Code completion

Code completionis anautocompletionfeature in manyintegrated development environments(IDEs) that speeds up the process of coding applications by fixing common mistakes and suggesting lines of code. This usually happens through popups while typing, querying parameters of functions, and query hints related to syntax errors. Modern code completion software typically usesgenerative artificial intelligencesystems to predict lines of code. Code completion and related tools serve as documentation and disambiguation forvariablenames,functions,andmethods,usingstatic analysis.

The feature appears in many programming environments.Implementations includeIntelliSenseinVisual Studio Code.The term was originally popularized as "picklist" and some implementations still refer to it as such.

Overview

Intelligent code completion, which is similar to otherautocompletionsystems, is a convenient way to access descriptions of functions—and in particular their parameter lists. The feature speeds up software development by reducing keyboard input and the necessity for name memorization. It also allows for users to refer less frequently to external documentation, as interactive documentation on many symbols (i.e. variables and functions) in the active scope appears dynamically in the form oftooltips.

Intelligent code completion uses an automatically generated in-memorydatabaseof classes, variable names, and other constructs that given computer code defines or references. The "classic" implementation of IntelliSense works by detectingmarker characterssuch as periods (or other separator characters, depending on the language). When the user types one of these characters immediately after the name of an entity having one or more accessible members (such as contained variables or functions), IntelliSense suggests matches in a pop-up dialog. The user can either accept the suggestion by typing a statement-completion character (Tab ↹or↵ Enter) or a language-specific marker (such as the semicolon forC++), or continue typing the name. Over time, IntelliSense determines which variable or function the user most likely needs. IntelliSense also displays a short description of a function in the pop-up window—depending on the amount of documentation in the function'ssource code.

The feature also lets users select from a number ofoverloaded functionsin languages that supportobject-oriented programming.Some code editing software provide intelligent code completion through aLanguage Server Protocol(LSP) server.

History

Research on intelligent code completion began in 1957, with spelling checkers forbitmapimages ofcursive writingand special applications to find records in databases despite incorrect entries. In 1961,Les Earnest,who headed the research on this budding technology, saw it necessary to include the first spell checker that accessed a list of 10,000 acceptable words.Ralph Gorin, a graduate student under Earnest at the time, created the first true spell-check program written as an application (rather than research) for general English text. SPELL, for the DEC PDP-10 at Stanford University's Artificial Intelligence Laboratory (SAIL), was published in February 1971.Gorin wrote the program inassemblyfor faster action; he made it by searching a word list for plausible correct spellings that differ by a single letter or adjacent-letter transpositions, and presenting them to the user. Gorin made SPELL publicly accessible, as was done with most SAIL programs, and it soon spread around the world via the then-newARPANET,about a decade before personal computers came into general use.SPELL and its algorithms and data structures inspired theUnixprogramIspell.

Support in editors and IDEs

Visual Studio

IntelliSense isMicrosoft's implementation of code completion, best known inVisual Studio.It was first introduced as a feature of a mainstream Microsoft product in 1996building on many already invented concepts of code completion and syntax checking, with the Visual Basic 5.0 Control Creation Edition, which was essentially a publicly available prototype forVisual Basic5.0.Initially, Visual Basic IDE was the primary "test bed" for the technology, but IntelliSense was incorporated intoVisual FoxProandVisual C++in the Visual Studio 97 timeframe (one revision after first seen in Visual Basic). Because it was based on the introspection capabilities ofCOM,the Visual Basic versions of IntelliSense were always more robust and complete than the 5.0 and 6.0 (97 and 98 in the Visual Studio naming sequence) versions of Visual C++, which did not have the benefit of being entirely based on COM. These shortcomings (criticized by many VC++ developers since the 97 release) have been largely corrected in the.NETproduct lines. For example, one of the most requested capabilities missing from the pre-.NET products was support fortemplates,which is now fully implemented.

IntelliSense has entered a new phase of development with the unifiedVisual Studio.NETenvironment first released in 2001, augmented by the more powerful introspection and code documentation capabilities provided by the.NET framework. IntelliSense is now supported by the Visual Studio editors forC++,C#,J#,Visual Basic,XML,HTMLandXSLTamong others. As ofVisual Studio 2005,IntelliSense is now activated by default when the user begins to type, instead of requiring marker characters (though this behavior can be turned off). TheIDEhas the capability of inferring a greater amount of context based on what the developer is typing, to the point that basic language constructs such asforandwhileare also included in the choice list. In 2017 Microsoft announced IntelliCode,which uses machine learning to infer exactly which language or library feature is likely to be intended at every keystroke. Initially available as an extension for C# only, it is expected to be built in to future releases of Visual Studio.

Visual Studio 2022 includes artificial-intelligence features, such asGitHub Copilot,which can automatically suggest entire lines of code based on surrounding context.

Other Microsoft products that incorporate IntelliSense includeExpression Web,FrontPage 2003,Small Basic,theVisual Basic for ApplicationsIDEs in theMicrosoft Officeproducts,Visual Studio Codeand many others.SQL Server 2008 Management Studiohas autocomplete for the SQL syntax.

Eclipse

TheEclipse IDEhas code completion tools that come packaged with the program.It includes notable support for Java, C++, and JavaScript code authoring. The Code Recommenders Eclipse project used to provide powerful intelligent completion,but due to lack of resources, was dropped in Eclipse 2018–12, and then archived in July 2019.

Vim

Vim Intellisenseis an advanced code completion system for theVimeditor.

Example

Assume a C++ application being edited in Visual Studio has a classFoowith some member functions:

classFoo{
public:
voidbar();
voidfoo_bar(charc,intn);
};

When the developer references this class in source code, e.g.:

Foofoo;
foo.

as soon as the user types the period afterfoo,IntelliSense automatically lists all the available member functions (i.e.bar()andfoo_bar()) and all the available member attributes (private and protected members can be identified by a padlock picture beside their names). The user can then select one by using the arrow keys and hitting a completion character when the correct member function is highlighted. When available, IntelliSense displays a short description of the member function as given in the source code documentation.

IntelliSense goes further by indicating the required parameters in another pop-up window as the user fills in the parameters. As the user types a variable name, the feature also makes suggestions to complete the variable as they are typed. IntelliSense continues to show parameters, highlighting the pertinent one, as the user types.

The user can "force" IntelliSense to show its pop-up list without context by usingCtrl+JorCtrl+Space.InVisual Studiothis displays the entire application domain object model available to the developer.