[bubuga]

> Virtual inheritance means you have a diamond pattern and haven't separated your components properly.

I've seen this argument pop out time and again as if it was a mantra of sorts, and more often than not it comes from someone with a background almost exclusively founded on Java.

Their line of reasoning essentially boils down to "Java doesn't support it, the people behind Java said something about it, therefore it's bad".

But C++ isn't Java, nor does Java dictate what is correct or what makes sense.

Particularly when the only assertion you could come up with to criticize multiple inheritance was a comment that had nothing to do with C++ or even OO programming, but only to do with your personal taste regarding your superficial impressions regarding software design.

[vvanders]

Ooo, that Java remark burns. I've only touched Java seriously in the last few years of my career, I'm a hardcore C++/native type.

Here's the issues I take with virtual inheritance:

1. It complicates the vtable and function/member calls leading to another level of indirection, so from a performance perspective it's a negative mark.

2. The more fundamental problem is that you've botched the hasA vs isA association. If you have two classes that share a base class then that base class should be refactored into a component that can then be used as a member without polluting the class hierarchy. This leads to better composability since you can now pass this object around without pulling a whole class hierarchy with it.

You're welcome to use virtual inheritance all you want in your projects but much like knowing what subset of C++ to use(and what not) you'll never see it in code I work on.

[quotemstr]

> you've botched the hasA vs isA association

No you haven't.

In many senses, C++ inheritance is just shorthand for composition. That's why things like private inheritance are useful. Through this lens, virtual inheritance is automatic composition in which multiple composed objects each have a pointer to some shared object (the virtual base). It doesn't mess with hasA vs. isA at all.

> knowing what subset of C++ to use

This idea that a good coding standard necessarily bans parts of C++ has done massive damage to the C++ community. The correct subset of C++ to use is C++. Turning off parts of the language is just a cheap way to look sagacious while infantilizing developers. Every feature has its place.