Inheritance problems in arduino c++

Question

I'm having trouble with inherited methods in a class definitions in the Arduino environment. I have a base class portal which is inherited from by class gauge and then meter inherits from gauge. The base class has a definition for a method, but the compiler says it cant find a definition of meter::method. Header file:

#ifndef UIelements_h
#define UIelements_h

#include "Arduino.h"
#include "UTFT.h"
#include "URTouch.h"
#include "UTFT_Geometry.h"
    class portal
{
  public:
    UTFT* myDisplay;
    int origin[2]={0,0};
    int portalSize[2]={10,10};
    int BGColor=VGA_BLACK;
    int borderSize=1;
    int borderColour=VGA_WHITE;

    portal();
    portal(UTFT* myDisplay);
    void draw(void);
    void drawContents(void);

    private:
    bool firstdraw=true;

};

class guage :public portal
{
  public:   
    UTFT_Geometry* myGeometry;
    float scaleMin=0.0;
    float scaleMax=100.0;
    float tick=20;
    bool logScale=false;
    int scaleColour=VGA_WHITE;
    int foreColour=VGA_YELLOW;
    float redBegin=80.0;
    int redColour=VGA_RED;
    float value=0;
    float lastValue=0;

    guage();
    guage(UTFT*,UTFT_Geometry*);
    void setNewValue(float);
};

class meter :public guage
{
  public:
    float startAngle=-40.0;
    float endAngle=40.0;
    float scaleRadius=80.0;

    meter();
    meter(UTFT*,UTFT_Geometry*);
    void setValueAndDraw(float);

  private:
    void PointerDraw(float);

};

.cpp

#include "Arduino.h"
#include "UIelements.h"

portal::portal()
{
}
    portal::portal(UTFT* UTFT)
{
    // constructor: save the passed in method pointers for use in the class
    myDisplay=UTFT;
}

void portal::draw(void)
{
    // draw the contents
    if (firstdraw)
    {
    // draw background and border
    }
    else
    {
    drawContents();
    }
}

void portal::drawContents(void)
{
    //portal class has no contents to draw, but subclasses will have..
}

...

meter::meter(UTFT* UTFT,UTFT_Geometry* Geometry)
{
    // constructor save the passed in method pointers for use in the class
    myDisplay=UTFT;
    myGeometry=Geometry;
}

void meter::setValueAndDraw(float newValue)
{
    setNewValue(newValue);
    draw();
}

void meter::drawContents(void)
{
    float xcentre=origin[1]+portalsize[1]/2;
    float ycentre=origin[2]+portalSize[2]-1;
    if (firstdraw=true)
//...more code in here..
}

error message

error: no 'void meter::drawContents()' member function declared in class 'meter'

I've asked a couple of people but everyone seemed to think that the class inheritance looked OK - is this an Arduino thing, or is there something fundamental that I don't understand? Any help gratefully received. I fear that its some silly typo or omission of ; etc.

declaration of void drawContents(void) is part of portal, not meter... — Jarod42
– Jarod42, Commented May 5, 2017 at 11:53
The error message is quite clear, there is no drawContents member declared in meter class even though it does inherit drawContents member from portal base class. — user7860670
– user7860670, Commented May 5, 2017 at 11:55

Serve Laurijssen · Accepted Answer · 2017-05-05 12:06:42Z

1

in C++ when you want to override behavior in subclasses you must mark the function as virtual and declare them in the subclass again.

So you must do this:

class portal
{
  public: virtual void drawContents();
};

void portal::drawContents()
{
//do stuff
}

class meter : public portal
{
public:
    virtual void drawContents() override; // virtual may be omitted
};

void meter::drawContents()
{
// override behavior
}

The keyword override after drawContents is a relatively new C++ feature. It might not be implemented by the arduino compiler. You can just omit it then.

edited May 5, 2017 at 12:06

answered May 5, 2017 at 12:03

Serve Laurijssen

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2 Comments

BrightSparkey Over a year ago

Thanks for the explanation. I guess I expected the compiler to work it out for itself. Seems rather clumsy to have to define it all over again, makes the override a bit pointless in my example.

BrightSparkey Over a year ago

Once again thanks for the coming to my rescue so quickly.

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