C++ template parent class constructor on child class

Question

I have this

template<class PayloadType>
    class Event {
    protected:
        PayloadType payload;
    public:
        Event(PayloadType payload);
    };

This is the definition:

template<class PayloadType>
Event<PayloadType>::Event(PayloadType payload) {
    this->payload = payload;
}

And this:

class SimplePayload {
protected:
    int number;
public:
    SimplePayload(int number) : number(number) { }
    int getNumber() { return number; };
};

template<class PayloadType>
class SimpleEvent : public Event<PayloadType> {
protected:
    PayloadType payload;
public:
    SimpleEvent(PayloadType payload) : Event<PayloadType>(payload) { }
};

Trying to use it:

SimplePayload simplePayload;
SimpleEvent<SimplePayload> *simpleEvent = dynamic_cast<SimpleEvent<SimplePayload>*>(new Event<SimplePayload>(simplePayload));

And I'm getting this error:

error: member initializer 'Event' does not name a non-static data member or base class

How can I properly construct the object?

Your casting makes no sense. You allocate an `Event` object instance, it's not going to be a `SimpleEvent` whatever you say. — Some programmer dude, May 13 '16 at 14:25
`dynamic_cast` is to change the type of an existing variable. It's pretty pointless to use it in variable creation. It'll fail, too, because the argument (`new Event...`) is not the type you're casting to (`SimpleEvent...`), it's the base type. Just use `= new SimpleEvent<...>(...)` if you want a derived type, or better, a `std::unique_ptr` if you need the polymorphism or a regular non-pointer `SimpleEvent` if not. — chris, May 13 '16 at 14:26
The base class is `Event`, not `Event` which is the name of a template. Fix the initializer list. Either way as it's been pointed out the cast makes no sense. — user657267, May 13 '16 at 14:27
Would you mind explaining why? I'm getting started into C++... — Daniel Ribeiro, May 13 '16 at 14:29
I've updated the question. Definition in .h and implementation in .cpp — Daniel Ribeiro, May 13 '16 at 14:32
@DanielRibeiro move the implementation to a header file and all should compile and link... — W.F., May 13 '16 at 14:33
@DanielRibeiro read the comments above, you can't downcast an instance of a base class. — user657267, May 13 '16 at 14:37
Yes, that makes perfect sense. I was doing it the reverse way. Thanks! — Daniel Ribeiro, May 13 '16 at 14:39

score 2 · Answer 1 · edited May 23 '17 at 12:16

2

You need to specify the template argument:

template<class PayloadType>
class SimpleEvent : public Event<PayloadType> {
protected:
    PayloadType payload;
public:
    SimpleEvent(PayloadType payload) : Event<PayloadType>(payload) { }
                                            ~~~~~~~~~~~~~
};

EDIT

For the link error, try to move the definition to the header file.

See Why can templates only be implemented in the header file?

edited May 23 '17 at 12:16

Community

1
1

answered May 13 '16 at 14:27

songyuanyao

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SimpleEvent(PayloadType payload) : Event(payload) { } and SimpleEvent *simpleEvent = new SimpleEvent(simplePayload); works but I end up with Undefined symbols for architecture x86_64: "Event::Event(SimplePayload)", referenced from: SimpleEvent::SimpleEvent(SimplePayload) in main.cpp.o – Daniel Ribeiro May 13 '16 at 14:28
4

@DanielRibeiro you haven't defined the constructor for Event<>. – Richard Hodges May 13 '16 at 14:29
I did, but like this: Event(PayloadType payload); – Daniel Ribeiro May 13 '16 at 14:30
And the implementation is : template Event::Event(PayloadType payload) { this->payload = payload; } – Daniel Ribeiro May 13 '16 at 14:30
1

@SergeyA I saw that and removed my comment. The op really needs to post real code. All of these typos really makes the question a moving target. – NathanOliver May 13 '16 at 14:41
@NathanOliver, agreed. This is why I downvoted it. Is that this hard to post **real** code? – SergeyA May 13 '16 at 14:43
I'm sorry, as a human I make mistakes. I've updated everything to the real code. – Daniel Ribeiro May 13 '16 at 14:45
2

@DanielRibeiro It would be a good idea to post a [mcve](http://stackoverflow.com/help/mcve) when asking questions. – songyuanyao May 13 '16 at 14:47

score 0 · Answer 2 · answered May 13 '16 at 14:36

I see couple of errors.

Error 1

template<class PayloadType>
Event<PayloadType>::Event(PayloadType payload) {

    // this->payload is of type PayloadType*
    // payload is of type PayloadType
    // Assigning to a pointer using an object is a problem.
    this->payload = payload;
}

The easiest resolution will be to make this->payload an object of type PayloadType.

Error 2

You are not using the template parameter when using the base class constructor from the derived class.

  SimpleEvent(PayloadType payload) : Event(payload) { }

needs to be:

  SimpleEvent(PayloadType payload) : Event<PayloadType>(payload) { }

Update

The OP's question has been updated since this answer. So, the first error is not an error any longer.

C++ template parent class constructor on child class

2 Answers2