Pure virtual definitions
It’s possible to provide a definition for
a pure virtual function in the base class. You’re still telling the compiler
not to allow objects of that abstract base class, and the pure virtual
functions must still be defined in derived classes in order to create objects.
However, there may be a common piece of code that you want some or all of the
derived class definitions to call rather than duplicating that code in every
function.
Here’s what a pure virtual definition
looks like:
//: C15:PureVirtualDefinitions.cpp
// Pure virtual base definitions
#include <iostream>
using namespace std;
class Pet {public:
virtual void speak() const = 0;
virtual void eat() const = 0;
// Inline pure virtual definitions illegal:
//! virtual void sleep() const = 0 {}};
// OK, not defined inline
void Pet::eat() const { cout << "Pet::eat()" << endl;
}
void Pet::speak() const { cout << "Pet::speak()" << endl;
}
class Dog : public Pet {public:
// Use the common Pet code:
void speak() const { Pet::speak(); } void eat() const { Pet::eat(); }};
int main() { Dog simba; // Richard's dog
simba.speak();
simba.eat();
} ///:~
The slot in the Pet VTABLE is still
empty, but there happens to be a function by that name that you can call in the
derived class.
The other benefit to this feature is that
it allows you to change from an ordinary virtual to a pure virtual without
disturbing the existing code. (This is a way for you to locate classes that
don’t override that virtual function.)
