C++17


C++17 is a version of the ISO/IEC 14882 standard for the C++ programming language. C++17 replaced the prior version of the C++ standard, called C++14, and was later replaced by C++20.
WWW iso.org/standard/68564.ht...

History

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 called C++1z, following C++0x or C++1x for C++11 and C++1y for C++14. The C++17 specification reached the Draft International Standard stage in March 2017. This DIS was unanimously approved, with only editorial comments, and the final standard was published in December 2017. Few changes were made to the C++ Standard Template Library, although some algorithms in the header were given support for explicit parallelization and some syntactic enhancements were made.

New features

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

Language

  • Making the text message for optional
  • Allow in a template template parameter
  • New rules for [Type inference|] deduction from braced-init-list
  • Nested namespace definitions, e.g., instead of
  • Allowing attributes for namespaces and enumerators
  • New standard attributes, and
  • UTF-8 character literals
  • Hexadecimal floating-point literals
  • Use of as the type for a non-type template parameter
  • Constant evaluation for all non-type template arguments
  • Fold expressions, for variadic templates
  • A compile-time static with the form
  • Structured binding declarations, allowing
  • Initializers in and statements
  • copy-initialization and direct-initialization of objects of type from prvalue expressions of type shall result in no copy or move constructors from the prvalue expression. See copy elision for more information.
  • Some extensions on over-aligned memory allocation
  • , introducing constructor deduction guides, e.g. allowing instead of requiring explicit constructor arguments types or an additional helper template function.
  • Inline variables, which allows the definition of variables in header files without violating the one definition rule. The rules are effectively the same as inline functions
  • , allowing the availability of a header to be checked by preprocessor directives
  • Value of changed to
  • Exception specifications were made part of the function type
  • Lambda expressions can capture "*this" by value

Library

  • Most of Library Fundamentals TS I, including:
  • * header: std::string_view, a read-only non-owning reference to a character sequence or string-slice
  • * : std::optional, for representing optional objects, a data type that may not always be returned by a given algorithm with support for non-return
  • * : std::any, for representing the any type, which holds a single value of any typestd::uncaught_exceptions, as a replacement of std::uncaught_exception in exception handling. Located in . uncaught_exceptions counts the number of exceptions in the current thread that have been thrown/rethrown and not yet entered a matching catch clause, whereas uncaught_exception only detects whether stack unwinding is in progress.
  • New insertion functions try_emplace and insert_or_assign for std::map and std::unordered_map key-value associative data structures
  • Uniform container access: std::size, std::empty and std::data
  • Definition of "contiguous iterators": a file system library, std::filesystem
  • Parallel versions of STL algorithms: additional mathematical special functions, including elliptic integrals and Bessel functions: std::variant, a tagged union containerstd::byte, allowing char to be replaced for data types intending to model a byte of data as a byte rather than a character
  • Logical operator traits: std::conjunction, std::disjunction and std::negation, header, for polymorphic memory resources

Removed features

This revision of C++ not only added new features but also removed a few.
  • Trigraphs were removed.
  • Some deprecated types and functions were removed from the standard library, including std::auto_ptr, std::random_shuffle, and old function adaptors. These were superseded in C++11 by improved facilities such as std::unique_ptr, std::shuffle, std::bind, and lambdas.
  • The use of the keyword as a storage class specifier was removed. This keyword is still reserved but now unused.

Compiler support

Library support

  • libstdc++ since version 9.1 has complete support for C++17
  • libc++ as of version 9 has partial support for C++17, with the remainder "in progress"
  • Visual Studio 2017 15.8 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 in Visual Studio 2019 16.4