Comment (computer programming)

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In computer programming, a comment is a programming language construct used to embed information in the source code of a computer program. In most cases, when the source code is processed by a compiler or interpreter, comments are ignored.^[1]

Comments have a wide range of potential uses: from augmenting program code with basic descriptions, to generating extensive documentation.^[2] Comments are also used for integration with source code management systems and other kinds of external programming tools.

Contents 1 Overview 2 Uses 2.1 Code description 2.2 Algorithmic description 2.3 Resource inclusion 2.4 Debugging 2.5 Automatic documentation generation 2.6 Metadata and annotations 3 Philosophy 3.1 Necessity of comments 3.2 Offensive comments 3.3 Comments in web templates 4 Styles 4.1 Tags 5 Examples 5.1 Comparison 5.2 In-context 5.2.1 C 5.2.2 Classic BASIC 5.2.3 Fortran 90 5.2.4 Java 6 Unique variants 6.1 Audio comments 7 See also 8 Notes and references 9 External links

[edit] Overview

Source code can be divided into program code (which consists of machine-translatable instructions); and comments (which include human-readable notes and other kinds of annotations in support of the program code).^[3] The syntax and rules for these are defined in a programming language specification.

Comments are generally formatted as block comments (also called prologue comments) or line comments (also called inline comments).^[4] Block comments are delimited by a sequence of characters that mark the start of the comment and continue until the sequence of characters that mark the end of the comment. Block comments are allowed to span multiple lines of the source code. Typically, block comments do not nest, so any comment start delimiter encountered within a comment body is ignored. Some languages allow nested block comments to facilitate using comments to comment-out blocks of code that may itself contain block comments. Line comments either start with a comment delimiter and continue until the end of the line, or in some cases, start at a specific column (character line offset) in the source code and continue until the end of the line.

Some programming languages employ both block and line comments with different comment delimiters. For example, C++ has block comments delimited by /* and */ that can span multiple lines and line comments delimited by //. Other languages support only one type of comment. For example, Ada comments are line comments: they start with -- and continue to the end of the line.

[edit] Uses

How best to make use of comments is subject to dispute, different commentators have offered varied and sometimes opposing viewpoints.^[5][6] There are many different ways of writing comments and almost no empirical research on the effectiveness of the different uses.^{[citation needed]}

[edit] Code description

Comments can be used to summarise code or to explain the programmer's intent. This is called the why rather than how approach. According to this school of thought, restating the code in plain English is a waste of time; the need to explain the code may be a sign that it is too complex and should be rewritten.

"Don't document bad code – rewrite it" (The Elements of Programming Style, Kernighan & Plauger).

"Good comments don't repeat the code or explain it. They clarify its intent. Comments should explain, at a higher level of abstraction than the code, what you're trying to do." (Code Complete, McConnell)

Comments may also be used to explain why a block of code does not seem to fit conventions or best practices. This is especially true of projects involving very little development time, or in bug fixing. For example:

 ' Second variable dim because of server errors produced when reuse form data. No
 ' documentation available on server behavior issue, so just coding around it.

[edit] Algorithmic description

Sometimes source code contains a novel or noteworthy solution to a specific problem. In such cases, comments may contain an explanation of the methodology. Such explanations may include diagrams and formal mathematical proofs. This contravenes the why rather than how approach, but may be crucial to those who will be maintaining the code base. This might especially be true in the case of highly-specialized problem domains; or rarely-used optimizations, constructs or function-calls.^[7]

For example, a programmer may add a comment to explain why an insertion sort was chosen instead of a quicksort, as the former is, in theory, slower than the latter. This could be written as follows:

list = [f (b), f (b), f (c), f (d), f (a), ...];
// Need a stable sort. Besides, the performance really does not matter.
insertion_sort (list);

[edit] Resource inclusion

Logos, diagrams, and flowcharts consisting of ASCII art constructions can be inserted into source code formatted as a comment. Additionally, copyright notices can be embedded within source code as comments. Binary data may also be encoded in comments through a process known as binary to text encoding, although such practice is uncommon and typically relegated to external resource files.

[edit] Debugging

A common developer practice is to comment out a code snippet as a comment, such that it will not be executed in the final program.

[edit] Automatic documentation generation

Some programming tools read structured information in comments and automatically generate documentation. Automatic documentation generation from the comments in the source code provides a means of maintaining consistency between the documentation of the interface and use of the code. Keeping the documentation within the code makes it easier, and thus more likely, to keep the documentation up to date with changes in the code.

Examples of documenation generators include the javadoc program, designed to be used with the Java programming language, Ddoc for the D programming language and doxygen, to be used with C++, C, Java, IDL and others.

C# implements a similar feature called "XML Comments" which are read by IntelliSense from the compiled .NET assembly. The next revision of Visual Basic will also feature this.

[edit] Metadata and annotations

Developer tools sometimes store metadata in comments, such as insert positions for automatic header file inclusion, commands to set the file's syntax highlighting mode, or the file's revision number. These functional control comments are commonly referred to as annotations.

Comments are often employed for these and related methods because they allow the use of syntax and lexical conventions that might otherwise conflict with those of the enclosing programming language. This is another sense in which it is helpful that compilers and interpreters "ignore" comments.

[edit] Philosophy

There are various normative and philosophical approches regarding the proper use of comments in source code.

[edit] Necessity of comments

Some contend that comments are unnecessary because well-written source should be self explanatory; others contend that source code should be extensively commented (it is not uncommon for over 50% of the non-whitespace characters in source code to be contained within comments).

In between these views is the philosophy that comments are neither beneficial nor harmful by themselves, and what matters is that they are correct and kept in synch with the source code, and omitted if they are superfluous, excessive, difficult to maintain or otherwise unhelpful.^[8]

[edit] Offensive comments

Sometimes comments in source code are used as a way to relieve stress or to speak unfavorably about development tools, competitors, employers, or working conditions. Some commentators deem this highly inappropriate and recommend against including potentially offensive remarks in comments, especially if there is any possibility that the source code may later be viewed by anyone besides the original developer responsible for it.^[9] The occurrence of this phenomenon can be easily seen from online resources that track profanity in source code.^[10]

[edit] Comments in web templates

Web development presents a special security challenge related to comments, because it is not uncommon for HTML comments to be viewable in plain text by any user of the application. Sections of code that are "commented out" in HTML templates may therefore present a security vulnerability.^[11]

[edit] Styles

Stylistically, comments can appear in source code in various ways: as brief end-of-line remarks to explain or document a single line of code; as one-line descriptions that preceed or follow a line of source code; or as multi-line paragraphs that document something between adjacent code blocks. A common use for a paragraph comment is to explain in some detail what the following block of code statements does.

Comment styles are often agreed upon before a project starts. Usually programmers prefer styles that are consistent, non-obstructive, easy to modify, and difficult to break.

For example, C-style comments could look like this

/*

    This is the comment body.

*/

or maybe this

/***************************\
*                           *
* This is the comment body. *
*                           *
\***************************/

If a programmer's editor doesn't manage the second variant automatically, it may discourage changes to the comments, thus leading to comments which are out of date with respect to the code. On the other hand, the visibility of the second comment variant is much higher.

Some people, such as Allen Holub (in his book Enough Rope to Shoot Yourself in the Foot, ISBN 0-07-029689-8, 1995, McGraw-Hill), advocate aligning the left edges of comments:

/* This is the style recommended by Holub for C and C++.
 * It is demonstrated in Enough Rope, in rule 29.
 */

/* This is another way to do it, also in C.
** It is easier to do in editors that do not automatically indent the second
** through last lines of the comment one space from the first.
** It is also used in Holub's book, in rule 31.
*/

Different styles can be chosen for different areas of code, from individual lines to paragraphs, routines, files, and programs. If the syntax supports both line comments and block comments, one approach is to use line comments only for minor comments (declarations, blocks and edits) and to use block comments to describe higher-level abstractions (functions, classes, files and modules).

Certain projects try to define rules like "one comment every ten lines". They can be counterproductive when adhered to with too much rigor, but might still be of use when one wants to quickly judge after writing a body of code whether one needs to check if enough comments are in place.

[edit] Tags

Certain tags are used in comments to assist in indexing common issues. Such tags can be searched with common programming tools, such as the UNIX grep utility.

• FIXME is used to mark potential problematic code that requires special attention and/or review.
• NOTE is used to document inner workings of code and indicate potential pitfalls.
• TODO is used to indicate planned enhancements.

[edit] Examples

[edit] Comparison

See main article: Programming Language Comparison: Comments.

The typographical conventions for specifying comments varies widely. Additionally, individual programming languages sometimes provide unique variants. For a detailed review, please consult the programming language comparison article.

[edit] In-context

[edit] C

/*
 * Check if we are over our maximum process limit, but be sure to
 * exclude root. This is needed to make it possible for login and
 * friends to set the per-user process limit to something lower
 * than the amount of processes root is running. -- Rik
 */
if (atomic_read(&p->user->processes) >= p->rlim[RLIMIT_NPROC].rlim_cur
    && !capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_RESOURCE))
    goto bad_fork_free;

(This is from the fork.c file from the Linux kernel source.)

[edit] Classic BASIC

10 REM This is a classic BASIC program for novice programmers.
15 REM It fills the screen with the word "WIKIPEDIA"
20 PRINT "WIKIPEDIA"
30 GOTO 20

[edit] Fortran 90

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* Any non-numeric character in the first column comments out  *
*  the entire line.                                           *
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

      PRINT "WIKIPEDIA" ! All characters after an exclamation
                        ! mark are an inline comment.
      END

[edit] Java

/**
 * Registers the text to display in a tool tip.   The text 
 * displays when the cursor lingers over the component.
 *
 * @param text  the string to display.  If the text is null, 
 *              the tool tip is turned off for this component.
 */
public void setToolTipText(String text) {

(From the Sun Microsystems javadoc documentation. The comment is designed to be read by the javadoc processor.)

[edit] Unique variants

Some developers have either implemented or begun work on providing alternate modalities for specifying comments in source code. This type of extension is possible since any type of resource can be specified in text (given the proper encoding), and because Integrated Development Environments provide increasingly flexible options for linking to related external content.

[edit] Audio comments

One example of this variability of comment types is the use of audio files to include voice annotations in source code.^[12]

[edit] See also

• Comment (computing) for a general discussion on the use of comments in computing.
• Docstring, a specific type of comment that is parsed and retained throughout the runtime of the program

[edit] Notes and references

1. ^ Comments must be indicated in a way that allows a source code processor to recognize them as such. This is often simplified by saying comments are "ignored" (after they have been recognized) by the processor.
2. ^ Comments can be processed in various ways to generate documentation. See e.g., Comparison of documentation generators.
3. ^ Penny Grubb, Armstrong Takang (2003). Software Maintenance: Concepts and Practice. World Scientific. ISBN 981238426X. 
4. ^ Dixit, J.B. (2003). Computer Fundamentals and Programming in C. Laxmi Publications. ISBN 8170088828. 
5. ^ W. R., Dietrich (2003). Applied Pattern Recognition: Algorithms and Implementation in C++. Springer. 3528355581.  offers viewpoints on proper use of comments in source code. p. 66.
6. ^ Keyes, Jessica (2003). Software Engineering Handbook. CRC Press. 0849314798.  discusses comments and the "Science of Documentation" p. 256.
7. ^ Spinellis, Diomidis (2003). Code reading: The Open Source Perspective. Addison-Wesley. ISBN 0201799405. 
8. ^ Dewhurst, Stephen C (2002). C++ Gotchas: Avoiding Common Problems in Coding and Design. Addison-Wesley Professional. ISBN 0321125185. 
9. ^ Doar, Matthew B. (2005). Practical Development Environments. O'Reilly. ISBN 0596007965. 
10. ^ (see e.g., http://www.vidarholen.net/contents/wordcount/ Linux Swear Count, http://www.google.com/codesearch?hl=en&lr=&q=fucking Google Code Search).
11. ^ Andress, Mandy (2003). Surviving Security: How to Integrate People, Process, and Technology. CRC Press. ISBN 0849320429. 
12. ^ http://dev.devbiz.com/blogs/hakan/archive/2005/08/03/voice_comments.aspx Voice Comments

[edit] External links

• How to Write Comments by Denis Krukovsky
• How to Write Comments for the Javadoc Tool
• Doxygen, a documentation system for C, C++, Java, Objective-C, Python, IDL and to some extent PHP, C#, and D