Question

I was assigned the task of updating a very old project a while back. The first thing I had to do was to expand the existing code to incorporate a new feature. As part of this I modified existing macros to print JSON representations of incoming messages (over CORBA, into C++ structs). I then incorporated boost program_options and a new logger and now I want to modernise the macros.

The problem is that I have no idea how to implement what I did with the macros with templates. The key problem is that I use the name of the parameters to the macros to access the fields of the struct:

//Defines the string that precedes the variable name in a JSON name-value pair (newline,indent,")
#define JSON_PRE_VNAME      _T("%s,\n\t\t\t\t\"")
//Defines the string that follows the variable name in a JSON name-value pair (":) preceding the value
#define JSON_SEP            _T("\":")
#define printHex(Y,X)       _tprintf(_T("%02X"), (unsigned char)##Y->##X );

// ******** MACRO **********
// printParam (StructureFieldName=X, ParamType=Y)
// prints out a json key value pair.
// e.g. printParam(AgentId, %s) will print "AgentId":"3910"
// e.g. printParam(TempAgent, %d) will print "TempAgent":1

#define printParam(X,Y)         if(strcmp(#Y,"%s")==0){\
                                    _byteCount += _stprintf(_logBuf,JSON_PRE_VNAME _T(#X) JSON_SEP _T("\"%s\""),_logBuf,myEvent->##X);\
                                }else{\
                                    _byteCount += _stprintf(_logBuf,JSON_PRE_VNAME _T(#X) JSON_SEP _T(#Y),_logBuf,myEvent->##X);\
                                }\
                                printBufToLog();

And it is used like this:

//CORBA EVENT AS STRUCT "event"
else if(event.type == NI_eventSendInformationToHost ){
    evSendInformationToHost *myEvent;
    event.data >>= myEvent;  //demarshall
    printParam(EventTime,%d);
    printParam(Id,%d);
    printParam(NodeId,%d);
}

and this results in JSON like this:

“EventTime”:1299239194,
“Id”:1234567,
“NodeId”:3

etc…

Obviously I have commented these macros fairly well, but I am hoping that for the sake of anyone else looking at the code that there is a nice way to achieve the same result with templates. I have to say the macros do make it very easy to add new events to the message logger.

Basically how do I do “#X” and ##X with templates?

Any pointers would be appreciated.

Thanks!

Answer 1

There are some things that you cannot really do without macros, and for some specific contexts macros are the solution. I would just leave the macros as they are and move on to the next task.

Well, I would actually try to improve a bit the macros. It is usually recommended not to use ; inside macros, and with macros that contain more than a single statement wrap them in do {} while(0) loops:

#define printHex(Y,X)       _tprintf(_T("%02X"), (unsigned char)##Y->##X ) 
//                                                               remove ; ^

// add do while here:
#define printParam(X,Y)         do { if(strcmp(#Y,"%s")==0){\
                                    _byteCount += _stprintf(_logBuf,JSON_PRE_VNAME _T(#X) JSON_SEP _T("\"%s\""),_logBuf,myEvent->##X);\
                                }else{\
                                    _byteCount += _stprintf(_logBuf,JSON_PRE_VNAME _T(#X) JSON_SEP _T(#Y),_logBuf,myEvent->##X);\
                                }\
                                printBufToLog();\
                                } while (false)

This might help avoid small mistakes that would otherwise be hard to fix, as, for example uses of the macros with if:

if (condition) printHex(a,b);
else printHex(c,d);

// looks good, but would originally expand to a syntax error:
if (condition) _tprintf(_T("%02X"), (unsigned char)##Y->##X );;
else ...

Similarly

if (condition) printParam(a,b);
else ... 

would expand to a whole lot of non-sense for the compiler even if it looks correct enough to the casual eye.