I'm creating a library driver for AdaFruit's GFX library. During Constructor initialization, all the variables (set in another .cpp) are returned as zero. This code has several modular files to separate graphics functions from graphics primitives from device interfacing. What am I doing wrong?

In the Arduino sketch I call the class constructor: MB_GFX_SPI (in file MB_GFX_SPI.cpp). In the Constructor I'm call a function: mbR.SetMatrixSize (in another file raster.cpp). Also in the Constructor I call a function: clearScreen (in the same file MB_GFX_SPI.cpp).

The test variable mbR.r_totNumChannels set in clearScreen is returned with 99. All the variables set in mbR.SetMatrixSize return zero.

//Test Sketch
//Initialize both the Adafruit library and the current library.
//Instantiate an object mbGFX so all Adafruit GFX and MB_GFX add-on functions are addressed as mbGFX.xxx(); 
MB_GFX_SPI mbGFX = MB_GFX_SPI(Period, numCathodeLeds, numAnodeLeds, CathodeSSPin, AnodeSSPin);

//In file MB_GFX_SPI.cpp

MB_GFX_SPI::MB_GFX_SPI(int Period, int numCathodeLEDS, int numAnodeLEDS,
      int CathodeSSPin, int AnodeSSPin) 
      : Adafruit_GFX(numCathodeLEDS, numAnodeLEDS) {

  mbR.SetMatrixSize(numCathodeLEDS, numAnodeLEDS);
void MB_GFX_SPI::clearScreen(uint16_t color) {
   mbR.r_totNumChannels = 99;   //test set value

//In file mb_raster.cpp

void mbRasterClass::SetMatrixSize(int numCathodeLeds, int numAnodeLeds){
//cathodes require 1 byte per channel
//anodes require 1 bit per channel

    int numAnodeChannels = numAnodeLeds;
    int numCathodeChannels = mbX.pin2pin * numCathodeLeds; //times 1 or 3   

    r_numCathodeRegisters = (int)((numCathodeChannels+7)/8);   //select next higher number of registers
    r_numCathodes = 8 * r_numCathodeRegisters;  //must be whole 8 bit registers - is every single chatode pin
    r_numAnodeRegisters = (numAnodeChannels+7)/8;       //round up to next register number
    r_numAnodes = 8 * r_numAnodeRegisters; //must be whole 8 bit registers
    r_PWMBufferSize = r_totNumChannels = r_numAnodes * r_numCathodes;

    r_PWMBuffer = (uint8_t *) realloc(r_PWMBuffer, r_totNumChannels); //resize array for PWMBuffer
  • 1
    can you give a function that shows what isn't working in code? something you would expect to do one thing, but isn't working out that way? I'm not certain I know what you mean by "the variables return zero". Also some more information about the members of an MB_GFX_SPI, and where mbR comes from. Commented Mar 9, 2015 at 2:49
  • 1
    Try initializing mbGFX with its regular constructor (GFX_SPI mbGFX(...);) instead of using copy-initialization (GFX_SPI mbGFX = MB_GFX_SPI(...);). Commented Mar 9, 2015 at 8:52

1 Answer 1


First, I would do what Edgar Bonet suggested, call the normal constructor:

MB_GFX_SPI mbGFX (Period, numCathodeLeds, numAnodeLeds, CathodeSSPin, AnodeSSPin);

Second, I'm not keen on what you are doing in the constructor for a variable which appears to be at global scope (mbGFX).

See What's the "static initialization order fiasco"?

Basically you cannot assume that constructors in different compilation units are called in any defined sequence, you also cannot assume that the system has been initialized yet (ie. init() called).

Thus, things that address the hardware (eg. clearScreen(0);) may not work properly, or at all.

  • 1
    thanks for additional suggestions. I've recently installed Visual Studio+Visual Micro and have been tracing the initialization process. Commented Aug 23, 2015 at 14:36

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