C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
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Section 25:
[25.12] What special considerations do I need to know about when I inherit from a class that uses virtual inheritance?

Initialization list of most-derived-class's ctor directly invokes the virtual base class's ctor.

Because a virtual base class subobject occurs only once in an instance, there are special rules to make sure the virtual base class's constructor and destructor get called exactly once per instance. The C++ rules say that virtual base classes are constructed before all non-virtual base classes. The thing you as a programmer need to know is this: constructors for virtual base classes anywhere in your class's inheritance hierarchy are called by the "most derived" class's constructor.

Practically speaking, this means that when you create a concrete class that has a virtual base class, you must be prepared to pass whatever parameters are required to call the virtual base class's constructor. And, of course, if there are several virtual base classes anywhere in your classes ancestry, you must be prepared to call all their constructors. This might mean that the most-derived class's constructor needs more parameters than you might otherwise think.

However, if the author of the virtual base class followed the guideline in the previous FAQ, then the virtual base class's constructor probably takes no parameters since it doesn't have any data to initialize. This means (fortunately!) the authors of the concrete classes that inherit eventually from the virtual base class do not need to worry about taking extra parameters to pass to the virtual base class's ctor.