C++ CRTP to avoid code duplication


Example

The example in Visitor Pattern provides a compelling use-case for CRTP:

struct IShape
{
    virtual ~IShape() = default;

    virtual void accept(IShapeVisitor&) const = 0;
};

struct Circle : IShape
{
    // ...        
    // Each shape has to implement this method the same way
    void accept(IShapeVisitor& visitor) const override { visitor.visit(*this); }
    // ...
};

struct Square : IShape
{
    // ...    
    // Each shape has to implement this method the same way
    void accept(IShapeVisitor& visitor) const override { visitor.visit(*this); }
    // ...
};

Each child type of IShape needs to implement the same function the same way. That's a lot of extra typing. Instead, we can introduce a new type in the hierarchy that does this for us:

template <class Derived>
struct IShapeAcceptor : IShape {
    void accept(IShapeVisitor& visitor) const override {
        // visit with our exact type
        visitor.visit(*static_cast<Derived const*>(this));
    }
};

And now, each shape simply needs to inherit from the acceptor:

struct Circle : IShapeAcceptor<Circle>
{
    Circle(const Point& center, double radius) : center(center), radius(radius) {}
    Point center;
    double radius;
};

struct Square : IShapeAcceptor<Square>
{
    Square(const Point& topLeft, double sideLength) : topLeft(topLeft), sideLength(sideLength) {}    
    Point topLeft;
    double sideLength;
};

No duplicate code necessary.