C++ header file and function declaration ending in "= 0"

Question

I have the following code inside the .h file and I'm not sure what does the assignment statement do and how is it called properly?

virtual void yield() = 0; 

I thought that the function returns a value of 0 by default but since this function returns void I am a little bit confused. Can anyone comment on this and maybe say how can I refer to this assignment, I mean how is it called in C++ jargon?

Thanks.

Answer 1

This is a pure virtual function. This means, that subclasses have to implement this function, otherwise they are abstract, meaning you cannot create objects of that class.

class ISomeInterface { public:     virtual std::string ToString( ) = 0; }  class SomeInterfaceImpl : public ISomeInterface { public:     virtual std::string ToString( ) {         return "SomeInterfaceImpl";     } } 

The idea is, that a class can expose a certain method, but subclasses have to implement it. In this example, ISomeInterface exposes a ToString method, but there is no sensible default implementation for that, so it makes the method pure virtual. Subclasses like SomeInterfaceImpl can then provide a fitting implementation.

Answer 2

The = 0 syntax declares a pure virtual function, it has nothing to do with the return value.

If a class contains at least one pure virtual function, that makes the class "abstract", which means it cannot be instantiated.

In practice, such classes need to be concretized by subclassing and implementing the virtual function(s).