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[[File:CodeCmmt002.svg|thumb|right|300px|[[Java (programming language)|Java]] source code with block comments in <span style="color:#f00;">red</span>, line comments in <span style="color:#0e850e;">green</span> and program code in <span style="color:#00f;"> blue</span>.]]
In [[computer programming]], a '''comment''' is text embedded in [[source code]] that a translator ([[compiler]] or [[interpreter (computing)|interpreter]]) ignores. Generally, a comment is an [[annotation]] intended to make the code easier for a [[programmer]] to understand {{endash}} often explaining an aspect that is not readily apparent in the program (non-comment) code.<ref name="PennyGrubb000">{{cite book | last = Penny Grubb | first = Armstrong Takang | title = Software Maintenance: Concepts and Practice | publisher = World Scientific | year = 2003 | isbn = 978-981-238-426-3 | pages = 7, plese start120–121}}</ref> For this article, ''comment'' refers to the same concept in a [[programming
The flexibility supported by comments allows for a wide degree of content style variability. To promote uniformity, style conventions are commonly part of a [[programming style]] guide. But, [[best practice]]s are disputed and contradictory.<ref name="Dietrich000">{{cite book
| last = W. R.
| first = Dietrich
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Support for code comments is defined by each programming language. The features differ by language, but there are several common attributes that apply throughout.
Most languages support multi-line
| title = MATLAB Guide
| first = Desmond
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| access-date = 2007-07-24
| date = 2000-03-04
}}</ref> A '''line comment''' ends at the end of the text line. In modern languages, a line comment starts with a delimiter but some older languages designate a column at which subsequent text is considered comment.<ref name="javadude000" /> Many languages support both block and line comments {{endash}} using different delimiters for each. For example, [[C (programming language)|C]], [[C++]] and their many derivatives support block comments delimited by <code>/*</code> and <code>*/</code> and line comments delimited by <code>//</code>. Other languages support only one type of comment.<ref name="javadude000" />
Comments can also be classified as either prologue or inline based on their position and content relative to program code. A '''prologue comment''' is a comment (or group of related comments) located near the top of an associated programming topic, such as before a symbol declaration or at the top of a file. An '''inline comment''' is a comment that is located on the same line as and to the right of program code to which is refers.<ref name="JBDixit000">{{cite book | last = Dixit | first = J.B. | title = Computer Fundamentals and Programming in C | publisher = Laxmi Publications | year = 2003 | isbn = 978-81-7008-882-0 }}</ref> Both prologue and inline comments can be represented as either line or block comments. For example:
<syntaxhighlight lang="c">
/*
* prologue block comment; if is about foo()
*/
bool foo() {
return true; /* inline block comment; if is about this return */
}
//
// prologue line comment; if is about bar()
//
bool bar() {
return false; // inline line comment; if is about this return
}
</syntaxhighlight>
==Examples of use==
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Comments can store [[metadata]] about the code. Common metadata includes the name of the original author and subsequent maintainers, dates when first written and modified, link to development and user documentation, and legal information such as [[copyright]] and [[software license]].
Some [[programming tools]] write metadata into the code as comments.<ref>See e.g., {{cite book |last=Wynne-Powell |first=Rod |year=2008 |title=Mac OS X for Photographers: Optimized Image Workflow for the Mac User |page=243 |publisher=Focal Press |___location=Oxford |isbn=978-0-240-52027-8 |url=https://archive.org/details/macosxforphotogr0000wynn}}</ref> For example, a [[version control]] tool might write metadata such as author, date and version number into each file when it's
=== Integrate with development tools ===
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| publisher=Cambridge University Press
| isbn=978-1-397-80521-8
}}</ref> Examples include [[Javadoc]],
=== Visualization ===
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=== Extend language syntax ===
Occasionally, code that is formatted as a comment is overloaded to convey additional information to the translator, such as
Other examples include interpreter [[Directive (programming)|directives]]:
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<syntaxhighlight lang="c">
/***************************
* *
* This is the comment body. *
* *
</syntaxhighlight>
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==Tags==
Programmers often use one of select words {{endash}} also
Commonly used tags include:
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===Curly brace languages===
Many of the [[curly brace language]]s such as C, C++ and their many derivatives delimit a line comment with {{code|//}} and a block comment with {{code|/*}} and {{code|*/}}. Originally, C lacked the line comment, but it was added in [[C99]]. Notable languages include: C, C++, [[C# (programming language)|C#]], [[D (programming language)|D]], [[Java (programming language)|Java]], [[
<syntaxhighlight lang="c">
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"""Method docstring"""
</syntaxhighlight>
===Browser markup===
Markup languages in general vary in comment syntax, but some of the notable internet markup formats such as [[HTML]] and [[XML]] delimit a block comment with <code><!--</code> and <code>--></code> and provide no line comment support. An example in XML:
<syntaxhighlight lang="xml">
<!-- select the context here -->
<param name="context" value="public" />
</syntaxhighlight>
For compatibility with [[SGML]], double-hyphen (--) is not allowed inside comments.
[[ColdFusion]] provides syntax similar to the [[HTML comment]], but uses three dashes instead of two. CodeFusion allows for nested block comments.
===Double dash===
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MySQL also supports a line comment delimited by <code>#</code>.
===
====APL====
[[APL (programming language)|APL]] uses <code>⍝</code> ("lamp") for a line comment. For example:
<syntaxhighlight lang="apl">
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<syntaxhighlight lang="AppleScript">
# line comment (in later versions)
(*
This program displays a greeting.
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</syntaxhighlight>
In later variations, including [[Quick Basic]], [[Q Basic]], [[Visual Basic]] (VB), [[Visual Basic .NET|VB.NET]], [[
<syntaxhighlight lang="vbnet">
Public Class Form1
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
'
MessageBox.Show("Hello, World") ' show dialog with a greeting
End Sub
End Class
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shut
exit
</syntaxhighlight>
====Fortran====
The following fixed-form [[Fortran
<syntaxhighlight lang="fortranfixed">
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<syntaxhighlight lang="Fortran">
! A comment
program comment_test
print '(A)', 'Hello world' ! also a comment
end program
</syntaxhighlight>
Free-form Fortran, also introduced with Fortran 90, only supports this latter style of comment.
Although not a part of the Fortran Standard, many Fortran compilers offer an optional C-like [[preprocessor]] pass. This can be used to provide block comments:
<syntaxhighlight lang="Fortran">
#if 0
This is a block comment spanning
multiple lines.
#endif
program comment_test
print '(A)', 'Hello world' ! also a comment
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====Nim====
[[Nim (programming language)|Nim]]
Nim also has documentation comments that use mixed [[Markdown]] and [[ReStructuredText]] markups.
The compiler can generate [[HTML]], [[LaTeX]] and [[JSON]] documentation from the documentation comments.
Documentation comments are part of the [[abstract syntax tree]] and can be extracted using macros.<ref>[https://nim-lang.github.io/Nim/macros.html#extractDocCommentsAndRunnables%2CNimNode macros.extractDocCommentsAndRunnables]</ref>
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====OCaml====
[[OCaml]] supports nestable comments. For
<syntaxhighlight lang="ocaml">
codeLine(* comment level 1(*comment level 2*)*)
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====Pascal, Delphi====
In [[Pascal (programming language)|Pascal]] and [[Delphi (software)|Delphi]], a block comment is delimited by <code>{</code> and <code>}</code>, and as an alternative for computers that do not support these characters, <code>(*</code> and <code>*)</code> are also supported. A line comment is delimited by <code>\\</code>.<ref>{{cite book|author=Kathleen Jensen, Niklaus Wirth|year=1985|title=Pascal User Manual and Report|publisher=Springer-Verlag|isbn=0-387-96048-1}}</ref> In [[Niklaus Wirth]]'s more modern family of languages (including [[Modula-2]] and [[Oberon (programming language)|Oberon]]), comments are delimited by <code>(*</code> and <code>*)</code>.<ref>{{cite book|author=Niklaus Wirth|year=1983|title=Programming in Modula-2|publisher=Springer-Verlag|isbn=0-387-15078-1}}</ref><ref>*{{cite book|author=Martin Reiser, Niklaus Wirth|year=1992|title=Programming in Oberon|publisher=Addison-Wesley|isbn=0-201-56543-9}}</ref> Comments can be nested. For example:
<syntaxhighlight lang="pascal">
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====PHP====
Comments in [[PHP]] can be either
<syntaxhighlight lang="php">
/**
* This class contains a sample documentation.
* @author Unknown
*/
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class MyAttribute {
const VALUE = 'value';
//
private $value;
#
public function __construct($value = null) {
$this->value = $value;
}
}
</syntaxhighlight>
==Security issues==
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==See also==
* [[Comparison of programming languages (syntax)#Comments]]
==Notes and references==
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