Preprocessor tricks to detect library include in .ino

Projects Programming
1

I have the following library (I just add the files to my sketch folder rather than stick them in a separate library)

I have constructors like the following:

CTime(RealTimeClockDS1307& rtc);

However there might be some sketches where I don't won't to use #include <RealTimeClockDS1307.h>

Are there any pre-processor tricks to detect whether RealTimeClockDS1307.h has been included in the .ino?

So that these constrcutors are only compiled if that library has been included in the sketch.

Of course I could just comment them out if I don't wish to use RealTimeClockDS1307.h.

But it would be rather convenient if there was a way not to have to do this manually.

DateTime.cpp (16.7 KB)

DateTime.h (6.56 KB)

2

Most well designed libraries will define an include guard macro in the header file. Something like this:

#ifndef RealTimeClockDS1307_h
#define RealTimeClockDS1307_h

If so, you can use that macro in your sketch to detect whether the the file has been included.

3

pert:
Most well designed libraries will define an include guard macro in the header file. Something like this:

#ifndef RealTimeClockDS1307_h

#define RealTimeClockDS1307_h



If so, you can use that macro in your sketch to detect whether the the file has been included.

That did occur to me but is RealTimeClockDS1307_h 'visible' in DateTime.cpp/h from the .ino?

Or only if I include RealTimeClockDS1307.h in DateTime.cpp/h?

It must be the latter because if I don't include RealTimeClockDS1307.h then I get compile errors.

I was thinking there might be other more sophisticated preprocessor constructs that I have not yet encountered.

To date I have only ever used the very common preporcessor constructs.

But I know there is a lot more available.

4

Does Arduino support #pragma?

Just read that, if the compiler does not support it, then you don't get any compiler error.

5

Well this was totally unexpected....

Just did a bit of an experiment.

If I put this in datetime.cpp:

#ifdef RealTimeClockDS1307_h
XXXXXX
#endif

And removed #include RealTimeClockDS1307.h

I get a compile error for XXXXXXX, so #define RealTimeClockDS1307_h from the .ini carries over into datetime.cpp.

But I still get a compile error at the point I try to use class RealTimeClockDS1307

6

boylesg:
so #define RealTimeClockDS1307_h from the .ini carries over into datetime.cpp.

Any file that does '#include RealTimeClockDS1307.h' will have the 'RealTimeClockDS1307_h' macro defined. That's the whole point of it.

Put your .cpp file back the way it was and put this in your .ino file:

#ifdef RealTimeClockDS1307_h
CTime(RealTimeClockDS1307& rtc);
#endif

If

#include RealTimeClockDS1307.h

preceeds that, then the code within will be compiled. I'm thinking you need to define the rtc variable also, so maybe it should be:

#ifdef RealTimeClockDS1307_h
RealTimeClockDS1307 rtc;
CTime(RealTimeClockDS1307& rtc);
#endif
7

boylesg:
Does Arduino support #pragma?

Yes, but it's recommended to use a standard include guard instead of #pragma once. The only thing I've used #pragma for is #pragma message to display a message for the user in the compilation output, rather than using #warning for that purpose.

8

I don't understand your question. Are there cases when you don't want to #include <RealTimeClockDS1307.h>? If so, can't you leave it to the programmer to include that header in the .ino file?

comments:

char strDayMonthName[10] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0};

const char* getMonthName(const uint8_t nMonthNum, const bool bShort)
{
  memset(strDayMonthName, 0, 10);

  if (nMonthNum == 1)
    strcpy(strDayMonthName, "January");
  else if (nMonthNum == 2)
    strcpy(strDayMonthName, "February");
  else if (nMonthNum == 3)
    strcpy(strDayMonthName, "March");
  else if (nMonthNum == 4)
    strcpy(strDayMonthName, "April");
  else if (nMonthNum == 5)
    strcpy(strDayMonthName, "May");
  else if (nMonthNum == 6)
    strcpy(strDayMonthName, "June");
  else if (nMonthNum == 7)
    strcpy(strDayMonthName, "July");
  else if (nMonthNum == 7)
    strcpy(strDayMonthName, "August");
  else if (nMonthNum == 7)
    strcpy(strDayMonthName, "September");
  else if (nMonthNum == 7)
    strcpy(strDayMonthName, "October");
  else if (nMonthNum == 7)
    strcpy(strDayMonthName, "November");
  else if (nMonthNum == 7)
    strcpy(strDayMonthName, "December");
  else
    strcpy(strDayMonthName, "Invalid day number");

  if ((strlen(strDayMonthName) > 3) && bShort)
    strDayMonthName[3] = 0;

  return strDayMonthName;
}

how does "Invalid day number" fit into a 10 element char array?

you should use strcpy's safer cousin: strncpy

actually, why build the string to return? you are wasting memory using that strDayMonthName buffer

why are you not using arrays? Just return a pointer to the string constant held in an arry...

9

Take a look at this link , my post #3

https://forum.arduino.cc/index.php?topic=530472.0

I did try my best, but basically each .h/.cpp pair gets compiled to object files first so each pair needs to have its own and correct #includes.

Standard #ifndef #includes guard or #pragma is still required if the #includes headers /cpp have multiple entries so the .h /.cpp pair uses them only once.