C++ passing class to constructor = not passing same instance? - c++

Question

It seems that, when I pass an class it is not passing a persistant (the same) instance of that class as I would expect. I'm assuming this has something to do with memory state but I would appreciate it if someone could explain exactly what is happening. The issue is easily demonstrated as follows :
Main.ino
#include "Debug.h"
#include "Box.h"
Debug debug;
Box box(debug);
void loop(){
debug.message("loop");
debug.line();
}
void setup(){
debug.init();
box.init();
debug.message("Setup Complete");
debug.line();
}
Debug.h
#ifndef DEBUG_H
#define DEBUG_H
class Debug {
private:
bool state;
public:
Debug();
void init();
void message(const char *str);
void message(int);
void line();
};
#endif
Debug.cpp
#include "Debug.h"
#include <Arduino.h>
Debug::Debug() : state(false) {}
void Debug::init() {
if (state == false){
Serial.begin(9600);
state = true;
}
}
void Debug::message(const char *messageChar) {
if (state){
const char *p;
p = messageChar;
while (*p) {
Serial.print(*p);
p++;
}
}
}
void Debug::message(int messageInt) {
if (state){
Serial.print(messageInt);
}
}
void Debug::line() {
if (state){
Serial.println();
}
}
Box.h
#ifndef BOX_H
#define BOX_H
#include "Debug.h"
class Box {
private:
Debug debug;
public:
Box(Debug &debug);
void init();
};
#endif
Box.cpp
#include "Box.h"
#include <Arduino.h>
Box::Box(Debug &debug):
debug(debug)
{}
void Box::init(){
// Switches
pinMode(28, INPUT_PULLUP);
debug.message("Box intialized");
debug.line();
}
So the above code outputs to serial:
Setup Complete
If I modify Box::init() to
void Box::init(){
// Switches
pinMode(28, INPUT_PULLUP);
debug.init();
debug.message("Box intialized");
debug.line();
}
I get what I want :
Box initialized
Setup Complete
If I get rid of Box constructor class and instead do
void Box::init(Debug &debug){
this->debug = debug;
// Switches
pinMode(28, INPUT_PULLUP);
debug.message("Box intialized");
debug.line();
}
Called via Main.ino like
void setup(){
debug.init();
box.init(debug);
debug.message("Setup Complete");
debug.line();
}
I get the desired response again. I don't understand why my first attempt doesn't work nor do I feel comfortable knowing what best practices are. I would appreciate any guidance.

Answer 1

You have two Debug values in your code. One global, one member of the Box class.
Those are two distinct values, since Box create or copy from a value to create its own, and there's the global one.
A solution would be to contain a reference or a pointer.
Here's the example with a reference:
class Box {
private:
Debug& debug;
// ^---- there
public:
Box(Debug &debug);
void init();
};
If you want Box to still be assignable, then use a pointer:
class Box {
private:
Debug* debug;
// ^---- now it's a star
public:
Box(Debug &debug);
void init();
};
Box::Box(Debug &debug):
debug(&debug)
{} // ^----- here, we take the address of the debug variable.
Since references are immutable, you loose some important feature of the language: assignment.
struct thing {
int& ref;
};
int main () {
int a, b;
thing t1{a}, t2{b};
t1 = t2; // ERROR!
}
The assignment would cause t1.ref to point to b after the assignment.
Pointer has a more difficult syntax and hard to guess semantics. However, they play very well with assignment since they give you more freedom:
struct thing {
int* ptr;
};
int main () {
int a, b;
thing t1{&a}, t2{&b};
t1 = t2; // Works, t1.ptr points to b
}