1 Revision history

R0 [P1902R0] of this paper was presented to SG10 in Belfast in November, 2019. Some of the macros were renamed to be more consistent with other design decisions, and a couple more were added for papers that were missed. We also came up with a constexpr policy.

2 Introduction

When [P0941R2] was adopted both into the standard and as policy, that paper thoroughly went through all the relevant features that existed at the time. At the Cologne meeting in July 2019, we added signs in both the Core and Library Working Group rooms to ensure that papers in flight did not forget - they had to either explicitly adopt a new feature-test macro or explicitly state why they did not need one.

In the intervening time-frame, several meetings’ worth of papers were adopted which did not have a feature-test macro. The goal is this paper is to go through all of those papers and add the missing ones.

As a general note, [SD6] has been updated and is current through Cologne and through a few LWG issues following that.

2.1 NB Comments

This paper resolves the following NB comments:

• FI015
• GB146: add a macro for concepts.
• GB147: add a macro for consteval.
• US150: add a macro for familiar syntax for generic lambdas.
• US167: add a macro for non-member ssize()
• DE168: providing a consistent policy for constexpr

3 constexpr policy

In the previous draft of this paper, there was a discussion on whether or not we want fine-grained or coarse-grained feature test macros for all the constexpr extensions.

The issue is: we keep extending both the language and the library functionality for what you can do at constant evaluation time. How do we test for those? If we only have coarse-grained macros, this is hostile to implementers (because it requires a strict, linear adoption of features, some of which may be both large and unrelated) and this is hostile to users (because they check this one huge umbrella macro). If we have overly fine macros, this is harmful to the implementation as it adds more macros that compilers need to synthesize, and just adds more things that we have to check.

The policy discussed and agreed upon by SG10 in Belfast is as follows:

• For the language, we will have a single __cpp_constexpr macro. It will be bumped every time we extend constexpr in the language.
• For the library, we will add a specific feature test macros for significant, special features. Otherwise, for those cases where we are just adding constexpr to more things in the library, we will have a dedicated test macro per header and just bump that header-specific macro on each change. That macro will be named __cpp_lib_constexpr_HEADER (with the exception of a few preexisting macros for array and algorithm which have slightly different names).

This policy leads to several changes from R0 of this paper.

4 2017

4.1 Toronto (201707)

[P0683R1] (Default member initializers for bit-fields): no macro necessary.

[P0704R1] (Fixing const-qualified pointers to members): no macro necessary.

[P0409R2] (Allow lambda capture [=, this]): no macro necessary.

[P0306R4] (Comma omission and comma deletion): no macro necessary.

[P0329R4] (Designated Initialization Wording): Marc Mutz suggests a use case where you might only provide a constructor if designated initialization is available (using designated initializers as a proxy for named parameters). The following is slightly reduced from his example:

class QSlider {
public:
#if __cpp_designated_initializers
   struct Properties {
       struct { int min, max; } range = {};
       bool showTickMarks = false;
   };
   explicit QSlider(Properties props);
#endif
   // traditional, pre-existing approach:
   void setShowTickMarks(bool);
   bool showTickMarks() const noexcept;

   void setRange(int min, int max);
   int minimum() const noexcept;
   int maximum() const noexcept;
};

Where a C++14 user could then do:

#ifdef __cpp_designated_initializers
   auto sl = new QSlider({ .range = {0, 100}, .showTickMarks = true}, this);
#else
   auto sl = new QSlider(this);
   sl->setRange(0, 100);
   sl->setShowTickMarks(true);
#endif

This paper proposes the feature test macro __cpp_designated_initializers.

[P0428R2] (Familiar template syntax for generic lambdas): this paper proposes to bump the macro __cpp_generic_lambdas. One of the things this feature allows for is, for instance, defaulting a template parameter on a lambda, which is arguably a feature enhancement:

struct X { int i, j; };

auto f = 
#if __cpp_generic_lambdas >= 201707
    []<class T=X>(T&& var)
#else
    [](auto&& var)
#endif
    {
    };
    
// having the macro allows this usage
f({1, 2});

[P0702R1] (Language support for Constructor Template Argument Deduction): no macro necessary.

[P0734R0] (Wording Paper, C++ extensions for Concepts): this paper proposes the macro __cpp_concepts, which will be bumped later, but for SD-6 purposes will start here with the value 201707.

[P0463R1] (Endian just Endian) to the C++ working paper: Already have a macro from [P1612R1].

[P0682R1] (Repairing elementary string conversions): Already have a macro from [LWG3137].

[P0674R1] (Extending make_shared to Support Arrays): this paper proposes to bump __cpp_lib_shared_ptr_arrays, since make_shared can save an allocation.

4.2 Albuquerque (201711)

[P0614R1] (Range-based for statements with initializer): no macro necessary.

[P0588R1] (Simplifying implicit lambda capture): no macro necessary.

[P0846R0] (ADL and Function Templates that are not Visible): no macro necessary.

[P0641R2] (Resolving Core Issue #1331 (const mismatch with defaulted copy constructor)): no macro necessary.

[P0859R0] (Core Issue 1581: When are constexpr member functions defined?): this paper proposes the macro __cpp_constexpr_in_decltype. This issue is a blocker for library being able to implement [P1065R2], so the library needs to know when to be able to do that. It’s unclear if users outside of standard library implementers will need this.

[P0515R3] (Consistent comparison) and [P0768R1] (Library Support for the Spaceship (Comparison) Operator): already have a macro.

[P0857R0] (Wording for “functionality gaps in constraints”): no macro necessary.

[P0692R1] (Access Checking on Specializations): no macro necessary.

[P0624R2] (Default constructible and assignable stateless lambdas): no macro necessary.

[P0767R1] (Deprecate POD): no macro necessary.

[P0315R4] (Wording for lambdas in unevaluated contexts): no macro necessary.

[P0550R2] (Transformation Trait remove_cvref): this paper proposes the macro __cpp_lib_remove_cvref.

[P0777R1] (Treating Unnecessary decay): no macro necessary.

[P0600R1] (nodiscard in the Library): no macro necessary.

[P0439R0] (Make std::memory_order a scoped enumeration): no macro necessary

[P0053R7] (C++ Synchronized Buffered Ostream): this paper proposes the macro __cpp_lib_syncbuf.

[P0653R2] (Utility to convert a pointer to a raw pointer): this paper proposes the macro __cpp_lib_to_address.

[P0202R3] (Add constexpr modifiers to functions in <algorithm> and <utility> Headers): a macro was already added.

[P0415R1] (Constexpr for std::complex): this paper proposes to introduce the macro __cpp_lib_constexpr_complex. That is, introducing a new macro for this header.

[P0718R2] (Atomic shared_ptr): this paper proposes the macro __cpp_lib_atomic_shared_ptr.

[P0020R6] (Floating Point Atomic): this paper proposes the macro __cpp_lib_atomic_float.

[P0616R0] (de-pessimize legacy <numeric> algorithms with std::move): no macro necessary.

[P0457R2] (String Prefix and Suffix Checking): this paper proposes the macro __cpp_lib_starts_ends_with.

5 2018

5.1 Jacksonville (201803)

[P0840R2] (Language support for empty objects: already has a macro.

[P0962R1] (Relaxing the range-for loop customization point finding rules): no macro necessary.

[P0969R0] (Allow structured bindings to accessible members): no macro necessary.

[P0961R1] (Relaxing the structured bindings customization point finding rules): no macro necessary.

[P0634R3] (Down with typename!): no macro necessary.

[P0780R2] (Allow pack expansion in lambda init-capture): this paper proposes to bump the macro __cpp_init_captures. Having such a macro would allow you to avoid using tuple where necessary. The motivating example in that paper could thus conditionally improve compile throughput:

template <class... Args>
auto delay_invoke_foo(Args... args) {
#if __cpp_init_captures >= 201803
    return [...args=std::move(args)]() -> decltype(auto) {
        return foo(args...);
    };
#else
    return [tup=std::make_tuple(std::move(args)...)]() -> decltype(auto) {
        return std::apply([](auto const&... args) -> decltype(auto) {
            return foo(args...);
        }, tup);
    };
#endif
}

[P0479R5] (Proposed wording for likely and unlikely attributes (Revision 5)): already have a macro.

[P0905R1] (Symmetry for spaceship): in a vacuum, maybe, but since there isn’t an implementation of <=> that includes just up to this point, it’s probably not worth it.

[P0754R2] (<version>): already checkable with __has_include.

[P0809R0] (Comparing Unordered Containers): no macro necessary.

[P0355R7] (Extending chrono to Calendars and Time Zones): this paper proposes to bump the macro __cpp_lib_chrono.

[P0966R1] (string::reserve Should Not Shrink): no macro necessary.

[P0551R3] (Thou Shalt Not Specialize std Function Templates!): no macro necessary.

[P0753R2] (Manipulators for C++ Synchronized Buffered Ostream): this paper proposes to bump the macro __cpp_lib_syncbuf, which is also added by this paper.

[P0122R7] (<span>): already has a macro by way of [LWG3274].

[P0858R0] (Constexpr iterator requirements): already has a macro.

5.2 Rapperswil (201806)

[P0806R2] (Deprecate implicit capture of this via [=]): no macro necessary.

[P1042R1] (__VA_OPT__ wording clarifications): no macro necessary.

[P0929R2] (Checking for abstract class types): no macro necessary.

[P0732R2] (Class Types in Non-Type Template Parameters): already has a macro.

[P1025R1] (Update The Reference To The Unicode Standard): no macro necessary.

[P0528R3] (The Curious Case of Padding Bits, Featuring Atomic Compare-and-Exchange): no macro necessary.

[P0722R3] (Efficient sized delete for variable sized classes): already has a macro.

[P1064R0] (Allowing Virtual Function Calls in Constant Expressions): this paper proposes to bump __cpp_constexpr. There are functions that include virtual calls that would be able to be made constexpr. This macro already has a higher value in the standard, so no wording changes necessary - just to track in SD-6.

[P1008R1] (Prohibit aggregates with user-declared constructors): no macro necessary.

[P1120R0] (Consistency improvements for <=> and other comparison operators): no macro necessary.

[P0542R5] (Support for contract based programming in C++): would have been checked by the attribute.

[P0941R2] (Integrating feature-test macros into the C++ WD (rev. 2)): very meta.

[P0892R2] (explicit(bool)): already has a macro.

[P0476R2] (Bit-casting object representations): already has a macro.

[P0788R3] (Standard Library Specification in a Concepts and Contracts World): no macro necessary.

[P0458R2] (Checking for Existence of an Element in Associative Containers): no macro necessary. The new algorithms are nicer to use, but if you need to support the old code, then the old code works just as well.

[P0759R1] (fpos Requirements): no macro necessary.

[P1023R0] (constexpr comparison operators for std::array): this paper proposes to bump the macro __cpp_lib_array_constexpr.

[P0769R2] (Add shift to <algorithm>): this paper proposes the macro __cpp_lib_shift.

[P0887R1] (The identity metafunction): this paper proposes the macro __cpp_lib_type_identity.

[P0879R0] (Constexpr for swap and swap related functions): already has a macro.

[P0758R1] (Implicit conversion traits and utility functions). this paper proposes the macro __cpp_lib_nothrow_convertible.

[P0556R3] (Integral power-of-2 operations): this paper proposes the macro __cpp_lib_int_pow2. [ Editor's note: There are NB comments about renaming the features introduced by this paper, which may necessitate renaming this macro ]

[P0019R8] (Atomic Ref): this paper proposes the macro __cpp_lib_atomic_ref.

[P0935R0] (Eradicating unnecessarily explicit default constructors from the standard library): no macro necessary.

[P0646R1] (Improving the Return Value of Erase-Like Algorithms): already has a macro.

[P0619R4] (Reviewing Deprecated Facilities of C++17 for C++20): no macro necessary.

[P0898R3] (Standard Library Concepts): already has a macro.

5.3 San Diego (201811)

[P0982R1] (Weaken Release Sequences): no macro necessary.

[P1084R2] (Today’s _return-type-requirement_s Are Insufficient): this paper proposes to bump __cpp_concepts, which was also added by this paper.

[P1131R2] (Core Issue 2292: simple-template-id is ambiguous between class-name and type-name): no macro necessary.

[P1289R1] (Access control in contract conditions): no macro necessary

[P1002R1] (Try-catch blocks in constexpr functions): this paper proposes to bump __cpp_constexpr, as per previous arguments to allow more functions to be declared constexpr. This one more important than the rest as actually writing try in a function already makes constexpr ill-formed.

[P1327R1] (Allowing dynamic_cast, polymorphic typeid in Constant Expressions): this paper proposes to bump __cpp_constexpr.

[P1236R1] (Alternative Wording for P0907R4 Signed Integers are Two’s Complement): no macro necessary.

[P0482R6] (char8_t: A type for UTF-8 characters and strings (Revision 6)): already has a macro.

[P1353R0] (Missing Feature Test Macros): already adopted.

[P1073R3] (Immediate functions): this paper proposes the macro __cpp_consteval. There are some functions that you really only want to run at compile time. You cannot enforce this in C++17, but having this macro would allow you to conditionally enforce it as it becomes available.

[P0595R2] (std::is_constant_evaluated()): already has a macro.

[P1141R2] (Yet another approach for constrained declarations): no macro necessary.

[P1094R2] (Nested Inline Namespaces): no macro necessary.

[P1330R0] (Changing the active member of a union inside constexpr): this paper proposes to bump __cpp_constexpr, as per previous arguments. This would allow making std::optional fully constexpr, for instance.

[P1123R0] (Editorial Guidance for merging P0019r8 and P0528r3): no macro necessary.

[P0487R1] (Fixing operator>>(basic_istream&, CharT*) (LWG 2499)): no macro necessary.

[P0602R4] (variant and optional should propagate copy/move triviality): no macro necessary.

[P0655R1] (visit<R>: Explicit Return Type for visit): no macro necessary.

[P0972R0] (<chrono> zero(), min(), and max() should be noexcept): no macro necessary.

[P1006R1] (Constexpr in std::pointer_traits): this paper proposes to bump the macro __cpp_lib_constexpr_memory.

[P1032R1] (Misc constexpr bits): This paper originally added a single feature test macro, __cpp_lib_constexpr_misc, that was since renamed by [P1424R1] to __cpp_lib_constexpr. Per the policy laid out in this paper, this paper proposes to remove __cpp_lib_constexpr and instead add separate macros for each header: __cpp_lib_constexpr_functional, __cpp_lib_constexpr_iterator, __cpp_lib_constexpr_string_view, __cpp_lib_constexpr_tuple, and __cpp_lib_constexpr_utility and to bump __cpp_lib_array_constexpr.

[P1148R0] (Cleaning up Clause 20): no macro necessary.

[P0318R1] (unwrap_ref_decay and unwrap_reference): this paper proposes the macro __cpp_lib_unwrap_ref.

[P0357R3] (reference_wrapper for incomplete types): no macro necessary.

[P0608R3] (A sane variant converting constructor): no macro necessary.

[P0771R1] (std::function move constructor should be noexcept): no macro necessary.

[P1007R3] (std::assume_aligned): this paper proposes the macro __cpp_lib_assume_aligned.

[P1020R1] (Smart pointer creation with default initialization): this paper proposes the macro __cpp_lib_smart_ptr_default_init.

[P1285R0] (Improving Completeness Requirements for Type Traits): no macro necessary.

[P1248R1] (Remove CommonReference requirement from StrictWeakOrdering (a.k.a Fixing Relations): no macro necessary.

[P0591R4] (Utility functions to implement uses-allocator construction): no macro necessary.

[P0899R1] (LWG 3016 is Not a Defect): no macro necessary.

[P1085R2] (Should Span be Regular?): no macro necessary.

[P1165R1] (Make stateful allocator propagation more consistent for operator+(basic_string)): no macro necessary.

[P0896R4] (The One Ranges Proposal): already has a macro.

[P0356R5] (Simplified partial function application): already has a macro.

[P0919R3] (Heterogeneous lookup for unordered containers): already has a macro.

[P1209R0] (Adopt Consistent Container Erasure from Library Fundamentals 2 for C++20): already has a macro.

6 2019

6.1 Kona (201902)

[P1286R2] (Contra CWG DR1778): no macro necessary.

[P1091R3] (Extending structured bindings to be more like variable declarations): no macro necessary.

[P1381R1] (Reference capture of structured bindings): no macro necessary.

[P1041R4] (Make char16_t/char32_t string literals be UTF-16/32): no macro necessary.

[P1139R2] (Address wording issues related to ISO 10646): no macro necessary.

[P1323R2] (Contract postconditions and return type deduction): contracts were removed.

[P0960R3] (Allow initializing aggregates from a parenthesized list of values): already has a macro.

[P1009R2] (Array size deduction in new-expressions): no macro necessary.

[P1103R3] (Merging Modules): already has a macro

[P1185R2] (<=> != ==): this paper proposes to bump __cpp_impl_three_way_comparison.

[P0339R6] (polymorphic_allocator<> as a vocabulary type): this paper proposes the macro __cpp_lib_polymorphic_allocator.

[P0340R3] (Making std::underlying_type SFINAE-friendly): no macro necessary.

[P0738R2] (I Stream, You Stream, We All Stream for istream_iterator): no macro necssary.

[P1458R1] (Mandating the Standard Library: Clause 16 - Language support library): no macro necessary.

[P1459R1] (Mandating the Standard Library: Clause 18 - Diagnostics library): no macro necessary.

[P1462R1] (Mandating the Standard Library: Clause 20 - Strings library): no macro necessary.

[P1463R1] (Mandating the Standard Library: Clause 21 - Containers library): no macro necessary.

[P1464R1] (Mandating the Standard Library: Clause 22 - Iterators library): no macro necessary.

[P1164R1] (Make create_directory() Intuitive): no macro necessary.

[P0811R3] (Well-behaved interpolation for numbers and pointers): already has a macro.

[P1001R2] (Target Vectorization Policies from Parallelism V2 TS to C++20): no macro necessary.

[P1227R2] (Signed ssize() functions, unsigned size() functions ): this paper proposes the macro __cpp_lib_ssize.

[P1252R2] (Ranges Design Cleanup): no macro necessary.

[P1024R3] (Usability Enhancements for std::span): already has a macro.

[P0920R2] (Precalculated hash values in lookup): already had a macro, which was subsequently removed.

[P1357R1] (Traits for [Un]bounded Arrays): no macro necessary

6.2 Cologne (201907)

[P1161R3] (Deprecate uses of the comma operator in subscripting expressions): no macro necessary.

[P1331R2] (Permitting trivial default initialization in constexpr contexts): already has a macro.

[P0735R1] (Interaction of memory_order_consume with release sequences): already has a macro.

[P0848R3] (Conditionally Trivial Special Member Functions): no macro necessary. Would people really write both?

[P1186R3] (When do you actually use <=>?): this paper proposed to bump __cpp_impl_three_way_comparison, but shouldn’t have. This paper doesn’t need a macro.

[P1301R4] ([[nodiscard("should have a reason")]]): already has a macro.

[P1099R5] (Using Enum): already has a macro

[P1630R1] (Spaceship needs a tune-up): this paper proposes that the bump of __cpp_impl_three_way_comparison is associated with this paper instead of P1186R3.

[P1616R1] (Using unconstrained template template parameters with constrained templates): no macro necessary.

[P1816R0] (Wording for class template argument deduction for aggregates): no macro necessary. Would you choose to not provide a deduction guide for an aggregate?

[P1668R1] (Enabling constexpr Intrinsics By Permitting Unevaluated inline-assembly in constexpr Functions): this paper proposes to bump __cpp_constexpr.

[P1766R1] (Mitigating minor modules maladies): already has a macro.

[P1811R0] (Relaxing redefinition restrictions for re-exportation robustness): already has a macro.

[P0388R4] (Permit conversions to arrays of unknown bound): no macro necessary.

[P1823R0]: no macro necessary.

[P1143R2] (Adding the constinit keyword): already has a macro.

[P1452R2] (On the non-uniform semantics of return-type-requirements): this paper proposes to bump the value of __cpp_concepts.

[P1152R4] (Deprecating volatile): no macro necessary.

[P1771R1] ([[nodiscard]] for constructors): already has a macro.

[P1814R0] (Wording for Class Template Argument Deduction for Alias Templates): already has a macro.

[P1825R0] (Merged wording for P0527R1 and P1155R3): no macro needed, just write std::move().

[P1703R1] (Recognizing Header Unit Imports Requires Full Preprocessing): no macro needed.

[P0784R7] (More constexpr containers): already has a macro. This is a significant library feature that merits its own macro (__cpp_lib_constexpr_dynamic_alloc).

[P1355R2] (Exposing a narrow contract for ceil2): no macro needed.

[P0553R4] (Bit operations): already has a macro.

[P1424R1] (constexpr feature macro concerns): already resolved.

[P0645R10] (Text Formatting): already has a macro.

[P1361R2] (Integration of chrono with text formatting): already has a macro.

[P1652R1] (Printf corner cases in std::format): already has a macro.

[P0631R8] (Math Constants): already has a macro.

[P1135R6] (The C++20 Synchronization Library), [P1643R1] (Add wait/notify to atomic_ref), and [P1644R0] (Add wait/notify to atomic<shared_ptr>): already have macros.

[P1466R3] (Miscellaneous minor fixes for chrono): already has a macro.

[P1754R1] (Rename concepts to standard_case for C++20, while we still can): this paper proposes to bump __cpp_lib_concepts, even though nobody implements this yet, because otherwise it’s confusing.

[P1614R2] (The Mothership has Landed): erroneously introduced the new macro __cpp_lib_spaceship, this paper proposes removing that macro and instead bumping __cpp_lib_three_way_comparison.

[P0325R4] (to_array from LFTS with updates): already has a macro.

[P0408R7] (Efficient Access to basic_stringbuf’s Buffer): no macro necessary. Trying to move out of the buffer will compile even without this feature, it just won’t be a move.

[P1423R3] (char8_t backward compatibility remediation): already has a macro.

[P1502R1] (Standard library header units for C++20): no macro necessary.

[P1612R1] (Relocate Endian’s Specification): already has a macro.

[P1661R1] (Remove dedicated precalculated hash lookup interface): already removes a macro.

[P1650R0] (Output std::chrono::days with ‘d’ suffix): no macro necessary.

[P1651R0] (bind_front should not unwrap reference_wrapper): no macro necessary.

[P1065R2] (Constexpr INVOKE): as above, this paper proposes to rename __cpp_lib_constexpr_invoke to __cpp_lib_constexpr_functional.

[P1207R4] (Movability of Single-pass Iterators): no macro necessary.

[P1035R7] (Input Range Adaptors): this paper proposes to bump the macro __cpp_lib_ranges.

[P1638R1] (basic_istream_view::iterator should not be copyable): no macro necessary.

[P1522R1] (Iterator Difference Type and Integer Overflow): no macro necessary.

[P1004R2] (Making std::vector constexpr): already has a macro.

[P0980R1] (Making std::string constexpr): already has a mcro.

[P0660R10] (Stop Token and Joining Thread, Rev 10): already has a macro.

[P1474R1] (Helpful pointers for ContiguousIterator): no macro necessary.

[P1523R1] (Views and Size Types): no macro necessary.

[P0466R5] (Layout-compatibility and Pointer-interconvertibility Traits): already has a macro.

[P1208R6] (Adopt source_location for C++20): already has a macro.

7 Wording

Modify table 17 in 15.10 [cpp.predefined] with the following added:

Modify table 36 in 17.3.1 [support.limits.general] with the following added:

8 References