What is "Checkpoint: The Code So Far (After OCP)" about?

A running code snapshot after the Open-Closed Principle: the complete Bird class hierarchy in C++, TypeScript, JavaScript, and Java, browsable file by file.

What topics does "Checkpoint: The Code So Far (After OCP)" cover?

This article covers: open closed principle, ocp checkpoint, solid principles, bird class hierarchy, abstract class example, polymorphism example.

SOLID Checkpoint: Full Code After Open-Closed Principle

This is a running snapshot, not a new lesson. It's the full Bird hierarchy as it stands after the previous post: one abstract Bird class, and one subclass per bird that can actually fly.

This design fixes both problems from before: single responsibility and open-closed. But it breaks again the moment a bird that can't fly shows up. A penguin still has to write a fly() method, and there's no honest way to write one. That's the next principle's problem.

main.cpp

#include <vector>
#include <memory>
#include "bird/Bird.h"
#include "bird/Crow.h"
#include "bird/Sparrow.h"
#include "bird/Owl.h"

int main() {
    std::vector<std::unique_ptr<Bird>> birds;
    birds.push_back(std::make_unique<Crow>());
    birds.push_back(std::make_unique<Sparrow>());
    birds.push_back(std::make_unique<Owl>());

    for (auto& bird : birds) {
        bird->walk();
        bird->fly();
    }

    return 0;
}