Comment
A comment is a spoken or written expression of an opinion, attitude, or explanatory remark regarding a particular subject.[1] The noun derives from Late Latin commentum, denoting an interpretation or contrivance, which evolved through Old French coment meaning commentary, entering Middle English around the late 14th century as a term for an explanatory note or gloss.[2] As a verb, to comment signifies the act of making such a statement or providing analysis, often to annotate, criticize, or discuss.[3] In specialized contexts, comments serve distinct functions, such as non-executable annotations in computer source code designed to enhance readability and documentation without affecting program execution.[4] This practice underscores their utility in clarifying intent amid complex logic, though excessive or poorly placed comments can obscure rather than illuminate code structure.[5]Computing
Source Code Comments
Source code comments are non-executable text annotations embedded within programming source files, intended to explain the logic, algorithms, intent, or context of the code to human readers without influencing program execution. Compilers or interpreters systematically ignore these annotations during translation to machine code, treating them as whitespace or directives for documentation. This separation ensures comments serve purely as aids for maintenance, debugging, and collaboration, distinct from operational instructions.[6] The practice originated in early low-level languages of the mid-20th century, with assembly languages employing simple delimiters like semicolons (;) to denote comments from the 1950s onward, allowing programmers to annotate machine instructions directly alongside opcodes. For instance, in assemblers for systems like the IBM 704, trailing text after a semicolon was disregarded, facilitating readability in otherwise opaque binary representations. This convention predates higher-level languages and reflects the need for human-readable aids in code handling punch cards or tape, where errors in interpretation could cascade into hardware faults. By the late 1950s, structured high-level languages formalized comments: ALGOL 58 introduced the "comment" keyword followed by text until a semicolon, enabling block-level explanations within algorithmic blocks. ALGOL 60 retained this syntax, embedding comments flexibly within begin-end blocks to clarify procedural intent.[7][8] Syntax evolved diversely across paradigms. Early BASIC dialects from 1964 used the REM (short for "remark") statement to designate entire lines as comments, ignored by interpreters and useful for labeling program sections in educational or simple scripting contexts. Procedural languages like C (1972) adopted bounded delimiters such as /* / for multi-line comments and later // for single-line variants in C++ (1985), influencing descendants including Java, JavaScript, and C#. These C-style conventions prioritize nesting avoidance and scanner efficiency, with / */ allowing comments within strings or other comments in some implementations. Assembly variants persist heterogeneously—semicolons in x86 GAS, # in ARM—reflecting assembler-specific parsers. Modern languages extend this: Python uses # for indentation-sensitive comments, while Rust combines // with /// for documentation attributes.[9][10] Comments fulfill multiple roles beyond explanation, including temporary deactivation ("commenting out") of code blocks for debugging, where entire sections are prefixed with delimiters to isolate faults without deletion. They also enable self-documentation, embedding metadata like authorship or revision history directly in source. Empirical analyses indicate that consistent, high-quality comments enhance software comprehension and maintenance: a decade-long review of studies links them to improved bug detection during reviews and reduced resolution times for issues, as annotative clarity aids in tracing causal paths in complex modules. However, quantitative correlations vary; one analysis of GitHub repositories found denser commenting associated with fewer reported issues, though causation remains tied to comment accuracy rather than volume.[11][12] Best practices emphasize precision over proliferation, advocating comments that elucidate "why" a construct exists—such as algorithmic trade-offs or domain constraints—rather than restating evident "what" operations, which clear code should convey intrinsically. Brevity mitigates redundancy: guidelines recommend limiting to essential insights, using consistent formatting (e.g., aligned end-of-line notes), and updating alongside code changes to prevent obsolescence. Over-commenting incurs costs, as outdated annotations introduce cognitive dissonance, increasing maintenance overhead by 10-20% in empirical team studies where synchronization lags. Conversely, sparse or absent comments in dense codebases correlate with higher defect densities in collaborative environments, underscoring the balance for causal efficacy in development workflows.[13][14][11]Markup and Documentation Comments
In markup languages such as HTML and XML, comments are delimited by the syntax<!-- and -->, a convention inherited from the Standard Generalized Markup Language (SGML), which was formalized as ISO 8879 in 1986.[15][16] These comments are stripped by parsers during document processing, preventing their display or interpretation in rendered output, which enables developers to embed explanatory notes, disable sections temporarily, or include browser-specific directives without altering the visible structure. Unlike inline code comments that aid during compilation or execution, markup comments primarily serve web development workflows, such as conditional inclusion via server-side rendering or debugging unstyled elements, as they remain accessible in the client's source view but do not influence DOM construction.[17]
Documentation comments extend this concept by integrating structured markup within source files to automate the generation of external references, such as API guides, directly tied to code elements like classes and functions. Javadoc, developed alongside early Java implementations and first available in the 1995 JDK alpha release, employs block comments prefixed with /** to parse tags (e.g., @param, @return) into hyperlinked HTML documentation, establishing causal mappings from implementation details to intended usage.[18] Similarly, Doxygen, initially released in 1997, processes specially formatted comments across languages like C++, Java, and Python to produce outputs including class diagrams and call graphs, facilitating comprehension of software architecture by extracting relational data from code proximity and annotations.[19] These tools underscore a direct linkage: well-formed comments reduce cognitive load in understanding causal dependencies, such as method inputs yielding specific outputs, far more effectively than ad-hoc prose.
Despite their utility, markup and documentation comments introduce risks, particularly in production environments where unstripped HTML comments can expose sensitive details like internal paths, debug flags, or version metadata visible to end-users via browser inspection, potentially aiding reconnaissance in security assessments.[20][21] Empirical analyses link inadequate or absent documentation comments to extended developer onboarding, with one survey reporting onboarding durations extending by up to 340% in teams lacking comprehensive docs, as new contributors expend disproportionate effort reverse-engineering undocumented interfaces.[22] Best practices thus emphasize stripping comments from deployed artifacts and enforcing structured formats to mitigate these issues, prioritizing parser-agnostic clarity over voluminous notes.[23]