The blog post argues that inheritance in object-oriented programming wasn't initially conceived as a way to model "is-a" relationships, but rather as a performance optimization to avoid code duplication in early Simula simulations. Limited memory and processing power necessitated a mechanism to share code between similar objects, like different types of ships in a harbor simulation. Inheritance efficiently achieved this by allowing new object types (subclasses) to inherit and extend the data and behavior of existing ones (superclasses), rather than replicating common code. This perspective challenges the common understanding of inheritance's primary purpose and suggests its later association with subtype polymorphism was a subsequent development.
The blog post "Inheritance was invented as a performance hack (2021)" by Catern elucidates the historical context surrounding the genesis of inheritance in Simula, arguing that its initial conception was primarily motivated by performance optimization rather than the conceptual elegance of code reuse or representing "is-a" relationships, as it is frequently understood today. The author posits that inheritance in Simula emerged as a solution to the computational limitations of the time, specifically addressing the challenges of simulating complex systems like shipping traffic. Simula's developers sought to model numerous ships, each possessing distinct characteristics and behaviors. Representing these individually as separate blocks of code with their own procedures would have resulted in substantial code duplication and inefficient memory usage, particularly considering the hardware constraints of the era.
The post details how Simula introduced the concept of "prefixes" (what we now know as inheritance) to mitigate these performance issues. By defining a general "ship" prefix containing common attributes and procedures, individual ship instances could then inherit and extend this prefix, adding specific characteristics unique to each ship. This mechanism significantly reduced code redundancy: instead of replicating common code for each ship, the prefix provided a shared blueprint, saving both memory and processing power. This strategy was particularly advantageous in scenarios involving many instances of similar objects, precisely the situation encountered in the shipping simulations Simula was designed for.
The author emphasizes that this early form of inheritance was fundamentally different from the way it is frequently used and perceived in modern object-oriented programming. In Simula, the focus was on optimizing code execution and memory usage, not necessarily on establishing a strict taxonomic hierarchy or representing "is-a" relationships. The post underscores this distinction by highlighting how inheritance in Simula could be applied even when the conceptual relationship between the prefix and the inheriting object was not a clear-cut "is-a" connection, further solidifying the argument that performance optimization was the primary driver behind its development.
Furthermore, the author argues that this performance-driven origin of inheritance has influenced its subsequent evolution and usage in other programming languages. Even though modern hardware limitations are vastly different from those of Simula's era, the concept of inheritance, initially conceived as a performance hack, has become a central tenet of object-oriented programming, often employed for purposes beyond its original intent. The post concludes by suggesting that understanding the historical context of inheritance can provide valuable insight into its proper usage and limitations, ultimately leading to more effective and efficient software design.
Summary of Comments ( 174 )
https://news.ycombinator.com/item?id=43903705
Hacker News users discussed the claim that inheritance was created as a performance optimization. Several commenters pushed back, arguing that Simula introduced inheritance for code organization and modularity, not performance. They pointed to the lack of evidence supporting the performance hack theory and the historical context of Simula's development, which focused on simulation and required ways to represent complex systems. Some acknowledged that inheritance could offer performance benefits in specific scenarios (like avoiding virtual function calls), but that this was not the primary motivation for its invention. Others questioned the article's premise entirely and debated the true meaning of "performance hack" in this context. A few users found the article thought-provoking, even if they disagreed with its central thesis.
The Hacker News post titled "Inheritance was invented as a performance hack (2021)" linking to an article on catern.com about inheritance has generated a moderate number of comments discussing the historical context, technical nuances, and alternative perspectives on the origins and purpose of inheritance in programming.
Several commenters delve into the historical context of Simula and Smalltalk, challenging or refining the article's claim about inheritance being solely a performance optimization. One commenter highlights that Simula introduced inheritance primarily for code structuring and organization relating to simulations, with performance benefits being a secondary consideration. Another commenter emphasizes the importance of the historical context of limited memory, suggesting that while not the sole driver, performance optimization was a significant factor in the early development of object-oriented concepts. This context is further reinforced by a comment mentioning the use of "class inheritance" in the segmented memory model of early machines, which directly addresses memory limitations of the time.
A few comments discuss the distinction between inheritance for code reuse (implementation inheritance) and inheritance for subtyping (interface inheritance), arguing that the article conflates the two. One comment points out that subtyping is crucial for polymorphism and dynamic dispatch, features important for flexible and maintainable code, and not necessarily tied to performance. This theme is echoed by another commenter who clarifies that inheritance is not inherently about performance, but about establishing "IS-A" relationships between types, fundamental to object-oriented design. The commenter further argues that conflating implementation inheritance (code reuse) with subtyping creates many of the issues people encounter with inheritance.
Another line of discussion revolves around alternatives to inheritance, particularly composition. A commenter suggests that composition, while often presented as a superior alternative, comes with its own set of tradeoffs, particularly in terms of verbosity and the potential for increased code complexity.
Finally, some comments offer practical examples or anecdotes illustrating how inheritance can be misused or lead to problems in software development. One commenter mentions the "fragile base class problem" and how it can arise from improper use of inheritance, leading to unexpected behavior when base classes are modified.
In summary, the comments on Hacker News provide a multi-faceted view on the article's claim, enriching the discussion with historical context, technical distinctions, and practical considerations related to inheritance in programming. While acknowledging performance considerations, the comments generally push back against the idea that inheritance was solely a performance hack, highlighting its role in code organization, typing, and polymorphism. They also emphasize the importance of distinguishing between inheritance for code reuse and inheritance for subtyping and exploring the tradeoffs associated with alternatives like composition.