No viable conversion between Derived Class (constructor has parameters) and Base Class (constructor is absent of parameters)

Question

I have an abstract class Handle() containing a handle() method (amongst others). This class is implemented several times, with each class handling a different kind of data (stored as bytes) in a different way, e.g.

class BaseHandle
{
   virtual void handle(uint8_t *ptr, uint8_t data_size) = 0;
}

and

class DerivedHandleA : public BaseHandle 
{
   void handle(uint8_t *ptr, uint8_t data_size)
   {
      //DO THINGS WITH DATA OF KIND "A"
   }
}


class DerivedHandleB : public BaseHandle 
{
   void handle(uint8_t *ptr, uint8_t data_size)
   {
      //DO THINGS WITH DATA OF KIND "B"
   }
}

My issue is that in handling certain kinds of data the handle() method needs access to instances of other classes to call their member functions, which I would ordinarily pass through the constructor, so currently my code is like this:

class DerivedHandleC : BaseHandle 
{
private:
   HelperClass helper;

   void handle(uint8_t *ptr, uint8_t data_size)
   {
      //DO THINGS WITH DATA OF KIND "C" WITH FUNCTIONS FROM helper
   }

public:
   DerivedHandleC(HelperClass helper) : BaseHandle() helper(helper){};
}

The issue arises because I have a std::vector containing each instance of the Handler class, which is iterated through each time I get data to find the correct handler and send on the data. This works fine with DerivedHandle without parameters, but with the constructor with parameters I receive a no viable conversion from 'DerivedHandleC' to 'BaseHandle' when pushing it to my vector, which is understandable.

My question is, what should I be doing instead of passing the parameters I need through the constructor? I would prefer not to simply add the same parameters to all derived classes where they are not needed, though I am aware this would solve my compilation error.

EDITS

Adding std::vector code on request:

Declarations

   typedef std::vector<std::unique_ptr<BaseHandler>> HandlerVec;
   HandlerVec handlers;

Push back method

registerHandler(std::unique_ptr<BaseHandler> handler)
{
   handlers.push_back(std::move(handler));
}

Examples call of pushback

registerHandler(std::make_unique<DerivedHandleA>());
registerHandler(std::make_unique<DerivedHandleC>(helper));

Please show a [mre], sounds like you need to read https://stackoverflow.com/questions/274626/what-is-object-slicing though — Alan Birtles, Jan 15 '20 at 14:02
Can you add the code that push back into the vector, including the declaration of the vector please? — Guillaume Racicot, Jan 15 '20 at 14:02
You probably want `std::vector`. For the reasons take a look at the above lined duplicate. — Lukas-T, Jan 15 '20 at 14:04
Thank you for your quick responses! I will have a look through your suggestions! — madmissmolly, Jan 15 '20 at 14:13

score 4 · Accepted Answer · answered Jan 15 '20 at 14:07

4

Class types declared using the class keyword have by default private inheritance, compared to class types declared with struct.

Here's an example:

struct A {};

struct B : A {
    // inherit publicly from A, the outside world can see the inheritance graph 
};

class C : A {
    // privately inherit, the outside world cannot see the inheritance
};

To inherit publicly, use a struct or use classes with a specifier to override the default behaviour:

class D : public A {
    // inherit publicly
};

answered Jan 15 '20 at 14:07

Guillaume Racicot

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Thank you, as it turns out specifying the public override solved my issue! (Using my example classes) I had declared DerivedHandleA and DerivedHandleB public but not DerivedHandleC, silly mistake on my part that had gone right by me! – madmissmolly Jan 15 '20 at 14:18

No viable conversion between Derived Class (constructor has parameters) and Base Class (constructor is absent of parameters)

EDITS

1 Answers1