How to pass Strem object to my class

Question

I'm trying to creaper class wraper to use an Streamobject but I've an error when I try to compile. My .hfile :

#include <Arduino.h>
class TestSerialListener {
public:
  TestSerialListener(Stream &serial);
  void testPrint(void);
private:
  bool _listener_start;
  Stream &_serial;
void startListener(void);
};

my cppfile

#include "TestSerialListener.h"
#include <Arduino.h>
TestSerialListener::TestSerialListener(Stream &serial) : _serial(serial)
{
}
void TestSerialListener::testPrint(){
    _serial.begin(115200);      <---------- error compilation 
    _serial.println("test");
}
void TestSerialListener::startListener(void)
{
    if(!_listener_start){
        _listener_start = true;
    }
}

And when i try to compile it I got this error : error: 'class Stream' has no member named 'begin'Why I can't use begin()in my classe ?

The compiler said: “you can't because the Stream class has no method named begin”. Does this answer your question? — Edgar Bonet, Jan 30 '23 at 08:43
Ok but why I can do SerialUSB.begin(115200)in my sketch ? @EdgarBonet — simon, Jan 30 '23 at 08:44
Because, SerialUSB does have a method named begin. Some Stream-derived classes can have such a method, It's just not something you can expect from any Stream object. — Edgar Bonet, Jan 30 '23 at 08:48
see https://arduino.stackexchange.com/questions/78719/can-you-use-serial-port-as-a-variable/78725#78725 — Juraj, Jan 30 '23 at 08:55

score 2 · Accepted Answer · answered Jan 30 '23 at 09:07

the begin() method is defined in the HardwareSerial class, not in Stream. You can look that up yourself in your Arduino installation. For me these files are placed under ~/.arduino15/packages/arduino/hardware/avr/1.8.6/cores/arduino/ and are named Stream.h and HardwareSerial.h.

And if you think about it, this is quite logical. The Stream class can represent any object, that can stream data. It doesn't know anything about how exactly the data is streamed on a hardware level. It just provides the convenient functions for handling the data. The HardwareSerial class then derives from Stream to get those data handling methods and adds its own methods to handle the hardware side of the problem, including setting a baudrate.

When providing a Stream reference to a class constructor you want to begin() the corresponding object outside of the class, before you start to stream any data through it. Doing it this way makes your class widely usable, since you could use any Stream object, even a file on an SD card.

So move the begin() line from your class to the setup() function in your main code.

For reference: Juraj showed the inheritance between the classes in his answer to this question. Have a look at it, since it is quite enlightening.

How to pass Strem object to my class

1 Answers1