Inheritance problem

Projects Programming
1

Hi there!

Im trying to implement a technique I learned from other languages. Im want to use a generic parent class to call overwritten (virtual) methods from children classes. Currently Im failing allready when trying to gather the classes in an array (simplyfied version):

#include "i2CScreenScan.h"

i2CScreenScan screenScan(tft,0);
SCTFTScreen * screens[1];

void setup()
{
screens[0] = screenScan;
}

But I get an error saying:

exit status 1
cannot convert 'i2CScreenScan' to 'SCTFTScreen*' in assignment

"i2CScreenScan" inherits from "SCTFTScreen" so it is actually a "SCTFTScreen". I want to call a generic virtual function "draw()" on this object later in code by looping over the objects in the array:

void drawScreens()
{
  for(int i=0;i<numScreens;i++)
  {
    SCTFTScreen * s = screens[i];
    if(s.index == activeScreen)
    {
      s.draw();
    }
  }
}

The inheritance is working so far when Im not trying to cast to the parent class.

Is this possible in C++?

Thank you very much for help!

2

Got it myself. Its simple casting to the object. Sorry.

In case anybody struggling with downcasting too:

screens[0] = (SCTFTScreen *) screenScan;

// upcast - implicit type cast allowed
  Parent *pParent = child; 

  // downcast - explicit type case required 
  Child *pChild =  (Child *) parent;

Got the example and the infos from this site: C++ Tutorial: Upcasting and Downcasting - 2020

Thanks anyway!

3

asuryan:

i2CScreenScan screenScan(tft,0);

SCTFTScreen * screens[1];
screens[0] = (SCTFTScreen *) screenScan;

This won't work. You're trying to assign an variable of type i2CScreenScan to a variable of type pointer to SCTFTScreen.
You are forcing the compiler to convert it anyway, using a C-style cast. You shouldn't use this in C++, use static_cast instead (it won't solve your problem here, but it'll only allow valid casts).

When you use the correct types, up-casting is implicit in C++:

i2CScreenScan screenScan;
SCTFTScreen *screens[1];
screens[0] = &screenScan;

Note the address-of operator used to get a pointer to screenScan.

Pieter

4

I see!

Thank you very much for your help!