# Lecture 21: Separate polymorphism, ownership, and graph links ## Store varied behavior behind one safe interface **Instructor:** [Dr. Debasish Pattanayak](https://drdebmath.github.io)

## A heterogeneous container owns varied objects through one interface ~~~cpp std::vector<std::unique_ptr<Shape>> shapes; shapes.push_back(std::make_unique<Circle>(2.0)); shapes.push_back(std::make_unique<Square>(3.0)); ~~~ Concrete types differ; ownership and the operation contract are uniform.

## Algorithms stay reusable through promised interface operations ~~~cpp double totalArea(const std::vector<std::unique_ptr<Shape>>& shapes) { double total = 0.0; for (const auto& shape : shapes) total += shape->area(); return total; } ~~~ Adding a new shape does not change this traversal.

## An object graph separates entities from their relationships ~~~text [Indore] --rail--> [Ujjain] | road v [Dewas] ~~~ Vertices model entities; edges model relationships. The graph is a structure, not an external visualization service.

## Graph storage follows the operations the application needs - Adjacency list: store outgoing neighbors for each vertex. - Edge list: store relationships as records. - Matrix: direct pair lookup but quadratic storage. Polymorphism is useful only when vertex or edge behavior genuinely varies.

## Ownership stays independent from graph connectivity Let the graph own vertices and edges with smart pointers. Store non-owning IDs or indices for relationships when possible. Avoid cycles of shared ownership; graph connectivity is not the same as memory ownership.

## Add one actor without changing the controller Add a <code>WeightedEdge</code> with a cost and a <code>ScheduledEdge</code> with departure time. Write one traversal that prints every edge through the base interface, then identify which operations do not require polymorphism.

## Polymorphism, ownership, and graph links stay independently testable - Heterogeneous containers combine a base contract with safe ownership. - Algorithms depend on operations, not concrete type names. - Object graphs separate entities from relationships. - Pick adjacency representation from required operations. - Keep graph links separate from ownership links.