Feature-testing recommendations for C++

Preface

At the July 2017 (Toronto) meeting, a poll was taken in EWG:

Do we feel that the plenary should formally approve SD-6 revisions?
4 | 9 | 14 | 4 | 1
Ville: That's 13-5, so there's some feeling that it might be a good idea.

To pursue this idea, it would make sense for EWG and LEWG (for starters) to consider the content of this document, probably more closely than they have in the past.

Contents

1. Introduction
2. Explanation and rationale for the approach
   1. Problem statement
   2. Status quo
   3. Characteristics of the proposed solution
3. Recommendations
   1. Introduction
   2. Testing for the presence of a header: __has_include
   3. Testing for the presence of an attribute: __has_cpp_attribute
   4. C++17 features
   5. C++14 features
   6. C++11 features
   7. C++98 features (STUB)
   8. Features published and later removed
4. Annex: Recommendations from Technical Specifications
5. Annex: Detailed explanation and rationale
   1. C++14 features
   2. C++17 features
6. Annex: Model wording for a Technical Specification
7. Revision history

Introduction

At the September 2013 (Chicago) meeting of WG21, there was a five-way poll of all of the C++ experts in attendance – approximately 80 – concerning their support for the approach described herein for feature-testing in C++. The results of the poll:

General support for these recommendations was reaffirmed at the 2015 October (Kona) meeting, as indicated by the following straw poll:

Should WG21 encourage, but not require, feature-test macros for proposals?

Strongly favor	Favor	Neutral	Against	Strongly against
50	13	7	3	2

This document was subsequently designated WG21's SD-6 (sixth standing document), which will continue to be maintained by SG10.

Explanation and rationale for the approach

Problem statement

The pace of innovation in the standardization of C++ makes long-term stability of implementations unlikely. Features are added to the language because programmers want to use those features. Features are added to (the working draft of) the standard as the features become well-specified. In many cases a feature is added to an implementation well before or well after the standard officially introducing it is approved.

This process makes it difficult for programmers who want to use a feature to know whether it is available in any given implementation. Implementations rarely leap from one formal revision of the standard directly to the next; the implementation process generally proceeds by smaller steps. As a result, testing for a specific revision of the standard (e.g. by examining the value of the __cplusplus macro) often gives the wrong answer. Implementers generally don't want to appear to be claiming full conformance to a standard revision until all of its features are implemented. That leaves programmers with no portable way to determine which features are actually available to them.

It is often possible for a program to determine, in a manner specific to a single implementation, what features are supported by that implementation; but the means are often poorly documented and ad hoc, and sometimes complex – especially when the availability of a feature is controlled by an invocation option. To make this determination for a variety of implementations in a single source base is complex and error-prone.

Status quo

Here is some code that attempts to determine whether rvalue references are available in the implementation in use:

#ifndef __USE_RVALUE_REFERENCES
  #if (__GNUC__ > 4 || __GNUC__ == 4 && __GNUC_MINOR__ >= 3) || \
      _MSC_VER >= 1600
    #if __EDG_VERSION__ > 0
      #define __USE_RVALUE_REFERENCES (__EDG_VERSION__ >= 410)
    #else
      #define __USE_RVALUE_REFERENCES 1
    #endif
  #elif __clang__
    #define __USE_RVALUE_REFERENCES __has_feature(cxx_rvalue_references)
  #else
    #define __USE_RVALUE_REFERENCES 0
  #endif
#endif

First, the GNU and Microsoft version numbers are checked to see if they are high enough. But then a check is made of the EDG version number, since that front end also has compatibility modes for both those compilers, and defines macros indicating (claimed) compatibility with them. If the feature wasn't implemented in the indicated EDG version, it is assumed that the feature is not available – even though it is possible for a customer of EDG to implement a feature before EDG does.

Fortunately Clang has ways to test specifically for the presence of specific features. But unfortunately, the function-call-like syntax used for such tests won't work with a standard preprocessor, so this fine new feature winds up adding its own flavor of complexity to the mix.

Also note that this code is only the beginning of a real-world solution. A complete solution would need to take into account more compilers, and also command-line option settings specific to various compilers.

Characteristics of the proposed solution

To preserve implementers' freedom to add features in the order that makes the most sense for themselves and their customers, implementers should indicate the availability of each separate feature by adding a definition of a macro with the name corresponding to that feature.

Important note: By recommending the use of these macros, WG21 is not making any feature optional; the absence of a definition for the relevant feature-test macro does not make an implementation that lacks a feature conform to a standard that requires the feature. However, if implementers and programmers follow these recommendations, portability of code between real-world implementations should be improved.

To a first approximation, a feature is identified by the WG21 paper in which it is specified, and by which it is introduced into the working draft of the standard. Not every paper introduces a new feature worth a feature-test macro, but every paper that is not just a collection of issue resolutions is considered a candidate; exceptions are explicitly justified.

For C++14, the feature-test macro name generally consists of some combination of words from the title of the paper. In the future, it is hoped that every paper will include its own recommendations concerning feature-test macro names.

The value specified for a feature-test macro is based on the year and month in which the feature is voted into the working draft. In a case where a feature is subsequently changed in a significant way, but arguably remains the same feature, the value of the macro is changed to indicate the “revision level” of the specification of the feature. However, in most cases it is expected that the presence of a feature can be determined by the presence of any non-zero macro value; for example:

template<typename T>
struct use_empty_base_opt :
	std::integral_constant<bool, 
		std::is_empty<T>::value
#if __cpp_lib_is_final
		&& !std::is_final<T>::value
#endif
	>
{ };

To avoid the user's namespace, names of macros for language features are prefixed by “__cpp_”; for library features, by “__cpp_lib_”. A library feature that doesn't introduce a new header is expected to be defined by the header(s) that implement the feature.

Recommendations

Introduction

For the sake of improved portability between partial implementations of various C++ standards, WG21 (the ISO technical committee for the C++ programming language) recommends that implementers and programmers follow the guidelines in this document concerning feature-test macros.

Implementers who provide a new standard feature should define a macro with the recommended name and value, in the same circumstances under which the feature is available (for example, taking into account relevant command-line options), to indicate the presence of support for that feature.

Programmers who wish to determine whether a feature is available in an implementation should base that determination on the state of the macro with the recommended name. (The absence of a tested feature may result in a program with decreased functionality, or the relevant functionality may be provided in a different way. A program that strictly depends on support for a feature can just try to use the feature unconditionally; presumably, on an implementation lacking necessary support, translation will fail. Therefore, if the most useful purpose for a feature-test macro would be to control the inclusion of a #error directive if the feature is unavailable, that is considered inadequate justification for the macro. Note that the usefulness of a test macro for a feature is completely independent of the usefulness of the feature itself.)

Testing for the presence of a header: __has_include

It is impossible for a C++ program to directly, reliably and portably determine whether or not a library header is available for inclusion. Conditionally including a header requires the use of a configuration macro, whose setting can be determined by a configuration-test process at build time (reliable, but less portable), or by some other means (often not reliable or portable).

To solve this general problem, WG21 recommends that implementers provide, and programmers use, the __has_include feature.

Syntax

h-preprocessing-token:

any preprocessing-token other than >

h-pp-tokens:

h-preprocessing-token

h-pp-tokens h-preprocessing-token

has-include-expression:

__has_include ( header-name )

__has_include ( string-literal )

__has_include ( < h-pp-tokens > )

Semantics

In the first form of the has-include-expression, the parenthesized header-name token is not subject to macro expansion. The second and third forms are considered only if the first form does not match, and the preprocessing tokens are processed just as in normal text.

A has-include-expression shall appear only in the controlling constant expression of a #if or #elif directive ([cpp.cond] 16.1). Prior to the evaluation of such an expression, the source file identified by the parenthesized preprocessing token sequence in each contained has-include-expression is searched for as if that preprocessing token sequence were the pp-tokens in a #include directive, except that no further macro expansion is performed. If such a directive would not satisfy the syntactic requirements of a #include directive, the program is ill-formed. The has-include-expression is replaced by the pp-number 1 if the search for the source file succeeds, and by the pp-number 0 if the search fails.

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

Example

This demonstrates a way to use a library optional facility only if it is available.

#ifdef __has_include
#  if __has_include(<optional>)
#    include <optional>
#    define have_optional 1
#  elif __has_include(<experimental/optional>)
#    include <experimental/optional>
#    define have_optional 1
#    define experimental_optional
#  else
#    define have_optional 0
#  endif
#endif

Testing for the presence of an attribute: __has_cpp_attribute

A C++ program cannot directly, reliably, and portably determine whether or not a standard or vendor-specific attribute is available for use. Testing for attribute support generally requires complex macro logic, as illustrated above for language features in general.

To solve this general problem, WG21 recommends that implementers provide, and programmers use, the __has_cpp_attribute feature.

Syntax

has-attribute-expression:

__has_cpp_attribute ( attribute-token )

Semantics

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.

Example

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
#ifdef __has_cpp_attribute
#if __has_cpp_attribute(deprecated)
# define ATTR_DEPRECATED(msg) [[deprecated(msg)]]
#else
# define ATTR_DEPRECATED(msg)
#endif
#endif

C++17 features

The following table itemizes all the changes that were made to the working draft for C++17 as specified in a WG21 technical document. (Changes that were made as specified in a core or library issue are not generally included.)

The table is sorted by the section of the standard primarily affected. The “Doc. No.” column links to the paper itself on the committee web site. The “Macro Name” column links to the relevant portion of the “Detailed explanation and rationale” annex of this document. When the recommendation is to change the value of a macro previously recommended to be defined, the “Value” column links to the table entry for the previous recommendation.

For library features, the “Header“ column identifies the header that is expected to define the macro, although the macro may also be predefined. For language features, the macro must be predefined.

C++14 features

The following table itemizes all the changes that were made to the working draft for C++14 as specified in a WG21 technical document. (Changes that were made as specified in a core or library issue are not generally included.)

The table is sorted by the section of the standard primarily affected. The “Doc. No.” column links to the paper itself on the committee web site. The “Macro Name” column links to the relevant portion of the “Detailed explanation and rationale” annex of this document. When the recommendation is to change the value of a macro previously recommended to be defined, the “Value” column links to the table entry for the previous recommendation.

For library features, the “Header“ column identifies the header that is expected to define the macro, although the macro may also be predefined. For language features, the macro must be predefined.

C++11 features

Conditionally-supported constructs

STUB

C++98 features

STUB: especially for exception handling and RTTI

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.

Features published and later removed

Features removed from C++17

Features removed from C++14

The intention is that an implementation which provides runtime-sized arrays as specified in the first CD should define __cpp_runtime_arrays as 201304. The expectation is that a later document specifying runtime-sized arrays will specify a different value for __cpp_runtime_arrays.

Annex: Recommendations from Technical Specifications (informative)

In this annex, feature-test macros from all WG21 Technical Specifications are collected, for convenience.

C++ Extensions for Library Fundamentals

The recommended macro name is "__cpp_lib_experimental_" followed by the string in the "Macro Name Suffix" column.

C++ Extensions for Parallelism

File System Technical Specification

C++ Extensions for Transactional Memory

The Technical Specification for transactional memory specifies a new attribute, but does not specify a value for a has-attribute-expression for that attribute.

Annex: Detailed explanation and rationale (informative)

Many of the examples here have been shamelessly and almost brainlessly plagiarized from the cited paper. Assistance with improving examples is solicited.

C++14 features

N3323: A Proposal to Tweak Certain C++ Contextual Conversions

This paper specifies a small change that is considered to be more of a bug fix than a new feature, so no macro is considered necessary.

N3421: Making Operator Functors greater<>

Example:

#if __cpp_lib_transparent_operators
	sort(v.begin(), v.end(), greater<>());
#else
	sort(v.begin(), v.end(), greater<valueType>());
#endif

N3462: std::result_of and SFINAE

The macro __cpp_lib_result_of_sfinae was originally erroneously recommended to be defined in the unrelated header <functional>. This error is being corrected, because it is believed that no implementation actually followed the erroneous recommendation.

Example:

template<typename A>
#if __cpp_lib_result_of_sfinae
  typename std::result_of<inc(A)>::type
#else
  decltype(std::declval<inc>()(std::declval<A>()))
#endif
try_inc(A a);

N3469: Constexpr Library Additions: chrono
N3470: Constexpr Library Additions: containers
N3471: Constexpr Library Additions: utilities

These papers just add constexpr to the declarations of several dozen library functions in various headers. It is not clear that a macro to test for the presence of these changes would be sufficiently useful to be worthwhile.

N3472: Binary Literals in the C++ Core Language

Example:

int const packed_zero_to_three =
#if __cpp_binary_literals
	0b00011011;
#else
	0x1B;
#endif

N3545: An Incremental Improvement to integral_constant

Example:

constexpr bool arithmetic =
#if __cpp_lib_integral_constant_callable
	std::is_arithmetic<T>{}();
#else
	static_cast<bool>(std::is_arithmetic<T>{});
#endif

N3624: Core Issue 1512: Pointer comparison vs qualification conversions

This paper contained the wording changes to resolve a core issue. It did not introduce a new feature, so no macro is considered necessary.

N3638: Return type deduction for normal functions

This paper describes two separate features: the ability to deduce the return type of a function from the return statements contained in its body, and the ability to use decltype(auto). These features can be implemented independently, so a macro is recommended for each.

Examples:

template<typename T>
auto abs(T x)
#ifndef __cpp_return_type_deduction
	-> decltype(x < 0 ? -x : x)
#endif
{
	return x < 0 ? -x : x;
}

N3639: Runtime-sized arrays with automatic storage duration

This was removed from C++14 to TS 19569.

Example:

#if __cpp_runtime_arrays
	T local_buffer[n];	// more efficient than vector
#else
	std::vector<T> local_buffer(n);
#endif

N3642: User-defined Literals for Standard Library Types

This paper specifies user-defined literal operators for two different standard library types, which could be implemented independently. Furthermore, user-defined literal operators are expected to be added later for at least one other library type. So for consistency and flexibility, each type is given its own macro.

Examples:

N3644: Null Forward Iterators

Example:

N3648: Wording Changes for Generalized Lambda-capture

Example:

N3649: Generic (Polymorphic) Lambda Expressions

Example:

N3651: Variable Templates

Example:

N3652: Relaxing constraints on constexpr functions / constexpr member functions and implicit const

The major change proposed by this paper is considered to be strictly a further development of the constexpr feature of C++11. Consequently, the recommendation here is to give an increased value to the macro indicating C++11 support for constexpr.

Example:

N3653: Member initializers and aggregates

Example:

N3654: Quoted Strings Library Proposal

Example:

N3655: TransformationTraits Redux

Example:

N3656: make_unique

Example:

N3657: Adding heterogeneous comparison lookup to associative containers

Example:

N3658: Compile-time integer sequences

Example:

N3659: Shared locking in C++

The original recommendation, of a macro defined in header <mutex>, was not useful, and has been retracted.

For new headers, we have a long-term solution that uses __has_include. There was not sufficient support and a number of objections against adding macros to existing library header files, as there was not consensus on a place to put them.

There is also a simple workaround for users that are not using libraries that define the header file: supplying their own header that is further down the search path than the library headers.

Example:

#if __has_include(<shared_mutex>)
#include <shared_mutex>

// code that uses std::shared_mutex
#endif

N3662: C++ Dynamic Arrays

This was removed from C++14 to TS 19569.

Example:

N3664: Clarifying Memory Allocation

The substantive change in this paper just relaxes a restriction on implementations. There is no new feature for a programmer to use, so no macro is considered necessary.

N3667: Drafting for Core 1402

This paper contained the wording changes to resolve a core issue. It did not introduce a new feature, so no macro is considered necessary.

N3668: exchange() utility function

Example:

N3669: Fixing constexpr member functions without const

This paper contained the wording changes to ensure that a minor change proposed by N3652 did not impact the standard library. It did not introduce a new feature, so no macro is considered necessary.

N3670: Wording for Addressing Tuples by Type

Example:

N3671: Making non-modifying sequence operations more robust

Example:

N3672: A proposal to add a utility class to represent optional objects

This was removed from C++14 to TS 19568.

See N3659 for rationale.

Example:

#if __has_include(<optional>)
#include <optional>

// code that uses std::optional
#endif

N3673: C++ Library Working Group Ready Issues Bristol 2013

This paper was just a collection of library issues. It did not introduce a new feature, so no macro is considered necessary.

N3760: [[deprecated]] attribute

Differentiating attribute availability based on compiler-specific macros is error- prone. Since vendors are allowed to provide implementation-specific attributes in addition to standards-based attributes, this feature provides a uniform, future-proof way to test the availability of attributes.

Example:

#ifdef __has_cpp_attribute
#  if __has_cpp_attribute(deprecated)
#    define ATTR_DEPRECATED(msg) [[deprecated(msg)]]
#  else
#    define ATTR_DEPRECATED(msg)
#  endif
#endif

ATTR_DEPRECATED("f() has been deprecated") void f();

N3776: Wording for ~future

The change made by this paper is a bug fix, and no macro is considered necessary.

N3778: C++ Sized Deallocation

Example:

N3779: User-defined Literals for std::complex

Example:

N3781: Single-Quotation-Mark as a Digit Separator

Writing code that uses digit separators conditionally when they are available would require:

• either duplicating constants, presumably containing large numbers of digits, which would be error-prone and difficult to maintain,
• or using a scheme based on macros and token concatenation, which would effectively take the place of digit separators as supported by the language.

Thus it is believed that a macro indicating support for digit separators in the language would not be sufficiently useful.

N3786: Prohibiting "out of thin air" results in C++14

The change made by this paper is a bug fix, and no macro is considered necessary.

N3789: Constexpr Library Additions: functional

See N3471 for rationale.

N3887: Consistent Metafunction Aliases

Example:

N3891: A proposal to rename shared_mutex to shared_timed_mutex

Example:

N3910: What can signal handlers do? (CWG 1441)

The change made by this paper is a bug fix, and no macro is considered necessary.

N3924: Discouraging rand() in C++14

The change made by this paper is not normative; no macro is necessary.

N3927: Definition of Lock-Free

The change made by this paper is a bug fix, and no macro is considered necessary.

LWG issue 2112: User-defined classes that cannot be derived from

Libraries that want to do empty-base optimization could reasonably want to use !is_final && is_empty if is_final exists, and fall back to just using is_empty otherwise.

Example:

template<typename T>
struct use_empty_base_opt :
	std::integral_constant<bool, 
		std::is_empty<T>::value
#if __cpp_lib_has_is_final
		&& !std::is_final<T>::value
#endif
	>
{ };

LWG issue 2247: Type traits and std::nullptr_t

Example:

LWG issue 2249: Remove gets from <cstdio>

Since portable, well-written code does not use the gets function at all, there is no need for a macro to indicate whether it is available.

LWG issue 2285: make_reverse_iterator

Example:

C++17 features

N3911: TransformationTrait Alias void_t

Example:

#if __cpp_lib_void_t
  using std::void_t;
#else
  template< class... >  using void_t = void;
#endif

N3922: New Rules for auto deduction from braced-init-list

This change is considered a fix to a problem in C++14, not a new feature, so no macro is recommended. Code that needs to work both with and without this change should not attempt to deduce a type from a direct braced-init-list initializer.

N3928: Extending static_assert

Example:

#if __cpp_static_assert
#if __cpp_static_assert > 201400
#define Static_Assert(cond) static_assert(cond)
#else
#define Static_Assert(cond) static_assert(cond, #cond)
#endif
#define Static_Assert_Msg(cond, msg) static_assert(cond, msg)
#else
#define Static_Assert(cond)
#define Static_Assert_Msg(cond, msg)
#endif

N4051: Allow typename in a template template parameter

This is a minor stylistic extension; there is no difference in functionality or verbosity between code that uses the class keyword and code that uses the typename keyword in this context. If portability is needed between implementations having and lacking this feature, generally it would be easier to continue to write code using class than to write and maintain conditional compilation to use typename when available. So a macro for this feature would not seem to be justified.

N4086: Removing trigraphs??!

This is a very low-level change, purely lexical. Writing code to use trigraphs when they are available and different characters when trigraphs are not available would be tedious, and the resulting code would be even uglier than code that just uses trigraphs.

Even in circumstances where use of a character not in the ISO/IEC 646 Basic Character Set is problematic, it would generally be easier to use alternative tokens (a.k.a. “digraphs”) than to conditionally use trigraphs.

N4089: Safe conversions in unique_ptr<T[]>

This is considered a fix for a library issue, to remove an unnecessary restriction; no macro is considered necessary.

N4169: A proposal to add invoke function template

Example:

template<typename F>
auto deref_fn(F&& f) 
{ 
  return [f](auto&&... args) {
#if __cpp_lib_invoke
    return *std::invoke(f, std::forward<decltype(args)>(args)...);
#else
    return *f(std::forward<decltype(args)>(args)...);
#endif
  };
}

In this example, member function pointers are supported only if invoke is available.

N4190: Removing auto_ptr, random_shuffle(), And Old <functional> Stuff

These library features are removed because superior alternatives to them were introduced in C++11. Because these alternatives are superior, there is little motivation to maintain code that uses one of these obsolescent features when available.

N4230: Nested namespace definition

This feature doesn't provide any new functionality; it just makes it somewhat easier to write code. For code that needs to be portable to an implementation that doesn't provide this feature, it would be easier just to avoid using the feature than it would be maintain the code to use it when available but not otherwise. Therefore, a macro to indicate the availability of this feature would not seem to be justified.

N4258: Cleaning-up noexcept in the Library

Example:

template <class T, class Allocator = std::allocator<T>>
class myContainer {
    typedef std::allocator_traits<Allocator> alloc_traits;
public:
    ...
    myContainer& operator=(myContainer&& x)
#ifdef __cpp_lib_allocator_traits_is_always_equal
      noexcept(alloc_traits::propagate_on_container_move_assignment::value ||
               alloc_traits::is_always_equal::value);
#else
      noexcept(alloc_traits::propagate_on_container_move_assignment::value);
#endif
    ...
};

Without the trait, the code will be correct, but container move-assignment would have fewer exception guarantees, especially with pre-C++11 stateless allocators. An implementation of a mutating algorithm might choose to use copy-assignment instead of move-assignment if the noexcept clause evaluates to false, because the implementation is unable to determine whether the allocator always compares equal.

N4259: Wording for std::uncaught_exceptions

Example:

bool throw_may_terminate()
{
#if __cpp_lib_uncaught_exceptions
    return std::uncaught_exceptions() > 0;
#else
    return std::uncaught_exception();
#endif
}

N4261: Proposed resolution for Core Issue 330: Qualification conversions and pointers to arrays of pointers

This paper fixes a core language issue; no new feature is introduced.

N4262: Wording for Forwarding References

The changes detailed in this paper are solely editorial. It does not introduce any new feature.

N4266: Attributes for namespaces and enumerators

Example:

enum {
old_val
#if __cpp_enumerator_attributes
	[[deprecated]]
#endif
, new_val };

N4267: Adding u8 character literals

This feature gives guaranteed access to the Unicode encoding for a 7-bit ASCII character even in an implementation that uses a different character encoding. In an application that needs this functionality, it would probably be easier simply to hard-code the numeric values of the needed encodings. Thus a macro for this feature would seem to be of limited usefulness.

N4268: Allow constant evaluation for all non-type template arguments

Example:

int n = 0, m = 0;
int &r = n;
template<int &> struct X {};
#if __cpp_nontype_template_args
  X<r> x;
#else
  std::conditional<&r == &n, X<n>, X<m>>::type x;
#endif

N4277: TriviallyCopyable reference_wrapper

This paper imposes a new requirement on implementations, which most implementations had already satisfied. A reliable indication of when this requirement was not satisfied would have been useful for programming; an indication that it is satisfied would not be so useful.

N4279: Improved insertion interface for unique-key maps

Example:

#if __cpp_lib_map_try_emplace
    m.insert_or_assign("foo", std::move(p));
#else
    m["foo"] = std::move(p);
#endif

In this example, if insert_or_assign is not available, the type of p must be default-constructible.

N4280: Non-member size() and more

Example:

auto n =
#if __cpp_lib_nonmember_container_access
    std::size(c);
#else
    c.size();
#endif

In this example, an array type is supported for c only if the nonmember size function is available.

N4284: Contiguous Iterators

The changes detailed in this paper are solely editorial. It does not introduce any new feature.

N4285: Cleanup for exception-specification and throw-expression

The changes detailed in this paper are solely editorial. It does not introduce any new feature.

N4295: Folding expressions

Example:

#if __cpp_fold_expressions
template<typename... T>
  auto sum(T... args)  { return (args + ...); }
#else
auto sum() { return 0; }
template<typename T>
  auto sum(T t) { return t; }
template(typename T, typename... Ts)
  auto sum(T t, Ts... ts) { return t + sum(ts...); }
#endif

N4366: LWG 2228 missing SFINAE rule

This change fixes a defect in the library. No new feature is added, so no feature-test macro is needed.

N4387: Improving pair and tuple

This paper resolves some library issues. There is no new feature; no macro is required.

N4389: Wording for bool_constant

Example:

#if __cpp_lib_bool_constant
  template <bool B>
  using my_bool_constant = std::bool_constant<B>;
#else
  template <bool B>
  using my_bool_constant = std::integral_constant<bool, B>;
#endif

N4508: A proposal to add shared_mutex (untimed)

Example:

Lenexa

N4510: Minimal incomplete type support for standard containers

Example:

Lenexa

P0001R1: Remove Deprecated Use of the register Keyword

The simplest way to write code to be portable between implementations that have and lack this change is to avoid using the register keyword. A feature-test macro would not be sufficiently useful.

P0002R1: Remove Deprecated operator++(bool)

The simplest way to write code to be portable between implementations that have and lack this change is to avoid applying ++ to bool objects. A feature-test macro would not be sufficiently useful.

P0004R1: Remove Deprecated iostreams aliases

The simplest way to write code to be portable between implementations that have and lack this change is not to use the previously deprecated features. A feature-test macro would not be sufficiently useful.

P0006R0: Adopt Type Traits Variable Templates from Library Fundamentals TS for C++17

Example:

Kona

P0007R1: Constant View: A proposal for a std::as_const helper function template

Example:

Kona

P0012R1: Make exception specifications be part of the type system

Example:

Kona

P0013R1: Logical Operator Type Traits

Example:

template<typename... T>
  using condition =
#if __cpp_lib_logical_traits
  conjunction<is_pointer<decay_t<T>>...>;
#else
  bool_constant<(is_pointer<decay_t<T>>::value && ...)>;
#endif

template<typename... T>
  enable_if_t<condition<T...>::value>
  foo(T&&...) { }

template<typename... T>
  enable_if_t<!condition<T...>::value>
  foo(T&&...) { }

Using conjunction is more efficient at compile-time, because it short-circuits and doesn't evaluate every expression in the pack expansion if it doesn't need to.

P0061R1: __has_include for C++17

The presence of this feature can be determined by testing whether the __has_include macro is defined. However, because it is a function-like macro, it has no specific value that can be tested.

P0074R0: Making std::owner_less more flexible

Example:

#if __cpp_lib_transparent_operators >= 201510
using generic_owner_less = std::owner_less<>;
#else
struct generic_owner_less {
  template<typename T, typename U>
    bool
    operator()(const std::shared_ptr<T>& p1,
	       const std::shared_ptr<U>& p2) const
    { return p1.owner_before(p2); }
};
#endif

P0092R1: Polishing <chrono>

Example:

Kona

P0134R0: Introducing a name for brace-or-equal-initializers for non-static data members

This change is entirely editorial; no feature is added, and no test macro is needed.

P0136R1: Rewording inheriting constructors (core issue 1941 et al)

This change is considered a minor tweak to the inheriting constructors feature, and mostly just fixes issues with the original description. However, it can change the behavior of some code. Therefore the value of the __cpp_inheriting_constructors macro is adjusted.

Example:

Kona

P0156R0: Variadic lock_guard

In a previous revision, it was recommended that a test macro for this feature be defined in <thread>. That was a mistake; the relevant code is actually in <mutex>.

This feature was subsequently removed from C++17. The functionality it had provided is now provided by a scoped_lock.

template<class ObjectType>
void register_observers(ObjectType& obj)
{
#if __cpp_lib_enable_shared_from_this
    auto wptr = obj.weak_from_this();
#else
    auto sptr = obj.shared_from_this();
    // Fails if sptr is not std::shared_ptr
    // (e.g. std::experimental::shared_ptr or boost::shared_ptr)
    auto wptr = weak_ptr<decltype(sptr)::element_type>(sptr);
    sptr.reset(); // drop the strong reference as soon as possible
#endif
    register_observer_1(wptr);
    register_observer_2(wptr);
}

template<class T>
  void my_swap(T& x, T& y)
#if __cpp_lib_is_swappable
  noexcept(std::is_swappable_v<T>); // Covers all Lvalue Swappable cases
#else
  noexcept(
    std::is_nothrow_move_constructible_v<T> &&
    std::is_nothrow_move_assignable_v<T>
  );
#endif

struct optional {
  bool b;
  T t;
  constexpr T *get() {
    return b ?
#if __cpp_lib_addressof_constexpr
      std::addressof(t)
#else
      &t
#endif
      : nullptr;
  }
};

void update(std::set<X>& set, const X& elem, int val)
{
  auto pos = set.find(elem);
  if (pos == set.end())
    return;
#if __cpp_lib_node_extract
  auto next = std::next(pos);
  auto x = set.extract(pos);
  x.value().update(val);
  set.insert(next, std::move(x));
#else
  X tmp = *pos;
  pos = set.erase(pos);
  tmp.update(val);
  set.insert(pos, std::move(tmp));
#endif
}

class NoCopyNoMove {
public:
  NoCopyNoMove(const NoCopyNoMove&) = delete;
  NoCopyNoMove &operator=(const NoCopyNoMove&) = delete;
  NoCopyNoMove(int a, int b, int c);
};

NoCopyNoMove factory(int a) {
  return {a, 2, 3}; // constructs return value in place
}

#ifdef __cpp_guaranteed_copy_elision
NoCopyNoMove indirectFactory(int a) {
  return factory(a); // ill-formed prior to C++17
}
#endif

template<class ObjectType>
void register_observers(ObjectType& obj)
{
    auto sptr = obj.shared_from_this();
#if  __cpp_lib_shared_ptr_weak_type
    decltype(sptr)::weak_type wptr(sptr);
#else
    // Fails if sptr is not std::shared_ptr
    // (e.g. std::experimental::shared_ptr or boost::shared_ptr)
    auto wptr = weak_ptr<decltype(sptr)::element_type>(sptr);
#endif
    sptr.reset(); // drop the strong reference as soon as possible
    register_observer_1(wptr);
    register_observer_2(wptr);
}

#if __cpp_inline_variables >= 201703
#define INLINE_VAR_DECL inline
#else
#define INLINE_VAR_DECL /*fingers crossed*/
#endif

INLINE_VAR_DECL constexpr piecewise_construct_t piecewise_construct{};

#if __cpp_variadic_using >= 201611
template<typename ...T> struct Callable : T... {
  using T::operator() ...;
};
#else
template<typename ...T> struct Callable;
template<typename T, typename ...U> struct Callable<T, U...> : T,
Callable<U...> {
  using T::operator();
  using Callable<U...>::operator();
};
template<typename T> struct Callable<T> : T {
  using T::operator();
};
template<> struct Callable<> {};
#endif

#if _cpp_lib_is_invocable >= 201703
using std::is_invocable;
#elif __cpp_lib_void_t >= 201411
template<typename R, typename = void>
  struct is_invocable_impl : std::false_type
  { };
template<typename R>
  struct is_invocable_impl<R, std::void_t<typename R::type>> : std::true_type
  { };
template<typename F, typename... Args>
  struct is_invocable : is_invocable_impl<std::result_of<F&&(Args&&...)>>::type
  { };
#else
#error either is_invocable or void_t is needed
#endif

#include <vector>
template<typename T, typename... Args>
T make(Args&&... args)
{
#if __cpp_lib_is_aggregate
    if constexpr (std::is_aggregate_v<T>)
        return { std::forward<Args>(args)... };
    else
#endif
        return T(std::forward<Args>(args)...);
}
struct Agg { int i; };
int main()
{
    auto v = make<std::vector<int>>(1, 2);
#if __cpp_lib_is_aggregate
    // make<> only supports aggregates if std::is_aggregate is available
    auto a = make<Agg>(1);
#endif
}

Annex: Model wording for a Technical Specification

It is recommended that Technical Specifications include the wording below. yyyymm indicates the year and month of the date of the TS.

1.x Feature testing [intro.features]

An implementation that provides support for this Technical Specification shall define the feature test macro(s) in Table X.

Table X — Feature Test Macro(s)

Name	Value	Header
__cpp_name	yyyymm	“predefined” OR “<name>”

Revision history