Flocking written in C++ using SFML. Check out Youtube demo at https://youtu.be/xtSEIrXISDw.

RMouseBtn: add boids; Shift+T: reduce boids; Ctrl+S: add separation; Shift+S: reduce separation; Ctrl+C: add cohesion; Shift+C: reduce cohesion; Ctrl+A: add align; Shift+A: reduce align; P: toggle path

Used bin-lattice spacial partition for optimization which can allow 500 boids running at around 90fps if the pathing is turned off;
Steering force includes separation, cohesion, align and pathing. For pathing I used vehicle's steering force but after adding flocking forces, the steering simulation doesn't look any different from a simple arriving AI. If I didn't do those additional calculation, as well as allocate memory in advance rather than each time dynamically, the performance could be improved further.

Chapter 6 - Autonomous Agents, in The Nature of Code by Daniel Shiffman. Provides well organized explanations and source code (in Processing, a JAVA-like language) for implementing flocking.