templates.cpp

template <HardwareSerial* T>
class SerialWriter
{
public:
    void        writeMessage(unsigned char *message, int length);
};

template <HardwareSerial* T>
void SerialWriter<T>::writeMessage(unsigned char *message, int length) {
    T->write(SafeSerial::MESSAGE_START);
    for (int ix = 0; ix < length; ++ix) {
        byte b = message[ix];
        if (b == SafeSerial::MESSAGE_START || b == SafeSerial::MESSAGE_END || b == SafeSerial::MESSAGE_ESCAPE) {
            T->write(SafeSerial::MESSAGE_ESCAPE);
            T->write(SafeSerial::MESSAGE_XOR ^ b);
        } else {
            T->write(b);
        }
    }
    T->write(SafeSerial::MESSAGE_END);
}

SerialWriter<&SERIAL> writer;
unsigned char inBuffer[BUFFER_SIZE];

void main() {
  // Error: undefined reference to `SerialWriter<&Serial>::writeMessage(unsigned char*, int)'
  writer.writeMessage(inBuffer, 10);
}