Multiple Inheritance - should you?

It is often argued that multiple inheritance in C++ is not a good practice. This is why Java doesn't have this ability. Of course, multiple inheritance in C++ for something that is similar to interface implementation as in Java is reasonable, this is the case when all classes inherited from are pure abstract with no data members and no implementaions, only pure virtual methods, except one base which is the true parent.

However, in some cases you do see in C++ multiple inheritance that is not just interface implementation. And in some cases it seems reasonable. The iostream library is using it quite a lot.

I had a code review in which the developer used multiple inheritance. He said he was considering whether to use it or not and decided it serves his goal. Indeed it was OK, there was a class "SomeSortOfEventHandler" that was of both types "A_CertainEventHandler" and "AnotherKindOfEventHandler", that is, the "SomeSortOfEventHandler" was in fact handler for two kinds of events. In this case there was no need for virtual inheritance from the top parent ("AbstractEventHandler"), as the special "SomeSortOfEventHandler" need to have the data and behavior of both its parents.

So far so good. But at a certain point there was a need to implement a virtual method in two flavors, one for being the child of one parent and the other for being the child of the other. This becomes nasty. One can add to the virtual method a parameter of type T (there is a template class at the top, that differentiate the different families under "AbstractEventHandler") - but this is not so elegant as there is no real need for this parameter. All other alternatives were also breaking the elegancy of the inheritance tree. Conclusion: better to create two separate classes and hold a pointer from one to the other to get the relation between two objects, rather than use multiple inheritance.

Array of Generic Type

I was asked why one cannot create an array of generic type, which means this line cannot compile:


// the below doesn't compile
ArrayList<Integer>[] arrayOfLists = new ArrayList<Integer>[7];


The compiler doesn't like the creation part, on the right, while the declaration alone, on the left, goes fine. This issue is widly discussed, e.g. nicely described here.

There are many ideas why it is forbidden and many suggestions for workarounds. But the nice colleague who asked me this question liked the most the workaround I suggested him, which is very simple.

I started by asking why he needs this.

And he explained that he needs to manage some info for 7 days a week:


// the below doesn't compile
ArrayList<SomeInfo>[] weeklyInfo = new ArrayList<SomeInfo>[7];


I suggested to create two new classes, DailyInfo and WeeklyInfo:


class DailyInfo {
    private ArrayList<SomeInfo> dailyInfo = new ArrayList<SomeInfo>();
    ...
}

class WeeklyInfo {
    private DailyInfo[] dailyInfo = new DailyInfo[7];
    ...
}


In most cases this would be the right object oriented approach, which by the way solves the problem of array of generic type.

m and n have switched on my keyboard...

The letters 'm' and 'n' have been switched. I type 'm' and it comes out as 'n'. I remember something about a virus so I run my anti-virus (no result) and search the web (some results).

       Cone om! I’n beconimg imsame!

It's so annoying. Maybe someone is fooling around with me.

A quick glance at another keyboard: it's not that 'm' types 'n' -- the keys themselves changed places... each sits at the other one's location. A quick pull of the keys, stucking them again back into their right position and everything is great!

After a short investigation it appears that my 6 years old son, who is using my computer for playing, accidentally popped out the 'm' and 'n', then put them again in place quickly before I'll notice. Well, almost in place... He didn't intentinally planned for giving me a hoax, though it came out quite a good one for his age.