Calling overridden virtual methods from constructor

Projects Programming
1

I'm new to C++ , although have programmed Arduinos using C before and have been programming Windows apps for 30 odd years. So I understand concepts like virtual methods and overridng them but I'm struggling to get an overridden virtual method in a derived class called by the constructor.
Here's the code

class VelbusNode
{
public:
  VelbusNode();
  virtual void init();
protected: // some data members
  byte nodeType = 0x05; // use this value unless changed
  byte mapVersion;
};

class VelbusNode_VMBGPO: public VelbusNode
{
  public:
  VelbusNode_VMBGPO();  
    virtual void init() override;
};

The implementation of the base class looks like this

VelbusNode::VelbusNode()
{ 
  init();
}

void VelbusNode::init()
{
  mapVersion = 1;
}

and the derived class like this

VelbusNode_VMBGPO::VelbusNode_VMBGPO()
:  VelbusNode::VelbusNode()
{}
void VelbusNode_VMBGPO::init() 
{  
  VelbusNode::init(); // call the base class version first
  nodeType = 0x21;   // now override the values
}

and finally the code is used like this

  VelbusNode_VMBGPO* node;
  node = new VelbusNode_VMBGPO();

When I inspect the value assigned the node->nodeType it is the value set in the initial declaration i.e. 0x05. mapVersion is 1 as expected because the base init code has been called.

Given that the declaration and call to the constructor is for the derived class (VelbusNode_VMBGPO) I'd be expecting to see the base constructor call the overridden init function. But it appears to only call its own function, which makes the concept of polymorphism a bit broken.

What do I need to change so that the constructor for the instance of the derived class calls the correct code ?

2

you would get better and faster answers if you would provide a full working example.

You could call the init in the constructor of the derived class.

class VelbusNode {
  public:
    VelbusNode();
    virtual void init();

    byte getNodeType() {
       return nodeType; 
    }
  protected: // some data members
    byte nodeType = 0x05; // use this value unless changed
    byte mapVersion;
};

class VelbusNode_VMBGPO: public VelbusNode {
  public:
    VelbusNode_VMBGPO();
    virtual void init() override;
};

VelbusNode::VelbusNode() {
  init();
}

void VelbusNode::init() {
  mapVersion = 1;
}

VelbusNode_VMBGPO::VelbusNode_VMBGPO()
  :  VelbusNode::VelbusNode()
{init();}  // !

void VelbusNode_VMBGPO::init() {
  VelbusNode::init(); // call the base class version first
  nodeType = 0x21;   // now override the values
}

void setup() {
  Serial.begin(115200);
  VelbusNode_VMBGPO* node;
  node = new VelbusNode_VMBGPO();
  Serial.println(node->getNodeType(), HEX); 
}

void loop() {
  // put your main code here, to run repeatedly:

}

but there are for sure better answers out there.

3

Thanks for the reply. I've just stumbled upon this but it seems to be counter intuitive.
I think it is because from what I've read C++ constructors are not virtual so they do not act in a polymorphic way, unlike normal functions. Which is not how my experience outside of C++ works.

So, a bit of a learning curve and trying not to trip myself up by assuming similar paradigms as other languages.

4

That's a known problem with C++ constructors. Many classes have a begin() method that does the intended initialization after and outside the constructor.

5

Thanks for the tip.
As a Delphi/Pascal and C# developer it is a different paradigm for me, but figuring out that programming "mental" model is about 50% of the task.
I'll look at using that rather than relying on the constructor to call it.

6

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