problem_27.md

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Problem 27 - Resolving Method Overrides at Compile-Time

2017-11-23

Recall: Template Method Pattern

class Turtle {
    public:
        void draw() {
            drawHead();
            drawShell();
            drawFeet();
        }
    private:
        void drawHead();
        virtual void drawShell() = 0;   // vtable lookup
        void drawFeet();
};

class RedTurtle: public Turtle {
    void drawShell() override;
};

Consider:

template<typename T> class Turtle {
    public:
        void draw() {
            drawHead();
            static_cast<T *>(this)->drawShell();
            drawFeet();
        }
    private:
        void drawHead();
        void drawFeet();
};

class RedTurtle: public Turtle<RedTurtle> {
    friend class Turtle;
    void drawShell();
};

class GreenTurtle: public Turtle<GreenTurtle> {
    friend class Turtle;
    void drawShell();
};

No virtual method methods, no vtable lookup

• Drawback: no relationship between RedTurtle & GreenTurtle
  • Can't store a mix of them in a container

Can give Turtle a parent:

template<typename T> class Turtle: public Enemy { ... };

Then can store RedTurtles and GreenTurtles

• But then can't access the draw method
• Could give Enemy a virtual draw method

class Enemy {
    public:
        virtual void draw() = 0;
};

But then there will be a vtable lookup

• On the other hand, if Turtle::draw calls several would-be virtual helpers, could trade away several vtable lookups for one

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