Go Down

Topic: How to control the #define in a library? I mean... (Read 1 time) previous topic - next topic


It's hard to describe in general terms so I'll use an example:

In a library I have a buffer. Its size should be the same as the width of the lcd. I have in the .h file:
Code: [Select]
#define phi_prompt_max_item_display_length 20

In the .cpp file:
Code: [Select]
char buffer[phi_prompt_max_item_display_length];

If someone uses a 40x2 LCD, I want to define phi_prompt_max_item_display_length as 40. I want the arduino sketch to be able to override the define in .h

I tried this in .h:
Code: [Select]
#ifndef phi_prompt_max_item_display_length      ///< Only define this if it has not been defined.
#define phi_prompt_max_item_display_length 20   ///< The render_list uses a buffer to render list item. 20 works for up to 20x4 displays to save SRAM.

I then have in arduino sketch #define phi_prompt_max_item_display_length 40 before it includes the .h
I get 40 in arduino sketch but I still get 20 in the library.

So are the define spaces separate between different .cpp files? How to do what I want to do? Thank you!


I'm not certain that you can do that given the way the Arduino build system works.

However if your library creates C++ classes, you could template the class, supplying the width of the LCD display as a template parameter.



You can add both - rows and colums - of the LCD as parameters of the constructor and malloc() the array.

Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)

Jack Christensen

MCP79411/12 RTC ... "One Million Ohms" ATtiny kit ... available at http://www.tindie.com/stores/JChristensen/


Evidently all great minds run in the same ruts XD


Thank you Jack! Seems like there is no simple solution as both of us initially experimented. I gather, every .cpp compiles separately. Without a make mechanism in arduino, such as define xxx as yyy in the make file, the only way to communicate with another .cpp is at execution time. I can add another parameter to my lib initialization to take a max_item_length parameter or just dare my lib users to change this define in the .h file.


Without a make mechanism in arduino, such as define xxx as yyy in the make file, the only way to communicate with another .cpp is at execution time.

Not so. C++ templates offer a compile time link.




I am not familiar with the term C++ templates. I'm reading wiki and yes the concept does sound familiar after some reading. I need to read on. Would you be willing to show me a line or two pseudo code on passing a parameter to a template? Thanks.



Does this help:

Code: [Select]
template <int length>
class Test
  int buffer[length];

void setup()
  Test<20> shortLine;
  Test<40> longLine;

void loop()



Go Up