C++ referencing instances created within a function's scope - c++

Question

Context
The context of the problem is that I am currently writing a small library for use with the Arduino in order to act as a game controller. The problem I am encountering has more to do with C++ than anything Arduino specific however.
I've included the libraries' header and source code below, followed by the Arduino code. I've truncated it where possible.
Problem
In short, only the last switch / action I define actually gets properly handles.
These actions get defined in the Arduino setup function. For example:
controller.addSwitchContinuous(10, 0); // Pin 10; btn index 0
means that pin 10 gets mapped to button 0. When pin 10 is switched closed this is treated as the button being pressed. This works fine for a single action but when I start adding more only the last action actually works. So in the following example only pin 9 is recognized:
controller.addSwitchContinuous(10, 0); // <-- Doesn't work
controller.addSwitchContinuous(9, 1); // <-- Works
This goes for any arbitrary number of actions:
controller.addSwitchContinuous(10, 0); // <-- Doesn't work
controller.addSwitchContinuous(9, 1); // <-- Doesn't work
controller.addSwitchContinuous(8, 2); // <-- Doesn't work
controller.addSwitchContinuous(7, 3); // <-- Works
Potential causes
I am fairly novice with C++ so this I suspect I'm doing something wrong with pointers. More specifically, something seems wrong with how the Joystick_ instance gets passed around.
I have been fiddling with the constructor and trying to use references instead of pointers but I couldn't get it to work properly.
I can confirm the iteration in JFSF::loop does iterate over all actions, if I modify it with:
void JFSF::loop()
{
for (int n = 0; n < _nextActionIndex; n++)
{
if (_actions[n])
{
_actions[n]->loop();
_joystick->setButton(n, PRESSED); // Debug: Set button pressed, regardless of switch.
}
}
if (_doSendState)
{
_joystick->sendState();
}
}
then buttons 0 through n get pressed as expected. It is possible that loop() isn't properly being called, but I would expect it to fail for the N = 1 case as well in that case. Furthermore the fact the last action always succeeds would suggest the iteration is ok.
Full code
// JFSF.h
#ifndef JFSF_h
#define JFSF_h
// ... include for Arduino.h and Joystick.h; bunch of defines
namespace JFSF_PRIV
{
class AbstractAction
{
public:
virtual void loop();
};
/* A Switch that essentially acts as a push button. */
class SwitchContinuousAction : public AbstractAction
{
public:
SwitchContinuousAction(Joystick_ *joystick, int pin, int btnIndex);
void loop();
private:
Joystick_ *_joystick;
int _pin;
int _btnIndex;
};
} // namespace JFSF_PRIV
class JFSF
{
public:
JFSF(Joystick_ *joystick, bool doSendState); // doSendState should be true if Joystick_ does not auto send state.
void loop();
void addSwitchContinuous(int inputPin, int btnIndex);
private:
Joystick_ *_joystick;
JFSF_PRIV::AbstractAction *_actions[MAX_ACTIONS];
int _nextActionIndex;
bool _doSendState;
};
#endif
Source file (trimmed):
// JFSF.cpp
#include "Arduino.h"
#include "Joystick.h"
#include "JFSF.h"
#define PRESSED 1
#define RELEASED 0
// Private classes
namespace JFSF_PRIV
{
SwitchContinuousAction::SwitchContinuousAction(Joystick_ *joystick, int pin, int btnIndex)
{
_joystick = joystick;
_pin = pin;
_btnIndex = btnIndex;
pinMode(_pin, INPUT_PULLUP);
}
void SwitchContinuousAction::loop()
{
int _state = digitalRead(_pin) == LOW ? PRESSED : RELEASED;
_joystick->setButton(_btnIndex, _state);
}
} // namespace JFSF_PRIV
JFSF::JFSF(Joystick_ *joystick, bool doSendState)
{
_joystick = joystick;
_nextActionIndex = 0;
_doSendState = doSendState;
}
void JFSF::addSwitchContinuous(int inputPin, int btnIndex)
{
JFSF_PRIV::SwitchContinuousAction newBtnAction(_joystick, inputPin, btnIndex);
_actions[_nextActionIndex++] = &newBtnAction;
}
void JFSF::loop()
{
for (int n = 0; n < _nextActionIndex; n++)
{
if (_actions[n])
{
_actions[n]->loop();
}
}
if (_doSendState)
{
_joystick->sendState();
}
}
For completeness sake, this is the code for the Arduino, but it is pretty much just declarations:
#include <JFSF.h>
// ... A bunch of const declarations used below. These are pretty self explanatory.
// See: https://github.com/MHeironimus/ArduinoJoystickLibrary#joystick-library-api
Joystick_ joystick(HID_REPORT_ID,
JOYSTICK_TYPE_JOYSTICK, // _JOYSTICK, _GAMEPAD or _MULTI_AXIS
BTN_COUNT, HAT_SWITCH_COUNT,
INCLUDE_X_AXIS, INCLUDE_Y_AXIS, INCLUDE_Z_AXIS,
INCLUDE_RX_AXIS, INCLUDE_RY_AXIS, INCLUDE_RZ_AXIS,
INCLUDE_RUDDER, INCLUDE_THROTTLE,
INCLUDE_ACCELERATOR, INCLUDE_BRAKE, INCLUDE_STEERING);
JFSF controller(&joystick, !DO_AUTO_SEND_STATE);
void setup() {
joystick.begin(DO_AUTO_SEND_STATE);
controller.addSwitchContinuous(10, 0); // <-- Doesn't work
controller.addSwitchContinuous(9, 1); // <-- Works
}
void loop() {
controller.loop();
}
References
ArduinoJoystickLibrary (Source for Joystick_) can be found here: https://github.com/MHeironimus/ArduinoJoystickLibrary#joystick-library-api

Answer 1

I dont really understand your code. Please read How to create a Minimal, Complete and Verifiable example. Anyhow, the following is certainly wrong and likely the cause of your problem:
void JFSF::addSwitchContinuous(int inputPin, int btnIndex)
{
JFSF_PRIV::SwitchContinuousAction newBtnAction(_joystick, inputPin, btnIndex);
_actions[_nextActionIndex++] = &newBtnAction;
}
Lets rewrite it a bit for clarity:
void foo(){
T bar;
container[index] = &bar;
}
What happens here is that bar gets destroyed when it goes out of scope, hence the pointer you put into the container, points to garbage. Presumably somewhere else in your code you are dereferencing those pointers, which is undefined behaviour (aka anything can happen).
Long story short: It is a common pattern among c++ beginners to overuse pointers. Most likely you should make container a container of objects rather than pointers and make use of automatic memory managment instead of trying to fight it.

Answer 2

Thanks to #user463035818 and #drescherjm for identifiying the actual problem.
So in the end I fixed it by simply moving the Action object creation up to the Arduino code (where it's essentially global) and passing references to those objects to the controller.
In code this translates to:
JFSF.cpp
void JFSF::addAction(JFSF_PRIV::AbstractAction *action){
_actions[_nextActionIndex++] = action;
}
Arduino code (ino)
// See code in original post
JFSF controller(&joystick, !DO_AUTO_SEND_STATE);
JFSF_PRIV::SwitchContinuousAction btnOne(&joystick, 10, 0);
JFSF_PRIV::SwitchContinuousAction btnTwo(&joystick, 9, 1);
void setup() {
joystick.begin(DO_AUTO_SEND_STATE);
// controller.addSwitchContinuous(10, 0); // Pin 10; btn index 0
// controller.addSwitchContinuous(9, 1); // Pin 9 ; btn index 1
controller.addAction(&btnOne);
controller.addAction(&btnTwo);
}
// loop() is unchanged