Show Lecture.Mixins as a slide show.

CS253 Mixins

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Origin

The name “mixin” comes from adding various treats to ice cream, such as the McFlurry, Blizzard, or the ground-breaking research at Cold Stone Creamery.

Is-A

We know the is-a relationship, which comes from class inheritance:

class Dog {
  public:
    virtual const char * drink() { return "Lap lap lap."; }
    virtual const char * bark() { return "Woof!"; }
    virtual ~Dog() = default;
};

class Chihuahua : public Dog {
  public:
    virtual const char * bark() { return "¡Yip!"; }
};

Dog *koko = new Chihuahua;
cout << koko->drink() << '\n' << koko->bark();
delete koko;

Lap lap lap.
¡Yip!

Mixins

• Mixins also use inheritance, and could certainly be considered an is-a relationship, but don’t think about it that way.
• Focus on the base class providing methods to the derived class.

class PowerRing {
  public:
    auto zap() { return "green beam"; }
    auto fly() { return "up, up, and … ?"; }
};

class GreenLantern : public PowerRing { };

GreenLantern john_stewart;
cout << john_stewart.zap();

green beam

• Don’t think of John as being a power ring. Instead, he uses the abilities that the power ring gives him.

Usefulness

• Beginning programmers put everything in one giant main().
• Experienced programmers break up a big program into several functions.
• This helps to make each individual function manageable.
• Beginning O-O programmers put everything into one giant class.
• Experienced programmers, break up a big class into several classes.
• This helps to make each individual class manageable.
• Classes can be combined in several ways. Has-a and is-a are two common methods. Mixins are a certain type of is-a.