# 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 mainline GCC, not in any particular release.

The following table lists new library features that are included in the C++17 standard. 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 Library Features

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`
C++17 should refer to C11 instead of C99 | P0063R3 | 9.1 |
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 | 9.1 | `__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 (kept for backwards compatibility) |
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 | `__cpp_lib_raw_memory_algorithms >= 201606L`
`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_invocable >= 201703`
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 | 8.1 | `__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 | 9.1 | `__has_include(<execution>)` , `__cpp_lib_execution >= 201603` , `__cpp_lib_parallel_algorithm >= 201603` (requires linking with `-ltbb`, see Note 3)
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 4)
Adopt the File System TS for C++17 | P0218R1 | 8.1 | `__has_include(<filesystem>)` , `__cpp_lib_filesystem >= 201603` (GCC 8.x requires linking with `-lstdc++fs`)
Relative Paths for Filesystem | P0219R1 | 8.1 | `__cpp_lib_filesystem >= 201606`
Adapting string_view by filesystem paths | P0392R0 | 8.1 | `__cpp_lib_filesystem >= 201606`
Directory Entry Caching for Filesystem | P0317R1 | 8.1 | `__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 | 8.1 (only integral types supported) | `__has_include(<charconv>)` `__cpp_lib_to_chars >= 201611`
Homogeneous interface for variant, any and optional | P0032R3 | 7.1 | `__cpp_lib_any >= 201606` , `__cpp_lib_optional >= 201606` , `__cpp_lib_variant >= 201606`
Making Optional Greater Equal Again | P0307R2 | 7.1 | `__cpp_lib_optional >= 201606`

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 Parallel Algorithms have an external dependency on Intel TBB 2018 or later. If the `<execution>` header is included then `-ltbb` must be used to link to TBB.

Note 4: 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`.

The following status table is based on the table of contents of ISO/IEC 14882:2017. Some subclauses are not shown in the table where the content is unchanged since C++14 and the implementation is complete.

C++ 2017 Implementation Status

Section | Description | Status | Comments
--- | --- | --- | ---
21 | Language support
21.1 | General |  |
21.2 | Common definitions |  |
21.3 | Implementation properties |  |
21.3.1 | General |  |
21.3.2 | Header `<limits>` synopsis |  |
21.3.3 | Floating-point type properties |  |
21.3.3.1 | `float_round_style` | N |
21.3.3.2 | `float_denorm_style` | N |
21.3.4 | Class template `numeric_limits` | Y |
21.3.5 | Header `<climits>` synopsis | Y |
21.3.6 | Header `<cfloat>` synopsis | Y |
21.4 | Integer types |  |
21.4.1 | Header `<cstdint>` synopsis | Y |
21.5 | Start and termination | Partial | C library dependency for quick_exit, at_quick_exit
21.6 | Dynamic memory management |  |
21.6.1 | Header `<new>` synopsis |  |
21.6.2 | Storage allocation and deallocation | Y |
21.6.3 | Storage allocation errors | Y |
21.6.4 | Pointer optimization barrier | Y |
21.6.5 | Hardware interference size | N |
21.7 | Type identification | Y |
21.8 | Exception handling |  |
21.8.1 | Header `<exception>` synopsis |  |
21.8.2 | Class exception | Y |
21.8.3 | Class bad_exception | Y |
21.8.4 | Abnormal termination | Y |
21.8.5 | `uncaught_exceptions` | Y |
21.8.6 | Exception Propagation | Y |
21.8.7 | `nested_exception` | Y |
21.9 | Initializer lists | Y |
21.10 | Other runtime support | Y |
22 | Diagnostics
22.1 | General |  |
22.2 | Exception classes | Y |
22.3 | Assertions | Y |
22.4 | Error numbers | Y |
22.5 | System error support |  |
23 | General utilities
23.1 | General |  |
23.2 | Utility components |  |
23.2.1 | Header `<utility>` synopsis |  |
23.2.2 | Operators | Y |
23.2.3 | `swap` | Y |
23.2.4 | `exchange` | Y |
23.2.5 | Forward/move helpers | Y |
23.2.6 | Function template `as_const` | Y |
23.2.7 | Function template `declval` | Y |
23.2.8 | Primitive numeric output conversion | Partial | Only integer types supported, not floating-point types
23.2.9 | Primitive numeric input conversion | Partial | Only integer types supported, not floating-point types
23.3 | Compile-time integer sequences |  |
23.4 | Pairs | Y |
23.5 | Tuples | Y |
23.6 | Optional objects | Y |
23.7 | Variants | Y |
23.8 | Storage for any type | Y |
23.9 | Bitsets | Y |
23.10 | Memory | Y |
23.10.1 | In general |  |
23.10.2 | Header `<memory>` synopsis | Y |
23.10.3 | Pointer traits | Y |
23.10.4 | Pointer safety | Y |
23.10.5 | Align | Y |
23.10.6 | Allocator argument tag | Y |
23.10.7 | `uses_allocator` | Y |
23.10.8 | Allocator traits | Y |
23.10.9 | The default allocator | Y |
23.10.10 | Specialized algorithms | Y |
23.10.11 | C library memory allocation | Y |
23.11 | Smart pointers |  |
23.11.1 | Class template `unique_ptr` | Y |
23.11.2 | Shared-ownership pointers | Y |
23.12 | Memory resources |  |
23.12.1 | Header `<memory_resource>` synopsis | Y |
23.12.2 | Class `memory_resource` | Y |
23.12.3 | Class template `polymorphic_allocator` | Y |
23.12.4 | Access to program-wide `memory_resource` objects | Y |
23.12.5 | Pool resource classes | Y |
23.12.6 | Class `monotonic_buffer_resource` | Y |
23.13 | Class template `scoped_allocator_adaptor` | Y |
23.14 | Function objects |  |
23.14.1 | Header `<functional>` synopsis |  |
23.14.2 | Definitions |  |
23.14.3 | Requirements |  |
23.14.4 | Function template `invoke` | Y |
23.14.5 | Class template `reference_wrapper` | Y |
23.14.6 | Arithmetic operation | Y |
23.14.7 | Comparisons | Y |
23.14.8 | Logical operations | Y |
23.14.9 | Bitwise operations | Y |
23.14.10 | Function template `not_fn` | Y |
23.14.11 | Function object binders | Y |
23.14.12 | Function template `mem_fn` | Y |
23.14.13 | Polymorphic function wrappers | Y |
23.14.14 | Searchers | Y |
23.14.15 | Class template `hash` | Y |
23.15 | Metaprogramming and type traits |  |
23.15.1 | Requirements |  |
23.15.2 | Header `<type_traits>` synopsis | Y |
23.15.3 | Helper classes | Y |
23.15.4 | Unary Type Traits | Y |
23.15.5 | Type property queries | Y |
23.15.6 | Relationships between types | Y |
23.15.7 | Transformations between types | Y |
23.15.8 | Logical operator traits | Y |
23.16 | Compile-time rational arithmetic | Y |
23.17.1 | In general |  |
23.17.2 | Header `<chrono>` synopsis |  |
23.17 | Time utilities |  |
23.17.3 | Clock requirements | Y |
23.17.4 | Time-related traits | Y |
23.17.5 | Class template `duration` | Y |
23.17.6 | Class template `time_point` | Y |
23.17.7 | Clocks | Y |
23.17.8 | Header `<ctime>` synopsis | Y |
23.18 | Class `type_index` | Y |
23.19 | Execution policies |  |
23.19.1 | In general |  |
23.19.2 | Header `<execution>` synopsis |  |
23.19.3 | Execution policy type trait | Y |
23.19.4 | Sequenced execution policy | Y |
23.19.5 | Parallel execution policy | Y |
23.19.6 | Parallel and unsequenced execution policy | Y |
23.19.7 | Execution policy objects | Y |
24 | Strings
24.1 | General |  |
24.2 | Character traits | Y |
24.3 | String classes | Y |
24.4 | String view classes | Y |
24.4.1 | Header `<string_view>` synopsis | Y |
24.4.2 | Class template `basic_string_view` | Y |
24.4.3 | Non-member comparison functions | Y |
24.4.4 | Inserters and extractors | Y |
24.4.5 | Hash support | Y |
24.4.6 | Suffix for `basic_string_view` literals | Y |
24.5 | Null-terminated sequence utilities | Partial | C library dependency.
25 | Localization
25.1 | General | Y |
25.2 | Header `<locale>` synopsis | Y |
25.3 | Locales | Y |
25.4 | Standard `locale` categories | Y |
25.5 | C Library Locales | Y |
26 | Containers
26.1 | General |  |
26.2 | Container requirements | Y |
26.3 | Sequence containers | Y |
26.4 | Associative containers | Y |
26.5 | Unordered associative containers | Y |
26.6 | Container adaptors | Y |
27 | Iterators
27.1 | General | Y |
27.2 | Iterator requirements | Y |
27.3 | Header `<iterator>` synopsis | Y |
27.4 | Iterator primitives | Y |
27.5 | Iterator adaptors | Y |
27.6 | Stream iterators | Y |
27.7 | Range access | Y |
27.8 | Container access | Y |
28 | Algorithms
28.1 | General |  |
28.2 | Header `<algorithm>` synopsis |  |
28.3 | Algorithms requirements |  |
28.4 | Parallel algorithms |  | Using PSTL
28.5 | Non-modifying sequence operations | Y |
28.6 | Mutating sequence operations | Y |
28.7 | Sorting and related operations | Y |
28.8 | C library algorithms | Y |
29 | Numerics
29.1 | General |  |
29.2 | Definitions |  |
29.3 | Numeric type requirements | Y |
29.4 | The floating-point environment | Y |
29.5 | Complex numbers | Y |
29.6 | Random number generation | Y |
29.7 | Numeric arrays | Y |
29.8 | Generalized numeric operations |  |
29.8.1 | Header `<numeric>` synopsis |  |
29.8.2 | Accumulate | Y |
29.8.3 | Reduce | Y |
29.8.4 | Inner product | Y |
29.8.5 | Transform reduce | Y |
29.8.6 | Partial sum | Y |
29.8.7 | Exclusive scan | Y |
29.8.8 | Inclusive scan | Y |
29.8.9 | Transform exclusive scan | Y |
29.8.10 | Transform inclusive scan | Y |
29.8.11 | Adjacent difference | Y |
29.8.12 | Iota | Y |
29.8.13 | Greatest common divisor | Y |
29.8.14 | Least common multiple | Y |
29.9 | Mathematical functions for floating-point types |  |
29.9.1 | Header `<cmath>` synopsis |  |
29.9.2 | Absolute values | Y |
29.9.3 | Three-dimensional hypotenuse | Y |
29.9.4 | Classification / comparison functions | Y |
29.9.5 | Mathematical special functions | Y |
30 | Input/output library
30.1 | General | Y |
30.2 | Iostreams requirements | Y |
30.2.1 | Imbue Limitations | Y |
30.2.2 | Positioning Type Limitations | Y |
30.2.3 | Thread safety | Partial |
30.3 | Forward declarations | Y |
30.4 | Standard iostream objects | Y |
30.5 | Iostreams base classes | Y |
30.6 | Stream buffers | Y |
30.7 | Formatting and manipulators | Y |
30.8 | String-based streams | Y |
30.9 | File-based streams | Y |
30.10 | File systemss | Y |
30.11 | C library files | Y |
31 | Regular expressions
31.1 | General | Y |
31.2 | Definitions | Y |
31.3 | Requirements | Y |
31.4 | Header `<regex>` synopsis | Y |
31.5 | Namespace `std::regex_constants` | Y |
31.6 | Class `regex_error` | Y |
31.7 | Class template `regex_traits` | Partial | `transform_primary` is not correctly implemented |
31.8 | Class template `basic_regex` | Y |
31.9 | Class template `sub_match` | Y |
31.10 | Class template `match_results` | Y |
31.11 | Regular expression algorithms | Y |
31.12 | Regular expression Iterators | Y |
31.13 | Modified ECMAScript regular expression grammar | Y |
32 | Atomic operations
32.1 | General | Y |
32.2 | Header `<atomic>` synopsis | Y |
32.3 | Type aliases | Y |
32.4 | Order and consistency | Y |
32.5 | Lock-free property | Y |
32.6 | Class template `<atomic>` | Y |
32.7 | Non-member functions | Y |
32.8 | Flag Type and operations | Y |
32.9 | Fences | Y |
33 | Thread support
33.1 | General | Y |
33.2 | Requirements | Y |
33.3 | Threads |  |
33.3.1 | Header `thread` synopsis |  |
33.3.2 | Class `thread` | Y |
33.3.2.1 | Class `thread` | Partial | `thread::id` comparisons not well-defined
33.3.3 | Namespace `this_thread` | Y |
33.4 | Mutual exclusion |  |
33.4.3 | Mutex requirements |  |
33.4.3.1 | In general |  |
33.4.3.2 | Mutex types |  |
33.4.3.2.1 | Class `mutex` | Y |
33.4.3.2.2 | Class `recursive_mutex` | Y |
33.4.3.3 | Timed mutex types |  |
33.4.3.3.1 | Class `timed_mutex` | Y |
33.4.3.3.2 | Class `recursive_timed_mutex` | Y |
33.4.3.4 | Shared mutex types |  |
33.4.3.4.1 | Class `shared_mutex` | Y |
33.4.3.5 | Shared timed mutex types |  |
33.4.3.5.1 | Class `shared_timed_mutex` | Y |
33.4.4 | Locks |  |
33.4.4.1 | Class template `lock_guard` | Y |
33.4.4.2 | Class template `scoped_guard` | Y |
33.4.4.3 | Class template `unique_lock` | Y |
33.4.4.4 | Class template `shared_lock` | Y |
33.4.5 | Generic locking algorithms | Y |
33.4.6 | Call once |  |
33.4.6.1 | Struct `once_flag` | Y |
33.4.6.2 | Function `call_once` | Broken | See PR 66146
33.5 | Condition variables | Y |
33.5.1 | Class `condition_variable` | Y |
33.5.2 | Class `condition_variable_any` | Y |
33.6 | Futures |  |
33.6.1 | Overview |  |
33.6.2 | Header `<future>` |  |
33.6.3 | Error handling | Y |
33.6.4 | Class `future_error` | Y |
33.6.5 | Shared state | Y |
33.6.6 | Class template `promise` | Y |
33.6.7 | Class template `future` | Y |
33.6.8 | Class template `shared_future` | Y |
33.6.9 | Function template `async` | Y |
33.6.10 | Class template `packaged_task` | Y |
Appendix D | Compatibility features
D.4 | C++ standard library headers | Y |
D.4.1 | Header `<ccomplex>` synopsis | Y |
D.4.1 | Header `<cstdalign>` synopsis | Y |
D.4.1 | Header `<cstdbool>` synopsis | Y |
D.4.1 | Header `<ctgmath>` synopsis | Y |
D.5 | C standard library headers | Y |
D.6 | `char*` streams | Y |
D.7 | `uncaught_exception` | Y |
D.8 | Old adaptable function bindings | Y |
D.9 | The default allocator | Y |
D.10 | Raw storage iterator | Y |
D.11 | Temporary buffers | Y |
D.12 | Deprecated type traits | Y |
D.13 | Deprecated iterator primitives | Y |
D.14 | Deprecated `shared_ptr` observers | Y |
D.15 | Deprecated standard code conversion facets | Y |
D.16 | Deprecated convenience conversion interfaces | Y |

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 (components from V1 are still in namespace `fundamentals_v1`) | Library Fundamentals 2 TS
P0013R1 | Logical Operator Type Traits (revision 1) | Y | Library Fundamentals 2 TS
N4531 | std::rand replacement, revision 3 | 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.

20.5.1.2 [headers] Whether names from Annex K are declared by C++ headers depends on whether the underlying C library supports Annex K and declares the names. For the GNU C library, there is no Annex K support and so none of its names are declared by C++ headers.

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

23.7.3 [variant.variant] variant supports over-aligned types.

23.7.10 [variant.bad.access] `what()` returns one of the strings `"std::get: variant is valueless"`, `"std::get: wrong index for variant"`, `"std::visit: variant is valueless"`, or `"std::visit<R>: variant is valueless"`.

23.12.5.2 [memory.resource.pool.options] Let S equal `numeric_limits<size_t>::digits`. The limit for maximum number of blocks in a chunk is given by 2N-1, where N is min(19, 3 + S/2). The largest allocation size that will be allocated from a pool is 222 when S > 20, otherwise 3072 when S > 16, otherwise 768.

23.12.6.1 [memory.resource.monotonic.buffer.ctor] The default `next_buffer_size` is `128 * sizeof(void*)`. The default growth factor is `1.5`.

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 supported on some targets.

30.10.5 [fs.filesystem.syn] The clock used for file times is an unspecified type with a signed 64-bit representation, capable of representing timestamps with nanosecond resolution. The clock's epoch is unspecified, but is not the same as the system clock's epoch.

30.10.7.1 [fs.path.generic] dot-dot in the root-directory refers to the root-directory itself. On Windows, a drive specifier such as `"C:"` or `"z:"` is treated as a root-name. On Cygwin, a path that begins with two successive directory separators is a root-name. Otherwise (for POSIX-like systems other than Cygwin), the implementation-defined root-name is an unspecified string which does not appear in any pathnames.

30.10.10.1 [fs.enum.path.format] The character sequence is always interpreted in the native pathname format.

30.10.15.4 [fs.op.file_size] If `!is_regular_file(p)`, an error is reported.