# C++ 2017

ISO C++ 2017

In this implementation the `-std=gnu++17` or `-std=c++17` flag must be used to enable language and library features. See dialect options. The pre-defined symbol `__cplusplus` is used to check for the presence of the required flag.

This section describes the C++17 and library TS support in the GCC 7 series.

The following table lists new library features that have been accepted into the C++17 working draft. The "Proposal" column provides a link to the ISO C++ committee proposal that describes the feature, while the "Status" column indicates the first version of GCC that contains an implementation of this feature (if it has been implemented). The "SD-6 Feature Test" column shows the corresponding macro or header from SD-6: Feature-testing recommendations for C++.

C++ 2017 Implementation Status

Library Feature | Proposal | Status | SD-6 Feature Test
--- | --- | --- | ---
`constexpr std::hardware_{constructive,destructive}_interference_size` | P0154R1 | No | `__cpp_lib_hardware_interference_size >= 201603`
Core Issue 1776: Replacement of class objects containing reference members | P0137R1 | 7.1 | `__cpp_lib_launder >= 201606`
Wording for `std::uncaught_exceptions` | N4259 | 6.1 | `__cpp_lib_uncaught_exceptions >= 201411`
Variant: a type-safe union for C++17 | P0088R3 | 7.1 | `__has_include(<variant>)`, `__cpp_lib_variant >= 201603` (since 7.3, see Note 1)
Library Fundamentals V1 TS Components: `optional` | P0220R1 | 7.1 | `__has_include(<optional>)`, `__cpp_lib_optional >= 201603` (since 7.3, see Note 1)
Library Fundamentals V1 TS Components: `any` | P0220R1 | 7.1 | `__has_include(<any>)`, `__cpp_lib_any >= 201603` (since 7.3, see Note 1)
Library Fundamentals V1 TS Components: `string_view` | P0220R1 | 7.1 | `__has_include(<string_view>)`, `__cpp_lib_string_view >= 201603` (since 7.3, see Note 1)
Library Fundamentals V1 TS Components: `memory_resource` | P0220R1 | No | `__has_include(<memory_resource>)`, `__cpp_lib_memory_resource >= 201603`
Library Fundamentals V1 TS Components: `apply` | P0220R1 | 7.1 | `__cpp_lib_apply >= 201603`
Library Fundamentals V1 TS Components: `shared_ptr<T[]>` | P0220R1 | 7.1 | `__cpp_lib_shared_ptr_arrays >= 201603`
Library Fundamentals V1 TS Components: Searchers | P0220R1 | 7.1 | `__cpp_lib_boyer_moore_searcher >= 201603`
Library Fundamentals V1 TS Components: Sampling | P0220R1 | 7.1 | `__cpp_lib_sample >= 201603`
Constant View: A proposal for a `std::as_const` helper function template | P0007R1 | 7.1 | `__cpp_lib_as_const >= 201510`
Improving pair and tuple | N4387 | 6.1 | N/A
`make_from_tuple`: apply for construction | P0209R2 | 7.1 | `__cpp_lib_make_from_tuple >= 201606`
Removing `auto_ptr`, `random_shuffle()`, And Old `<functional>` Stuff | N4190 | No (kept for backwards compatibility) |
Deprecating Vestigial Library Parts in C++17 | P0174R2 | No |
Making `std::owner_less` more flexible | P0074R0 | 7.1 | `__cpp_lib_transparent_operators >= 201510`
`std::addressof` should be constexpr | LWG2296 | 7.1 | `__cpp_lib_addressof_constexpr >= 201603`
Safe conversions in `unique_ptr<T[]>` | N4089 | 6 |
LWG 2228: Missing SFINAE rule in unique_ptr templated assignment | N4366 | 6 |
Re-enabling `shared_from_this` | P0033R1 | 7.1 | `__cpp_lib_enable_shared_from_this >= 201603`
A proposal to add `invoke` function template | N4169 | 6.1 | `__cpp_lib_invoke >= 201411`
TriviallyCopyable `reference_wrapper` | N4277 | 5.1 |
Adopt `not_fn` from Library Fundamentals 2 for C++17 | P0005R4 | 7.1 | `__cpp_lib_not_fn >= 201603`
Fixes for `not_fn` | P0358R1 | 7.1 |
Fixing a design mistake in the searchers interface in Library Fundamentals | P0253R1 | 7.1 |
Extending memory management tools | P0040R3 | 7.1 | ``
`shared_ptr::weak_type` | P0163R0 | 7.1 | `__cpp_lib_shared_ptr_weak_type >= 201606`
Transformation Trait Alias `void_t` | N3911 | 6.1 | `__cpp_lib_void_t >= 201411`
Wording for `bool_constant`, revision 1 | N4389 | 6.1 | `__cpp_lib_bool_constant >= 201505`
Adopt Type Traits Variable Templates from Library Fundamentals TS for C++17 | P0006R0 | 7.1 | `__cpp_lib_type_trait_variable_templates >= 201510`
Logical Operator Type Traits | P0013R1 | 6.1 | `__cpp_lib_logical_traits >= 201510`
Adding [nothrow-]swappable traits | P0185R1 | 7 (`__is_swappable` available since 6.1) | `__cpp_lib_is_swappable >= 201603`
`is_callable`, the missing INVOKE related trait | P0077R2 | 7.1 | `__cpp_lib_is_callable >= 201603`
has_unique_object_representations | P0258R2 | 7.1 | `__cpp_lib_has_unique_object_representations >= 201606`
Polishing `<chrono>` | P0092R1 | 7.1 | `__cpp_lib_chrono >= 201510`
Adding more constexpr to `<chrono>` | P0505R0 | 7.1 | `__cpp_lib_chrono >= 201611` (since 7.3, see Note 2)
Constexpr for `std::char_traits` | P0426R1 | 7.3 | `__cpp_lib_constexpr_char_traits >= 201611`
Integrating `std::string_view` and `std::string` | P0254R2 | 7.1 | ``
Give 'std::string' a non-const '.data()' member function | P0272R1 | 7.1 | ``
Cleaning-up noexcept in the Library | N4258 | 6.1 | `__cpp_lib_allocator_traits_is_always_equal >= 201411`
Contiguous Iterators | N4284 | N/A |
Minimal incomplete type support for standard containers | N4510 | 3.0 | `__cpp_lib_incomplete_container_elements >= 201505` (since 6.2, see Note 2)
Emplace return type | P0084R2 | 7.1 |
Improved insertion interface for unique-key maps | N4279 | 6.1 | `__cpp_lib_map_try_emplace >= 201411`, `__cpp_lib_unordered_map_try_emplace >= 201411`
Splicing Maps and Sets | P0083R3 | 7.1 | `__cpp_lib_node_extract >= 201606`
Non-member `size()` and more | N4280 | 6.1 | `__cpp_lib_nonmember_container_access >= 201411`
A Proposal to Add Constexpr Modifiers to `reverse_iterator`, `move_iterator`, `array` and Range Access | P0031R0 | 7.1 | `__cpp_lib_array_constexpr >= 201603`
The Parallelism TS Should be Standardized | P0024R2 | No | `__has_include(<execution>)` , `__cpp_lib_execution >= 201603` , `__cpp_lib_parallel_algorithm >= 201603`
An algorithm to "clamp" a value between a pair of boundary values | P0025R0 | 7.1 | `__cpp_lib_clamp >= 201603`
Adopt Selected Library Fundamentals V2 Components for C++17 | P0295R0 | 7.1 | `__cpp_lib_gcd_lcm >= 201606`
Proposal to Introduce a 3-Argument Overload to `std::hypot` | P0030R1 | 7.1 | `__cpp_lib_hypot >= 201603`
Mathematical Special Functions for C++17 | P0226R1 | 7.1 | `__cpp_lib_math_special_functions >= 201603` (see Note 3)
Adopt the File System TS for C++17 | P0218R1 | No | `__has_include(<filesystem>)` , `__cpp_lib_filesystem >= 201603`
Relative Paths for Filesystem | P0219R1 | No | `__cpp_lib_filesystem >= 201606`
Adapting string_view by filesystem paths | P0392R0 | No | `__cpp_lib_filesystem >= 201606`
Directory Entry Caching for Filesystem | P0317R1 | No | `__cpp_lib_filesystem >= 201703`
constexpr `atomic<T>::is_always_lock_free` | P0152R1 | 7.1 | `__cpp_lib_atomic_is_always_lock_free >= 201603`
A proposal to add `shared_mutex` (untimed) (Revision 4) | N4508 | 6.1 | `__cpp_lib_shared_mutex >= 201505`
Variadic `lock_guard` (Rev. 5) | P0156R2 | 7.1 | `__cpp_lib_scoped_lock >= 201703`
A byte type definition | P0298R3 | 7.1 | `__cpp_lib_byte >= 201603` (since 7.3, see Note 2)
Elementary string conversions | P0067R5 | 8 (only integral types supported) | `__has_include(<charconv>)` `__cpp_lib_to_chars >= 201611`

Note 1: This feature is supported in GCC 7.1 and 7.2 but before GCC 7.3 the `__cpp_lib` macro is not defined, and compilation will fail if the header is included without using `-std` to enable C++17 support.

Note 2: This feature is supported in older releases but the `__cpp_lib` macro is not defined to the right value (or not defined at all) until the version shown in parentheses.

Note 3: The mathematical special functions are enabled in C++17 mode from GCC 7.1 onwards. For GCC 6.x or for C++11/C++14 define `__STDCPP_WANT_MATH_SPEC_FUNCS__` to a non-zero value and test for `__STDCPP_MATH_SPEC_FUNCS__ >= 201003L`.

C++ Technical Specifications Implementation Status

Paper | Title | Status | Comments
--- | --- | --- | ---
N4076 | A generalized callable negator | Y | Library Fundamentals 2 TS
N4273 | Uniform Container Erasure | Y | Library Fundamentals 2 TS
N4061 | Greatest Common Divisor and Least Common Multiple | Y | Library Fundamentals 2 TS
N4066 | Delimited iterators | Y | Library Fundamentals 2 TS
N4282 | The World's Dumbest Smart Pointer | Y | Library Fundamentals 2 TS
N4388 | Const-Propagating Wrapper | Y | Library Fundamentals 2 TS
N4391 | `make_array`, revision 4 | Y | Library Fundamentals 2 TS
N4502 | Support for the C++ Detection Idiom, V2 | Y | Library Fundamentals 2 TS
N4519 | Source-Code Information Capture | Y | Library Fundamentals 2 TS
N4521 | Merge Fundamentals V1 into V2 | N | Library Fundamentals 2 TS
P0013R1 | Logical Operator Type Traits (revision 1) | Y | Library Fundamentals 2 TS

## Implementation Specific Behavior

For behaviour which is also specified by previous standards, see C++ 1998/2003 Implementation Specific Behavior and C++ 2011 Implementation Specific Behavior. This section only documents behaviour which is new in the 2017 standard.

23.6.5 [optional.bad_optional_access] `what()` returns `"bad optional access"`.

23.7.3 [variant.variant] Whether variant supports over-aligned types should be documented here.

23.7.10 [variant.bad.access] `what()` returns `"Unexpected index"`.

23.12.5.2 [memory.resource.pool.options] The limits for maximum number of blocks and largest allocation size supported by pool_options should be documented here.

23.12.6.1 [memory.resource.monotonic.buffer.ctor] The default `next_buffer_size` and growth factor should be documented here.

23.15.4.3 [meta.unary.prop] The predicate condition for `has_unique_object_representations` is true for all scalar types except floating point types.

23.19.3 [execpol.type], 28.4.3 [algorithms.parallel.exec] There are no implementation-defined execution policies.

24.4.2 [string.view.template] basic_string_view<C, T>::iterator is `C*` and basic_string_view<C, T>::const_iterator is `const C*`.

28.4.3 [algorithms.parallel.exec] Threads of execution created by std::thread provide concurrent forward progress guarantees, so threads of execution implicitly created by the library will provide parallel forward progress guarantees.

29.4.1 [cfenv.syn] The effects of the `<cfenv>` functions depends on whether the `FENV_ACCESS` pragma is supported, and on the C library that provides the header.

29.6.9 [c.math.rand] Whether the `rand` function may introduce data races depends on the target C library that provides the function.

29.9.5 [sf.cmath] The effect of calling the mathematical special functions with large inputs should be documented here.

30.10.2.1 [fs.conform.9945] The behavior of the filesystem library implementation will depend on the target operating system. Some features will not be not supported on some targets.

30.10.5 [fs.filesystem.syn] The clock used for file times is std::chrono::system_clock.

30.10.7.1 [fs.path.generic] dot-dot in the root-directory refers to the root-directory itself.