QDebug, inheritance QObject issue ambiguous call

Dariusz

Hey

So this one is a bit unexpected. I decided to do another refactor of my class debug system. The base class works fine, but when it gets to printing inherited ones... it kinda flips...
Here is some code :


class base {
public:
     base();
     ~base();
     friend QDebug operator<<(QDebug stream, const base*object);
}

class inherA : public QObject, public base{
public
     inherA();
     ~inherA();
}

say I have

auto *class = inherA();
qDebug()<<class;

I'll get issue as c++ don't know which debug it should go for.... the native Qt one, or the one I implemented...

Woahz... any ideas on how to bite it?

TIA

jeremy_k

C++ friendship isn't inherited.

https://en.cppreference.com/w/cpp/language/friend note #2

Dariusz

@Dariusz said in QDebug, inheritance QObject issue ambiguous call:

friend QDebug operator<<(QDebug stream, const base*object);

Hmmmm thats good to know, but not sure if thats the issue?

This is the problem

base class:
friend QDebug operator<<(QDebug stream, const base*object);
Qt:
QDebug operator<<(QDebug stream, const QObject*object);
They both are viable options for the inheritedA class, and thust cause issue, for now I did qDebug()<<(base*)class; instead and that seems to work, but I do wonder if there is a more "proper" way of doing it.

jeremy_k

I was under the impression that friendship played a role in overload resolution, but am not finding a reference in https://en.cppreference.com/w/cpp/language/overload_resolution

Casting works. static_cast is generally preferred over the C-style (Type *).

Another option is to create an intermediate class that is closer to the final inherited class.

class Derived: public QObject, public base {};

QDebug operator<<(QDebug stream, const * Derived d) { return operator<<(stream, const_cast<const base *>(d)); }

class inherA: public Derived;

void test() {
  inherA instance;
  qDebug() << instance;
}

Dariusz

@jeremy_k Ohh I see, so re-implement debug call inside inherited class and do a cast there to correct type/etc/etc... nice! Will give it a go, thanks!

jeremy_k

Anything to make one definition more specific should work. My ability to recall the full set of rules is clearly less than perfect.

Unsolved QDebug, inheritance QObject issue ambiguous call