cpp core · advanced

CRTP: Static vs Dynamic Polymorphism

The base takes the derived as a template arg, so calls bind at compile time - zero-overhead static polymorphism vs runtime virtual dispatch.

The code

template <class Derived>
struct Shape {
    void draw() { // non-virtual!
        static_cast<Derived*>(this)->draw_impl();
    }
};
struct Circle : Shape<Circle> { // passes ITSELF as the arg
    void draw_impl() { /* draw a circle */ }
};
Shape<Circle> c;
c.draw(); // -> Circle::draw_impl(), inlined

What this lesson walks through

1. 01The CRTP pattern
2. 02Base calls down via static_cast
3. 03Resolved at COMPILE time -> inlinable
4. 04Contrast: virtual binds at RUNTIME
5. 05The cost difference
6. 06The trade-off

In the Curiously Recurring Template Pattern, a class derives from a base templated on itself: struct Circle : Shape<Circle>. The base now knows its derived type as a template parameter.