Skip to main content

When is virtual inheritance needed?

Multiple inheritance is a powerful and useful feature in C++, but it can lead to a problem known as the DDD or "Deadly Diamond of Derivation", as in this case:
class ElectricAppliance{ int voltage, int Hertz ; public: //...constructor and other useful methods int getVoltage () const { return voltage; } int getHertz() const {return Hertz; } };
class Radio : public ElectricAppliance {...}; class Tape : public ElectricAppliance {...};
class RadioTape: public Radio, public Tape { //multiple inheritance //... }; void main() {
RadioTape rt;
int voltage = rt.getVoltage(); //compilation Error -- ambiguous //call; //two copies getVoltage() exist in rt: //one from Radio and one from Tape. //Also: which voltage value should be //returned? }//end main()
The problem is clear: rt is derived simultaneously from two base classes, each having its own copy of the methods and data members of ElecctricAppliance. As a result, rt has two copies of ElectricAppliance. This is the DDD. However, giving up multiple inheritance will lead to a design compromise. So in cases where reduplication of data and methods from a common base class is undesirable, virtual inheritance should be used:
class Radio : virtual public ElectricAppliance {...}; class Tape : virtual public ElectricAppliance {...}; class RadioTape: public Radio, public Tape { //multiple inheritance };
As a result, class RadioTape contains a single instance of ElectricAppliance shared by Radio and Tape, so there are no ambiguities, no memory waste, and no need to give up the powerful tool of multiple inheritance:
void main() { RadioTape rt; int voltage = rt.getVoltage(); //now OK }//end main()
Labels:

Comments

Post a Comment

Popular posts from this blog

Explain Polymorphism and Flavors of Polymorphism...

Polymorphism is the ability of different objects to react in an individual manner to the same message. This notion was imported from natural languages. For example, the verb "to close" means different things when applied to different objects. Closing a door, closing a bank account, or closing a program's window are all different actions; their exact meaning is determined by the object on which the action is performed. Most object-oriented languages implement polymorphism only in the form of virtual functions. But C++ has two more mechanisms of static (meaning: compile-time) polymorphism: Operator overloading. Applying the += operator to integers or string objects, for example, is interpreted by each of these objects in an individual manner. Obviously, the underlying implementation of += differs in every type. Yet, intuitively, we can predict what results are. Templates. A vector of integers, for example, reacts differently from a vector of string objects when it receives ...
17 comments
Read more

Function name mangling for C++ and Java

G++ internals - Mangling Both C++ and Jave provide overloaded function and methods, which are methods with the same types but different parameter lists. Selecting the correct version is done at compile time. Though the overloaded functions have the same name in the source code, they need to be translated into different assembler-level names, since typical assemblers and linkers cannot handle overloading. This process of encoding the parameter types with the method name into a unique name is called name mangling . The inverse process is called demangling . It is convenient that C++ and Java use compatible mangling schemes, since the makes life easier for tools such as gdb, and it eases integration between C++ and Java. Note there is also a standard "Jave Native Interface" (JNI) which implements a different calling convention, and uses a different mangling scheme. The JNI is a rather abstract ABI so Java can call methods written in C or C++; we are concerned he...
5 comments
Read more