class ObjResult>
struct ConvertIfHasBrief <Obj, ObjResult,
std::void_t<decltype(std::declval<ObjResult>().xData.xBrief)>> {
static auto Convert(Obj const &xFrom, ObjResult &xTo) {
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;
}
std::void_t is not yet part of the standard, but the implementation is simple and can be found on the linked page. Just be sure not to declare it on the std namespace.
proof it works