Need Help Overriding Virtual Function

I'm in the process of abstracting the transport used for queueing in the OSS project ZDB. I've built an inheritance diagram that looks like this:

https://docs.google.com/drawings/d/1C-b5hCN0SwKREtbx0xIN0c37EGWcm-dWSvj8mpudi_w/edit?usp=sharing

Here is the error I'm getting when building:

Undefined symbols for architecture x86_64:
"zdb::Connection::connect(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)", referenced from:
construction vtable for zdb::ConsumerConnection-in-zdb::KafkaConsumerConnection in kafka_connection.o
construction vtable for zdb::ProducerConnection-in-zdb::KafkaProducerConnection in kafka_connection.o

I get other errors of the same variety, for methods that I've declared virtual in an abstract class and am overriding in a concrete class. But I think if we solve this one it will point me to the right answers for the others.

Here's the specific definitions that I think are relevant for this case.

Connection.h defines virtual connect method:

https://github.com/bitmage/zdb/blob/68756e843817bb2b85583de877b6c1cf2956e68d/src/transport/connection.h#L53-L55

KafkaConnection.h defines an override:

https://github.com/bitmage/zdb/blob/68756e843817bb2b85583de877b6c1cf2956e68d/src/transport/kafka/kafka_connection.h#L31-L33

KafkaConnection.cc defines the implementation:

https://github.com/bitmage/zdb/blob/68756e843817bb2b85583de877b6c1cf2956e68d/src/transport/kafka/kafka_connection.cc#L53-L122

Given that the concrete class being instantiated is KafkaConsumerConnection, and that it inherits from KafkaConnection which provides the implementation, I don't understand why the symbol cannot be found during linking. What am I doing wrong?

Thanks!
Brandon
Last edited on
¿have you provided an implementation for `Connection::connect()'?
If you have no intention on doing so, then declare it as pure virtual.
Thank you, that helped me fix the problem!

C++ won't let you declare constructors as virtual, so the solution was not exactly as you described. But I did two things.

1) Had to call the Connection() constructor. I did so from the ConsumerConnection and ProducerConnection constructors, which are also abstract. Is this a bad practice? Should I instead leave it to the concrete class to call all constructors or is it ok to have a chain like this?

2) Made all the functions that I was having trouble overriding pure virtual. I don't understand why this worked... maybe because since I had declared them as just virtual, C++ assumed I was providing a definition, and then was erroring because it could not find that definition?

Thanks ne555, you put me on the right path.
1) The chain is perfectly fine, however you should keep in mind that the base part always is constructed first. So this won't work:
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class Base
{
private:
    int x;
public:
    Base(int z) : x(z)
    {
    }
}

class Derived : public Base
{
private:
    int y;
public:
    Derived() : y(100), Base::Base(y) /*WRONG! Base::Base() will always be executed first, so y still contains garbage. You compiler will throw a warning at you (wReorder if I'm right)*/
    {

    }
}


2) I guess so.
Last edited on
I'm not well versed on virtual inheritance

for (1) read https://isocpp.org/wiki/faq/multiple-inheritance#virtual-inheritance-ctors

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ConsumerConnection::ConsumerConnection() = default;
ConsumerConnection::ConsumerConnection(): Connection() {};
those lines should be equivalent.
and because you've got virtual inheritance, I think that that invocation to `Connection()' will be ignored, it will be executed in `KafkaConsumerConnection'.
you seem to alway use the default constructor, you shouldn't need to write anything

by the way, https://isocpp.org/wiki/faq/multiple-inheritance#virtual-inheritance-where
class KafkaConsumerConnection : /*virtual*/ public KafkaConnection, /*virtual*/ public ConsumerConnection {
dunno what difference does it make.

> maybe because since I had declared them as just virtual, C++ assumed I was
> providing a definition
it didn't assume anything, by not declaring them pure virtual you say that you'll provide a definition.
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class base{
	public:
		virtual void foo() = 0;
		virtual void bar(); //no definition provided
};
class derived{
	public:
		virtual void foo() override; //provided a definition, so it isn't an abstract class
};

derived asdf; //can be instantiated because it isn't an abstract class
asdf.bar(); //¿what should this do?
base *fdsa = &asdf;
fdsa->bar(); //¿what should this do? 



one last thing, you should declare the destructor virtual
https://isocpp.org/wiki/faq/virtual-functions#virtual-dtors
Last edited on
C++ won't let you declare constructors as virtual
You are correct. Simply omit constructors that do nothing.

destructors however can and should be virtual in a class that serves as a base. Otherwise the chain of destruction may be broken. See:

http://en.cppreference.com/w/cpp/language/destructor
Thanks for the additional context, goldenchicken, ne555, coder777. It's more clear to me now. :-)
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