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Language: C++
Expertise: Intermediate
Oct 23, 1998



How to Help Your Business Become an AI Early Adopter

How is Virtual Inheritance Implemented?

When multiple inheritance is used, it is sometimes necessary to use virtual inheritance. A good example for this is the standard iostream class hierarchy:
//Note: this is a simplified description of iostream classes

class  ostream: virtual public ios { /*..*/ }
class  istream: virtual public ios { /*..*/ }

class iostream : public istream, public ostream { /*..*/ } //a single ios inherited
How does C++ ensure that only a single instance of a virtual member exists, regardless of the number of classes derived from it? C++ uses an additional level of indirection to access a virtual class, usually by means of a pointer. In other words, each object in the iostream hierarchy has a pointer to the shared instance of the ios object. The additional level of indirection has a slight performance overhead, but it's a small price to pay.
Danny Kalev
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