Editing C++ (section)

===With C===
{{Main|Compatibility of C and C++}}
C++ is often considered to be a superset of [[C (programming language)|C]] but this is not strictly true.<ref name="superset">{{cite web |url=http://www.stroustrup.com/bs_faq.html#C-is-subset |title=Bjarne Stroustrup's FAQ&nbsp;– Is C a subset of C++? |access-date=5 May 2014 |archive-date=6 February 2016 |archive-url=https://web.archive.org/web/20160206214150/http://www.stroustrup.com/bs_faq.html#C-is-subset |url-status=live }}</ref> Most C code can easily be made to compile correctly in C++ but there are a few differences that cause some valid C code to be invalid or behave differently in C++. For example, C allows implicit conversion from <syntaxhighlight lang="C++" inline>void*</syntaxhighlight> to other pointer types but C++ does not (for type safety reasons). Also, C++ defines many new keywords, such as <syntaxhighlight lang="C++" inline>new</syntaxhighlight> and <syntaxhighlight lang="C++" inline>class</syntaxhighlight>, which may be used as identifiers (for example, variable names) in a C program.

Some incompatibilities have been removed by the 1999 revision of the C standard ([[C99]]), which now supports C++ features such as line comments (<syntaxhighlight lang="C++" inline>//</syntaxhighlight>) and declarations mixed with code. On the other hand, C99 introduced a number of new features that C++ did not support that were incompatible or redundant in C++, such as [[variable-length array]]s, native complex-number types (however, the <syntaxhighlight lang="C++" inline>std::complex</syntaxhighlight> class in the C++ standard library provides similar functionality, although not code-compatible), designated initializers, [[C syntax#Compound literals|compound literals]], and the <syntaxhighlight lang="C++" inline>restrict</syntaxhighlight> keyword.<ref>{{cite web |url=http://home.datacomm.ch/t_wolf/tw/c/c9x_changes.html |title=C9X – The New C Standard |access-date=27 December 2008 |archive-date=21 June 2018 |archive-url=https://web.archive.org/web/20180621084656/http://home.datacomm.ch/t_wolf/tw/c/c9x_changes.html |url-status=live }}</ref> Some of the C99-introduced features were included in the subsequent version of the C++ standard, [[C++11#Improved C compatibility|C++11]] (out of those which were not redundant).<ref>{{cite web |title=C++0x Support in GCC |url=https://gcc.gnu.org/projects/cxx0x.html |access-date=12 October 2010 |archive-date=21 July 2010 |archive-url=https://web.archive.org/web/20100721215324/http://gcc.gnu.org/projects/cxx0x.html |url-status=live }}</ref><ref>{{cite web |title=C++0x Core Language Features In VC10: The Table |url=https://blogs.msdn.com/b/vcblog/archive/2010/04/06/c-0x-core-language-features-in-vc10-the-table.aspx |access-date=12 October 2010 |archive-date=21 August 2010 |archive-url=https://web.archive.org/web/20100821114635/http://blogs.msdn.com/b/vcblog/archive/2010/04/06/c-0x-core-language-features-in-vc10-the-table.aspx |url-status=live }}</ref><ref>{{cite web |url=https://clang.llvm.org/cxx_status.html |title=Clang - C++98, C++11, and C++14 Status |publisher=Clang.llvm.org |date=12 May 2013 |access-date=10 June 2013 |archive-date=4 July 2013 |archive-url=https://web.archive.org/web/20130704124639/http://clang.llvm.org/cxx_status.html |url-status=live }}</ref> However, the C++11 standard introduces new incompatibilities, such as disallowing assignment of a string literal to a character pointer, which remains valid C.

To intermix C and C++ code, any function declaration or definition that is to be called from/used both in C and C++ must be declared with C linkage by placing it within an <syntaxhighlight style=white-space:nowrap lang="C++" inline>extern "C" {/*...*/}</syntaxhighlight> block. Such a function may not rely on features depending on [[name mangling]] (i.e., function overloading).