I'm working on a proof of concept for using the Adafruit_Thermal printer library with an ATTiny85.

The ATTiny85 does not come with hardware serial capability, so I started looking into using SoftwareSerial to create the serial port necessary. I started down this path and ran into an error in the thermal library's constructor:

// thermal-printer.ino example file

#include "SoftwareSerial.h"
#define TX_PIN 3
#define RX_PIN 4 


SoftwareSerial mySerial(RX_PIN, TX_PIN);


#include "Adafruit_Thermal.h"

Adafruit_Thermal printer(&mySerial); // Pass addr to printer constructor  

// Adafruit_Thermal.h

class Adafruit_Thermal : public Print
{

public:
    // IMPORTANT: constructor syntax has changed from prior versions
    // of this library.  Please see notes in the example code!

    Adafruit_Thermal(Stream *s = &Serial, uint8_t dtr = 255);

Documents/Arduino/libraries/Adafruit_Thermal_Printer_Library/Adafruit_Thermal.h:125:33: error: 'Serial' was not declared in this scope Adafruit_Thermal(Stream *s = &Serial, uint8_t dtr = 255);

If I understand correctly, the verification process is failing because the compiler is looking for a global Serial value in the Adafruit_Thermal.hfile that doesn't exist because the ATTiny85 does not have HardwareSerial.

But, going by the Adafruit_Thermal class' constructor signature the reference passing of Serial is just a default value (right?) so I should be able to remove it and pass in my software serial reference. The SoftwareSerial class inherits from Stream, so it should still be a valid reference pass:

// this:
Adafruit_Thermal(Stream *s = &Serial, uint8_t dtr = 255);

// changes to this:
Adafruit_Thermal(Stream *s, uint8_t dtr = 255);

But when I do that, I start getting verification errors that make it look like the thermal instance wasn't constructed, like:

undefined reference to `Adafruit_Thermal::begin(unsigned char)'

and

undefined reference to `Adafruit_Thermal::Adafruit_Thermal(Stream*, unsigned char)'

Does this mean the class isn't getting constructed?

I looked at the underlying constructor definition to see if something was blowing up in there, but there doesn't seem to be anything fancy at the constructor level:

// Constructor
Adafruit_Thermal::Adafruit_Thermal(Stream *s, uint8_t dtr) :
  stream(s), dtrPin(dtr) {
  dtrEnabled = false;
}

This should still work with the software serial reference pass, right?

I'm a bit lost on how to debug this now. Any ideas?

Update

After a reply from Majenko I pulled up the cpp file and looked at the header reference:

So this means the cpp file was still referring to the include path header file instead of the lib version?

I updated my cpp file to use:

#include "lib/Adafruit_Thermal.h"

and I still get undefined reference errors, so it may still be something else, but I wanted to share this additional info.