Arduino IDE Initialize char[] in a separate file

Question

I have a large char[] that contains a bitmap font that I use to display numbers on an OLED display. The initialization of the array is at the beginning of my cpp file which just makes the file harder to read. I have to believe that there is a way to define this array in a separate file, but I am out of ideas.

I have an Arduino ino file and I have left the ino file mostly blank so that there is less chance of the arduino software being confused:

#include <SPI.h>
#include <Adafruit_SSD1306ms.h>
#include <Wire.h>
#include <Adafruit_GFX.h>
#include <RFduinoBLE.h>

I then have a new tab font.cpp that contains the font definition array:

#include <Arduino.h>

extern const unsigned char font[] PROGMEM = {
  B00000000, B00000000, B00000000, B00000000, B00000000,
  B00000000, B00000000, B00001111, B11110000, B00000000,
  B00000000, B00000000, B01111111, B11111110, B00000000,
  B00000000, B00000000, B11111111, B11111111, B00000000,
 ...
  B00000000, B00000000, B00000000, B00000000, B00000000
};

I then have another tab oled.cpp that contains the code for displaying the numbers on the display:

#include <SPI.h>
#include <Adafruit_SSD1306ms.h>
#include <Wire.h>
#include <Adafruit_GFX.h>
#include <RFduinoBLE.h>

const unsigned char font[];

void DrawNumber(char aValue[], uint size) {
 // draws the proper numbers on the display using the font
}

void setup() {
}
void loop {
}

The output from the compiler looks as follows:

Arduino: 1.6.6 (Windows 7), Board: "RFduino"

oled.cpp:7: error: uninitialized const 'font' [-fpermissive]
 const unsigned char font[];

                     ^
oled.cpp:7: error: storage size of 'font' isn't known
 const unsigned char font[];
                         ^
exit status 1
uninitialized const 'font' [-fpermissive]

  This report would have more information with
  "Show verbose output during compilation"
  enabled in File > Preferences.

When the code is all in the ino file, it works fine, but I prefer not to have the huge font bitmap char[] in the same file with the code for readability reasons.

I believe that I could create a library with the font, but is that the best way to solve this problem?

Answer 1

Try adding extern before

const unsigned char font[];

in oled.cpp

Without extern , the compiler try to define and allocate buffer for the variable. Then, not knowing how much memory should be allocated to the incomplete type, it fails.

Answer 2

put

extern const unsigned char font[];

in a header file and include it from both places instead of putting it everywhere you use it. This would prevent misalignment between declaration and definition.