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Abstract:

This video features a code review by The Cherno of "Greg Engine," a C++ game engine built from scratch by a 23-year-old developer (Axel) primarily for learning purposes, using minimal external libraries (Glad, STB Image). The review, conducted within a VM for safety, focuses on the engine's architecture, examining its level management, entity-component system, custom Win32 windowing implementation, math library, and OpenGL rendering pipeline. Key architectural critiques highlight potential issues with pointer/reference lifetime management for levels, significant memory fragmentation due to per-object/component unique pointers (contrasting with ECS principles), overuse of static singletons for core systems like physics and camera, and highly inefficient rendering practices (unique resources per sprite instead of shared geometry and batching). While acknowledging the project's educational value, The Cherno offers suggestions for improvement, emphasizing data locality, robust ownership models (e.g., using shared_ptr), and leveraging established libraries like GLFW and GLM for non-core, complex tasks.

Greg Engine Code Review: Architecture and Implementation Analysis

• 0:00:01 Introduction: The Cherno introduces a code review of "Greg Engine," created by a 23-year-old developer (Axel) working with Unreal Engine, who built this C++ engine from scratch using only Glad and STB Image libraries to deepen his C++ understanding.
• 0:01:18 Value vs. Effort: Discusses the trade-off between learning by building everything from scratch (like windowing) versus leveraging robust, community-tested libraries (like GLFW) which offer features and cross-platform support "for free."
• 0:02:41 Sponsorship: Mentions Code Rabbit AI as a tool for automated code reviews.
• 0:05:17 Project Setup & First Run: Clones the repository, runs the premake-based project generation script, opens the Visual Studio solution, and compiles the code (requiring a minor fix: adding #include <string>). Runs the "Basket Man" demo game.
• 0:06:05 Engine Structure: Notes the good practice of separating the core engine into a static library (Greg engine) and the game (Basket Man) into an executable. Compares this favorably to other structures like the Hazel engine.
• 0:10:15 Level Management Critique: Analyzes the makeLevel function which returns a reference to an internal unique_ptr. Critiques this approach due to potential lifetime issues and unexpected behavior if the engine internally changes the currentLevel. Suggests using std::shared_ptr for safer ownership sharing between engine and client code.
• 0:15:01 Component System Critique: Examines the addComponent template function. Criticizes the design of storing each component as a separate std::unique_ptr within a game object, leading to significant memory fragmentation and poor cache performance (violating data locality principles). Contrasts this with ECS approaches where components are stored contiguously by type. Also notes the return of raw pointers, requiring careful lifetime management by the user.
• 0:19:45 Custom Windowing Implementation: Reviews the code using raw Win32 API for window creation, message handling (WndProc), and OpenGL context setup. Acknowledges its value as a learning exercise but strongly recommends using a library like GLFW for practical development due to its robustness and cross-platform capabilities.
• 0:22:05 Game Loop and Object Management: Observes the main game loop structure (process messages, update, draw). Questions the practice of iterating through game objects in reverse for updates/draws. Discusses the destroyObject method which uses a swap-and-pop technique to potentially optimize vector element removal (though questions its benefit with unique pointers).
• 0:24:41 Static Systems Critique: Criticizes the implementation of Physics2D and Camera2D as static classes or singletons. Argues these should be regular objects, likely owned by the Level or Scene, to allow for better instance management (e.g., loading multiple levels), easier testing, cleaner resource management, and avoiding global state issues.
• 0:28:07 Custom Math Library: Notes the developer wrote their own matrix and vector classes. Points out the lack of obvious low-level optimizations (like SSE intrinsics) and the critical need for correctness (ideally verified by unit tests, which seem absent). Recommends using established, highly optimized, and tested math libraries like GLM.
• 0:29:41 Rendering Inefficiency Critique: Analyzes the sprite rendering code. Criticizes creating unique OpenGL vertex buffers, index buffers, and vertex array objects for every single sprite instance. Also critiques loading image data per sprite primitive. Strongly recommends using shared, static geometry for all quads, using transformation matrices (passed via uniforms) for positioning/scaling, and employing batch rendering techniques to drastically reduce draw calls and improve performance.
• 0:34:46 Conclusion: Summarizes the review, reiterating the architectural feedback regarding data-oriented design, resource management, and the practical benefits of using well-established libraries. Invites viewers to comment and submit their own code.