Integrating feature-test macros into the C++ WD

Preface

This revision of this paper proposes incorporating feature-testing macros into the working draft of the C++ standard.

Contents

1. Introduction
2. Wording
   1. Testing for the presence of an attribute: __has_cpp_attribute
   2. Feature-testing macros
3. Feedback question on RTTI and exceptions
4. Revision history

Introduction

At the November 2017 (Albuquerque) meeting of WG21, there was a five-way poll of all of the C++ experts in attendance concerning their support for "Feature macros should be standardized in the IS, and we want to invite a proposal to be reviewed by EWG to discss the rationale and granularity." The results of the poll:

The approach taken here is as follows:

• For the macros incorporated, use the latest value from SD-6, and do not bump the numbers. This is done in order to avoid pointless breakage of existing code that uses the SD-6 macros.
• Library macros are defined both in the corresponding header for the library facility and in <version>.

Wording

Testing for the presence of an attribute: __has_cpp_attribute

After [lex.literal], insert a new subsection of section [lex] with the following content:

has-attribute-expression:

__has_cpp_attribute ( attribute-token )

A has-attribute-expression shall appear only in the controlling constant expression of a #if or #elif directive ([cpp.cond] 16.1). The has-attribute-expression is replaced by a non-zero pp-number if the implementation supports an attribute with the specified name, and by the pp-number 0 otherwise.

For a standard attribute, the value of the __has_cpp_attribute macro is based on the year and month in which the attribute was voted into the working draft. In the case where the attribute is vendor-specific, the value is implementation-defined. However, in most cases it is expected that the availability of an attribute can be detected by any non-zero result.

The #ifdef and #ifndef directives, and the defined conditional inclusion operator, shall treat __has_cpp_attribute as if it were the name of a defined macro. The identifier __has_cpp_attribute shall not appear in any context not mentioned in this section.

This demonstrates a way to use the attribute [[deprecated]] only if it is available.

#ifndef __has_cpp_attribute
# define __has_cpp_attribute(x) 0
#endif
#if __has_cpp_attribute(deprecated)
# define ATTR_DEPRECATED(msg) [[deprecated(msg)]]
#else
# define ATTR_DEPRECATED(msg)
#endif
  

Feature-testing macros

Drafting note: these are intended to match the latest values in SD-6. Wording review should check that that is indeed so.

After [support.runtime], insert a new subsection of section [language.support] with the following content:

The macros that are not predefined are defined in the header <version> in addition to the header specified in the table.

Macro name	Value	Header
__cpp_unicode_characters	200704	predefined
__cpp_raw_strings	200710	predefined
__cpp_unicode_literals	200710	predefined
__cpp_user_defined_literals	200809	predefined
__cpp_threadsafe_static_init	200806	predefined
__cpp_lambdas	200907	predefined
__cpp_range_based_for	200907	predefined
__cpp_static_assert	200410	predefined
__cpp_decltype	200707	predefined
__cpp_attributes	200809	predefined
__has_cpp_attribute(noreturn)	200809	predefined
__has_cpp_attribute(carries_dependency)	200809	predefined
__cpp_rvalue_references	200610	predefined
__cpp_variadic_templates	200704	predefined
__cpp_initializer_lists	200806	predefined
__cpp_delegating_constructors	200604	predefined
__cpp_nsdmi	200809	predefined
__cpp_ref_qualifiers	200710	predefined
__cpp_alias_templates	200704	predefined
__cpp_binary_literals	201304	predefined
__cpp_init_captures	201304	predefined
__cpp_generic_lambdas	201304	predefined
__cpp_sized_deallocation	201309	predefined
__cpp_decltype_auto	201304	predefined
__cpp_return_type_deduction	201304	predefined
__has_cpp_attribute(deprecated)	201309	predefined
__cpp_aggregate_nsdmi	201304	predefined
__cpp_variable_templates	201304	predefined
__cpp_lib_integer_sequence	201304	<utility>
__cpp_lib_exchange_function	201304	<utility>
__cpp_lib_tuples_by_type	201304	<utility>
__cpp_lib_tuple_element_t	201402	<utility>
__cpp_lib_make_unique	201304	<memory>
__cpp_lib_transparent_operators	201210	<functional>
__cpp_lib_integral_constant_callable	201304	<type_traits>
__cpp_lib_transformation_trait_aliases	201304	<type_traits>
__cpp_lib_result_of_sfinae	201210	<functional> <type_traits>
__cpp_lib_is_final	201402	<type_traits>
__cpp_lib_is_null_pointer	201309	<type_traits>
__cpp_lib_string_udls	201304	<string>
__cpp_lib_generic_associative_lookup	201304	<map> <set>
__cpp_lib_null_iterators	201304	<iterator>
__cpp_lib_make_reverse_iterator	201402	<iterator>
__cpp_lib_robust_nonmodifying_seq_ops	201304	<algorithm>
__cpp_lib_complex_udls	201309	<complex>
__cpp_lib_quoted_string_io	201304	<iomanip>
__has_include(<shared_mutex>)	1	predefined
__cpp_lib_shared_timed_mutex	201402	<shared_mutex>
__cpp_hex_float	201603	predefined
__cpp_inline_variables	201606	predefined
__cpp_aligned_new	201606	predefined
__cpp_guaranteed_copy_elision	201606	predefined
__cpp_noexcept_function_type	201510	predefined
__cpp_fold_expressions	201411	predefined
__cpp_capture_star_this	201603	predefined
__cpp_constexpr	201603	predefined
__cpp_if_constexpr	201606	predefined
__cpp_range_based_for	201603	predefined
__cpp_static_assert	201411	predefined
__cpp_deduction_guides	201606	predefined
__cpp_deduction_guides	201611	predefined
__cpp_nontype_template_parameter_auto	201606	predefined
__cpp_namespace_attributes	201411	predefined
__cpp_enumerator_attributes	201411	predefined
__cpp_inheriting_constructors	201511	predefined
__cpp_variadic_using	201611	predefined
__has_cpp_attribute(fallthrough)	201603	predefined
__has_cpp_attribute(nodiscard)	201603	predefined
__has_cpp_attribute(maybe_unused)	201603	predefined
__cpp_structured_bindings	201606	predefined
__cpp_aggregate_bases	201603	predefined
__cpp_nontype_template_args	201411	predefined
__cpp_template_template_args	201611	predefined
__cpp_fold_expressions	201603	predefined
__has_include	defined	predefined
__cpp_lib_byte	201603	<cstddef>
__cpp_lib_hardware_interference_size	201703	<new>
__cpp_lib_launder	201606	<new>
__cpp_lib_uncaught_exceptions	201411	<exception>
__cpp_lib_as_const	201510	<utility>
__cpp_lib_make_from_tuple	201606	<tuple>
__cpp_lib_apply	201603	<tuple>
__has_include(<optional>)	1	predefined
__cpp_lib_optional	201603	<optional>
__has_include(<any>)	1	predefined
__cpp_lib_any	201603	<any>
__has_include(<memory_resource>)	1	predefined
__cpp_lib_memory_resource	201603	<memory_resource>
__cpp_lib_boyer_moore_searcher	201603	<functional>
__has_include(<string_view>)	1	predefined
__cpp_lib_string_view	201603	<string_view>
__cpp_lib_sample	201603	<algorithm>
__cpp_lib_any	201606	<any>
__cpp_lib_optional	201606	<optional>
__cpp_lib_optional	201606	<optional>
__has_include(<variant>)	1	predefined
__cpp_lib_variant	201606	<variant>
__cpp_lib_addressof_constexpr	201603	<memory>
__cpp_lib_raw_memory_algorithms	201606	<memory>
__cpp_lib_transparent_operators	201510	<memory> <functional>
__cpp_lib_enable_shared_from_this	201603	<memory>
__cpp_lib_shared_ptr_weak_type	201606	<memory>
__cpp_lib_shared_ptr_arrays	201611	<memory>
__cpp_lib_invoke	201411	<functional>
__cpp_lib_not_fn	201603	<functional>
__cpp_lib_void_t	201411	<type_traits>
__cpp_lib_bool_constant	201505	<type_traits>
__cpp_lib_type_trait_variable_templates	201510	<type_traits>
__cpp_lib_logical_traits	201510	<type_traits>
__cpp_lib_is_swappable	201603	<type_traits>
__cpp_lib_is_invocable	201703	<type_traits>
__cpp_lib_has_unique_object_representations	201606	<type_traits>
__cpp_lib_is_aggregate	201703	<type_traits>
__cpp_lib_chrono	201611	<chrono>
__has_include(<execution>)	1	predefined
__cpp_lib_execution	201603	<execution>
__cpp_lib_parallel_algorithm	201603	<algorithm> <numeric>
__cpp_lib_to_chars	201611	<utility>
__cpp_lib_string_view	201606	<string> <string_view>
__cpp_lib_allocator_traits_is_always_equal	201411	<memory> <scoped_allocator> <string> <deque> <forward_list> <list> <vector> <map> <set> <unordered_map> <unordered_set>
__cpp_lib_incomplete_container_elements	201505	<forwardlist> <list> <vector>
__cpp_lib_map_try_emplace	201411	<map>
__cpp_lib_unordered_map_try_emplace	201411	<unordered_map>
__cpp_lib_node_extract	201606	<map> <set> <unordered_map> <unordered_set>
__cpp_lib_array_constexpr	201603	<iterator> <array>
__cpp_lib_nonmember_container_access	201411	<iterator> <array> <deque> <forward_list> <list> <map> <regex> <set> <string> <unordered_map> <unordered_set> <vector>
__cpp_lib_clamp	201603	<algorithm>
__cpp_lib_gcd_lcm	201606	<numeric>
__cpp_lib_hypot	201603	<cmath>
__cpp_lib_math_special_functions	201603	<cmath>
__has_include(<filesystem>)	1	predefined
__cpp_lib_filesystem	201703	<filesystem>
__cpp_lib_atomic_is_always_lock_free	201603	<atomic>
__cpp_lib_shared_mutex	201505	<shared_mutex>
__cpp_lib_scoped_lock	201703	<mutex>

Feedback question on RTTI and exceptions

With very few exceptions, the features of C++98 have all been implemented in virtually every C++ compiler. But in many compilers, some of them can be enabled/disabled. It is recommended that the macros in the following table should be used to indicate whether one of these features is enabled in the current compilation.

Revision history