A pure virtual member function is declared with virtual and = 0. It keeps the declaring class abstract while requiring derived classes to provide their own implementation. section 6.1.1
Listing 6.1 A first attempt at a base class
namespace Race
{
class Blob
{
public:
virtual void step() = 0; #1
virtual int total_steps() const = 0; #2
};
}
A small Blob interface declares step and total_steps as pure virtual functions, creating a polymorphic base for different blob types. section 6.1.1
Because Blob has abstract functions, it cannot be instantiated directly. Derived classes become concrete by implementing each function with the exact same signature; override lets the compiler check that match. section 6.1.1 section 6.1.2
A stepping blob
StepperBlob overrides step to increase its private y value by two and overrides total_steps to return y. A small assertion verifies that one step produces two total steps. section 6.1.2
Virtual dispatch uses a v-table when a virtual function is called through a pointer or reference, allowing different derived classes to supply different behavior. section 6.1.1
