Is there a downside to making ALL inheritance virtual?

Question

I'm in the process of developing a framework for our internal use and thought making all inheritance virtual would avoid the diamond problem for other developers inheriting from my classes.

I looked around and found a lot of discussion on using virtual inheritance WHEN faced with the diamond issue, but nothing about preempting it.

Answer 1

Remember that with non-virtual inheritance, each constructor calls the constructors of just immediate base classes. But with virtual inheritance, the most derived class needs to call the constructors of all virtual base classes.

If your base classes need initialization, virtual inheritance will mean every class down the tree will need to correctly initialize it. And any class in the middle of the tree can't count on its own base class being initialized the way it would like.

Answer 2

Just what I found in the Standard:

• A virtual base class is initialized by the most-derived type (see aschepler's answer).
• You can't use a static_cast to convert to a derived class reference/pointer if there's virtual inheritance involved. [expr.static.cast]/2, 11
• You can't use C-style casts ("Explicit type conversion (cast notation)") to convert to a derived class pointer/reference ([expr.cast]), at least the example in [expr.dynamic.cast]/9 says so. (oooh no C-style casts ;)
• Copy/Move assignment and ctor cannot be trivial if there's a virtual base class. [class.copy]/12, 25
• [class.copy]/28 "It is unspecified whether subobjects representing virtual base classes are assigned more than once by the implicitly-defined copy assignment operator."
• You can't have constexpr ctors if there's a virtual base class. [dcl.constexpr]/4
• There are other subtleties, such as pointer-to-member conversions [conv.mem]/2, and reusing storage via placement-new on this [basic.life]/5,6.

Depending on the implementation of virtual base classes, there might be other drawbacks.