Lecture19.md

Lecture 19

Exceptions (cont'd), Design Patterns

Last Time: callchain.cc

Exception recovery can be done in stages

try {...}
catch (someExn e) { // catches someExn or a subclass
	// do part of recovery
  throw otherExn{...};
}

1. Throw some other exception
2. throw e; // throw the caught exception
3. throw; // throws original exception

All C++ library exceptions inherit from std::exception. C++ does not require exceptions to inherit from std::exception. In C++ you can throw anything.

Good Practice: Throw objects of existing exceptions or create your own exception classes

class BadInput {};

int n;
if (!(cin >> n)) throw BadInput {};

try {
	foo();
} catch(BadInput &) {
	n = 0;
}

Advice: catch by reference:

• out_of_range
• bad_alloc
• length_error

Observer Pattern

Publish/subscribe model

• Publish: Subject aggregates data
• Subscribe: Observer wants to be notified of new data

Decorator Pattern

• Scroll IS-A Window
• Scroll HAS-A Window

Window *w = new BasicWindow();
w = new Scroll{w};
w = new Menu{Scroll};
w->display();