C++ – Uses of Pure Virtual Functions

c++pure-virtualvirtual-functions

I'm learning about C++ in a class right now and I don't quite grok pure virtual functions. I understand that they are later outlined in a derived class, but why would you want to declare it as equal to 0 if you are just going to define it in the derived class?

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Briefly, it's to make the class abstract, so that it can't be instantiated, but a child class can override the pure virtual methods to form a concrete class. This is a good way to define an interface in C++.

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C++ – Use Cases of Pure Virtual Functions with Body

The classic is a pure virtual destructor:

class abstract {
  public: 
    virtual ~abstract() = 0;
};

abstract::~abstract() {}

You make it pure because there's nothing else to make so, and you want the class to be abstract, but you have to provide an implementation nevertheless, because the derived classes' destructors call yours explicitly. Yeah, I know, a pretty silly textbook example, but as such it's a classic. It must have been in the first edition of The C++ Programming Language.

Anyway, I can't remember ever really needing the ability to implement a pure virtual function. To me it seems the only reason this feature is there is because it would have had to be explicitly disallowed and Stroustrup didn't see a reason for that.

If you ever feel you need this feature, you're probably on the wrong track with your design.

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Related Solutions

C++ – Use Cases of Pure Virtual Functions with Body

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