Initialize a class (HX711) within a class with constructor and parameters

Question

I am far from being an Arduino specialist, so please bear with me if some of my code below is far from correct.

I am writing a sketch in which I need to use 3 instances of the HX711 library, as I am using 3 different weight sensors.

In order to try to clean up the code, I want to put basically all the code using HX711 in my own class.

Here is what my ScaleMeasure.h looks like:

// ScaleMeasure.h
#ifndef _SCALEMEASURE_h
#define _SCALEMEASURE_h

#if defined(ARDUINO) && ARDUINO >= 100
    #include "arduino.h"
#else
    #include "WProgram.h"
#endif

#include <CmdMessenger.h>
#include "HX711.h"

class ScaleMeasure
{
 public:
    ScaleMeasure(int Dout, int Clk, float CalibF, int ScaleID); 
    void Setup();   
    float Measure();    

private:
    int _dout_pin;
    int _clk_pin;
    float _calibration_factor;
    byte _scaleID;
    HX711 _scale = HX711(_dout_pin, _clk_pin);  
};

and .cpp

#include "ScaleMeasure.h"

ScaleMeasure::ScaleMeasure(int Dout, int Clk, float CalibF, int ScaleID)
{
    _clk_pin = Clk;
    _dout_pin = Dout;   
    _calibration_factor = CalibF;
    _scaleID = ScaleID;     
}

void ScaleMeasure::Setup()
{       
    _scale.power_up();
    _scale.set_scale();
    _scale.set_scale(_calibration_factor);
    _scale.tare();
    _scale.read();
}

float ScaleMeasure::Measure()
{   
    return _scale.get_units(5);
}

but there is obviously something wrong, as a call to the constructor lock the rest of the program. I believe that my instantiation, of HX711 in ScaleMeasure.h HX711 _scale = HX711(_dout_pin, _clk_pin); is not correct, or not in the right place. The code compile fine, though.

Thanks for your help.

Answer 1

The right way, in C++, to properly construct a field member b (of type class B) inside a class A is to do it as follows in A's constructor:

A::A() : b(...) {
    // Other init code here
}

In your situation that would mean:

class ScaleMeasure
{
 public:
    ScaleMeasure(int Dout, int Clk, float CalibF, int ScaleID); 
    void Setup();   
    float Measure();    

private:
    int _dout_pin;
    int _clk_pin;
    float _calibration_factor;
    byte _scaleID;
    HX711 _scale;  
};

Note that I have removed the init part for _scale.

ScaleMeasure::ScaleMeasure(int Dout, int Clk, float CalibF, int ScaleID)
: _scale(Dout, CLk)
{
    _clk_pin = Clk;
    _dout_pin = Dout;   
    _calibration_factor = CalibF;
    _scaleID = ScaleID;     
}

Answer 2

Unfortunatly HX711 has no default constructor. So the easiest way would be to add one in the header:

HX711() {} // default constructor, empty

Add member function to do the init job (done by original constructor), also in the header file

    void Init(byte dout, byte pd_sck, byte gain = 128) {
    PD_SCK  = pd_sck;
    DOUT    = dout;

    pinMode(PD_SCK, OUTPUT);
    pinMode(DOUT, INPUT);

    set_gain(gain);
}

Now your ScaleMeasure class will be like (header):

class ScaleMeasure
{
 public:
    ScaleMeasure(int Dout, int Clk, float CalibF, int ScaleID); 
    void Setup();   
    float Measure();    

private:
    int _dout_pin;
    int _clk_pin;
    float _calibration_factor;
    byte _scaleID;
    HX711 _scale;  
};

and cpp

ScaleMeasure::ScaleMeasure(int Dout, int Clk, float CalibF, int ScaleID)
{
    _clk_pin = Clk;
    _dout_pin = Dout;

    _scale.Init(_dout_pin, _clk_pin);  

    _calibration_factor = CalibF;
    _scaleID = ScaleID;     
}

Answer 3

The line

HX711 _scale = HX711(_dout_pin, _clk_pin);  

would be executed at construction time, which is the wrong time for it to run since _dout_pin and _clk_pin won't be set. Instead (much as it pains me to recommend it) you will have to use dynamic allocation:

HX711 *_scale;

Then you can, in your constructor:

_scale = new HX711(_dout_pin, _clk_pin);

Since you now have a pointer all your accesses to it need to change to -> instead of .:

_scale->power_up();
... etc ...

Also you should add a destructor to your class that deletes the object pointed to by _scale if it has been created.