Editing Inline function (section)

{{short description|Function in the C and C++ programming languages}}
{{About|inline functions in C and C++|inline expansion more generally|Inline expansion}}
{{distinguish|Inline assembler}}
<noinclude>{{Requested move notice|1=Inline (C and C++)|2=Talk:Inline function#Requested move 27 May 2025}}
</noinclude>
{{Update|reason=Meaning of inline changed in C++ (https://en.cppreference.com/w/cpp/language/inline)|date=April 2019}}

In the [[C (programming language)|C]] and [[C++]] [[programming language]]s, an '''inline function''' is one qualified with the [[Keyword (computer programming)|keyword]] <code>inline</code>; this serves two purposes:
# It serves as a [[compiler directive]] that suggests (but does not require) that the [[compiler]] substitute the body of the function inline by performing [[inline expansion]], i.e. by inserting the function code at the address of each function call, thereby saving the overhead of a function call. In this respect it is analogous to the <code>register</code> [[storage class specifier]], which similarly provides an optimization hint.<ref name="meyers2002">{{cite journal |title=The New C: Inline Functions |url=http://www.drdobbs.com/the-new-c-inline-functions/184401540 |first=Randy |last=Meyers |date=July 1, 2002}}</ref>
# The second purpose of <code>inline</code> is to change linkage behavior; the details of this are complicated. This is necessary due to the C/C++ separate compilation + linkage model, specifically because the definition (body) of the function must be duplicated in all [[Translation unit (programming)|translation unit]]s where it is used, to allow inlining during ''compiling'', which, if the function has external [[Linkage (software)|linkage]], causes a collision during ''linking'' (it violates uniqueness of external symbols). C and C++ (and dialects such as GNU C and Visual C++) resolve this in different ways.<ref name="meyers2002"/>