C++17 is a version of theISO/IEC 14882 standard for theC++ programming language. C++17 replaced the prior version of the C++ standard, calledC++14, and was later replaced byC++20.

Before the C++ Standards Committee fixed a 3-year release cycle, C++17's release date was uncertain. In that time period, the C++17 revision was also calledC++1z, following C++0x or C++1x forC++11 and C++1y forC++14. The C++17 specification reached the Draft International Standard (DIS) stage in March 2017.^[1][2] This DIS was unanimously approved, with only editorial comments,^[3] and the final standard was published in December 2017.^[4] Few changes were made to the C++Standard Template Library, although somealgorithms inthe<algorithm> header were given support for explicitparallelization and somesyntactic enhancements were made.

C++17 introduced many new features. The following lists may be incomplete.

• Making the text message forstatic_assert optional^[5]
• Allowtypename (as an alternative toclass) in a template template parameter^[6]
• New rules forauto deduction from braced-init-list^[7][8]
• Nested namespace definitions, e.g.,namespaceX::Y{…} instead ofnamespaceX{namespaceY{…}}^[8][9]
• Allowing attributes fornamespaces andenumerators^[10][11]
• New standard attributes[[fallthrough]],[[maybe_unused]] and[[nodiscard]]^[12]
• UTF-8 (u8) character literals^[10][13] (UTF-8 string literals have existed sinceC++11; C++17 adds the corresponding character literals for consistency, though as they are restricted to a single byte they can only store "Basic Latin" andC0 control codes, i.e.ASCII)
• Hexadecimalfloating-point literals^[14][15]
• Use ofauto as the type for a non-type template parameter^[16]
• Constant evaluation for all non-type template arguments^[10][17]
• Fold expressions, forvariadic templates^[10][18]
• A compile-time staticif with the formifconstexpr(expression)^[19]
• Structured binding declarations, allowingauto[a,b]=getTwoReturnValues();^[20]
• Initializers inif andswitch statements^[21]
• copy-initialization and direct-initialization of objects of typeT from prvalue expressions of typeT (ignoring top-level cv-qualifiers) shall result in no copy or move constructors from the prvalue expression. Seecopy elision for more information.
• Some extensions on over-aligned memory allocation^[22]
• Class template argument deduction (CTAD), introducing constructor deduction guides, e.g. allowingstd::pair(5.0,false) instead of requiring explicit constructor arguments typesstd::pair<double,bool>(5.0,false) or an additional helper template functionstd::make_pair(5.0,false).^[23][24]
• Inline variables, which allows the definition of variables in header files without violating theone definition rule. The rules are effectively the same asinline functions
• __has_include, allowing the availability of a header to be checked by preprocessor directives^[25]
• Value of__cplusplus changed to201703L^[26]
• Exception specifications were made part of the function type^[27]
• Lambda expressions can capture "*this" by value^[28]

• Most of Library Fundamentals TS I, including:^[29][30]
  • std::string_view, a read-only non-owning reference to a character sequence orstring-slice^[31]
  • std::optional, for representingoptional objects, a data type that may not always be returned by a given algorithm with support for non-return
  • std::any, for holding single values of any type
• std::uncaught_exceptions, as a replacement ofstd::uncaught_exception inexception handling^[32][10]
• New insertion functionstry_emplace andinsert_or_assign forstd::map andstd::unordered_mapkey-value associative data structures^[33][34]
• Uniformcontainer access:std::size,std::empty andstd::data^[34][35]
• Definition of "contiguousiterators"^[34][36]
• Afile system library based onboost::filesystem^[37]
• Parallel versions ofSTL algorithms^[38]
• Additional mathematicalspecial functions, includingelliptic integrals andBessel functions^[39]
• std::variant, atagged union container^[40]
• std::byte, allowingchar to be replaced for data types intending to model abyte of data as a byte rather than a character^[41]
• Logical operator traits:std::conjunction,std::disjunction andstd::negation^[42]
• <memory_resource> header, for polymorphic memory resources^[43]

This revision of C++ not only added new features but also removed a few.

• Trigraphs were removed.^[44][45]
• Some deprecated types and functions were removed from thestandard library, includingstd::auto_ptr,std::random_shuffle, and old function adaptors.^[8][46] These were superseded in C++11 by improved facilities such asstd::unique_ptr,std::shuffle,std::bind, and lambdas.
• The (formerly deprecated) use of the keywordregister as a storage class specifier was removed.^[47] This keyword is stillreserved but now unused.

• GCC has had complete support for C++17 language features since version 8.^[48]
• Clang 5 and later supports all C++17 language features.^[49]
• Visual Studio 2017 15.8 (MSVC 19.15) and later supports all C++17 language features.^[50][51]

• libstdc++ since version 9.1 has complete support for C++17 (8.1 without Parallelism TS and referring to C99 instead of C11)^[52]
• libc++ as of version 9 has partial support for C++17, with the remainder "in progress"^[53]
• Visual Studio 2017 15.8 (MSVC 19.15) Standard Library and later supports all C++17 library features except for "Elementary String Conversions" and referring to C99 instead of C11. "Elementary String Conversions" is added inVisual Studio 2019 16.4^[54]

• C++ compilers
• C11 (C standard revision)
• C17 (C standard revision)