C++ Support in Clang

Clang fully implements all published ISO C++ standards (C++98 / C++03, C++11, C++14, and C++17), and some of the upcoming C++20 standard.

The Clang community is continually striving to improve C++ standards compliance between releases by submitting and tracking C++ Defect Reports and implementing resolutions as they become available.

Experimental work is also under way to implement C++ Technical Specifications that will help drive the future of the C++ programming language.

The LLVM bug tracker contains Clang C++ components that track known bugs with Clang's language conformance in each language mode.

C++98 implementation status

Clang implements all of the ISO C++ 1998 standard (including the defects addressed in the ISO C++ 2003 standard) except for export (which was removed in C++11).

C++11 implementation status

Clang 3.3 and later implement all of the ISO C++ 2011 standard.

By default, Clang builds C++ code according to the C++98 standard, with many C++11 features accepted as extensions. You can use Clang in C++11 mode with the -std=c++11 option. Clang's C++11 mode can be used with libc++ or with gcc's libstdc++. List of features and minimum Clang version with support Language Feature C++11 Proposal Available in Clang? Rvalue references N2118 Clang 2.9 P1825R0 (DR) No Rvalue references for *this N2439 Clang 2.9 Initialization of class objects by rvalues N1610 Clang 2.9 Non-static data member initializers N2756 Clang 3.0 Variadic templates N2242 Clang 2.9 Extending variadic template template parameters N2555 Clang 2.9 Initializer lists N2672 Clang 3.1 P1009R2 (DR) Clang 9 P1957R2 (DR) Clang 11 Static assertions N1720 Clang 2.9 auto -typed variables N1984 Clang 2.9 Multi-declarator auto N1737 Clang 2.9 Removal of auto as a storage-class specifier N2546 Clang 2.9 New function declarator syntax N2541 Clang 2.9 Lambda expressions N2927 Clang 3.1 P0588R1 (DR) No Declared type of an expression N2343 Clang 2.9 Incomplete return types N3276 Clang 3.1 Right angle brackets N1757 Clang 2.9 Default template arguments for function templates DR226 Clang 2.9 Solving the SFINAE problem for expressions DR339 Clang 2.9 Alias templates N2258 Clang 3.0 Extern templates N1987 Clang 2.9 Null pointer constant N2431 Clang 3.0 Strongly-typed enums N2347 Clang 2.9 Forward declarations for enums N2764

DR1206 Clang 3.1 Standardized attribute syntax N2761 Clang 3.3 (1) Generalized constant expressions N2235 Clang 3.1 P0859R0 (DR) Clang 8 Alignment support N2341 Clang 3.3 Conditionally-support behavior N1627 Clang 2.9 Changing undefined behavior into diagnosable errors N1727 Clang 2.9 Delegating constructors N1986 Clang 3.0 Inheriting constructors N2540 Clang 3.3 P0136R1 (DR) Clang 3.9 Explicit conversion operators N2437 Clang 3.0 New character types N2249 Clang 2.9 Unicode string literals N2442 Clang 3.0 Raw string literals N2442 Clang 3.0 Universal character names in literals N2170 Clang 3.1 User-defined literals N2765 Clang 3.1 Standard Layout Types N2342 Clang 3.0 Defaulted functions N2346 Clang 3.0 P1286R2 (DR) Clang 9 Deleted functions N2346 Clang 2.9 Extended friend declarations N1791 Clang 2.9 Extending sizeof N2253

DR850 Clang 3.1 Inline namespaces N2535 Clang 2.9 Unrestricted unions N2544 Clang 3.1 Local and unnamed types as template arguments N2657 Clang 2.9 Range-based for N2930 Clang 3.0 P0962R1 (DR) Clang 8 Explicit virtual overrides N2928

N3206

N3272 Clang 3.0 Minimal support for garbage collection and reachability-based leak detection N2670 N/A (2) Allowing move constructors to throw [noexcept] N3050 Clang 3.0 Defining move special member functions N3053 Clang 3.0 Concurrency Sequence points N2239 Clang 3.3 Atomic operations N2427 Clang 3.1 Strong Compare and Exchange N2748 Clang 3.1 (3) Bidirectional Fences N2752 Clang 3.1 Memory model N2429 Clang 3.2 Data-dependency ordering: atomics and memory model N2664 Clang 3.2 (4) Propagating exceptions N2179 Clang 2.9 Allow atomics use in signal handlers N2547 Clang 3.1 Thread-local storage N2659 Clang 3.3 (5) Dynamic initialization and destruction with concurrency N2660 Clang 2.9 C99 Features in C++11 __func__ predefined identifier N2340 Clang 2.9 C99 preprocessor N1653 Clang 2.9 long long N1811 Clang 2.9 Extended integral types N1988 N/A (6) (1): The [[carries_dependency]] attribute has no effect.

(2): No compiler changes are required for an implementation such as Clang that does not provide garbage collection.

(3): All compare-exchange operations are emitted as strong compare-exchanges.

(4): memory_order_consume is lowered to memory_order_acquire .

(5): thread_local support requires a C++ runtime library providing __cxa_thread_atexit , such as libc++abi 3.6 or later, or libsupc++ 4.8 or later.

(6): No compiler changes are required for an implementation such as Clang that does not provide any extended integer types. __int128 is not treated as an extended integer type, because changing intmax_t would be an ABI-incompatible change.

C++14 implementation status

Clang 3.4 and later implement all of the ISO C++ 2014 standard.

You can use Clang in C++14 mode with the -std=c++14 option (use -std=c++1y in Clang 3.4 and earlier).

List of features and minimum Clang version with support Language Feature C++14 Proposal Available in Clang? Tweak to certain C++ contextual conversions N3323 Clang 3.4 Binary literals N3472 Clang 2.9 decltype(auto) N3638 Clang 3.3 Return type deduction for normal functions Clang 3.4 Initialized lambda captures N3648 Clang 3.4 Generic lambdas N3649 Clang 3.4 Variable templates N3651 Clang 3.4 Relaxing requirements on constexpr functions N3652 Clang 3.4 Member initializers and aggregates N3653 Clang 3.3 Clarifying memory allocation N3664 Clang 3.4 [[deprecated]] attribute N3760 Clang 3.4 Single quotation mark as digit separator N3781 Clang 3.4 C++ Sized Deallocation N3778 Clang 3.4 (7) (7): In Clang 3.7 and later, sized deallocation is only enabled if the user passes the -fsized-deallocation flag. The user must supply definitions of the sized deallocation functions, either by providing them explicitly or by using a C++ standard library that does. libstdc++ added these functions in version 5.0, and libc++ added them in version 3.7.

C++17 implementation status

Clang 5 and later implement all the features of the ISO C++ 2017 standard.

You can use Clang in C++17 mode with the -std=c++17 option (use -std=c++1z in Clang 4 and earlier).

List of features and minimum Clang version with support Language Feature C++17 Proposal Available in Clang? static_assert with no message N3928 Clang 3.5 Disabling trigraph expansion by default N4086 Clang 3.5 typename in a template template parameter N4051 Clang 3.5 New auto rules for direct-list-initialization N3922 Clang 3.8 (8) Fold expressions N4295 Clang 3.6 P0036R0 Clang 3.9 u8 character literals N4267 Clang 3.6 Nested namespace definition N4230 Clang 3.6 Attributes for namespaces and enumerators N4266 Clang 3.6 Allow constant evaluation for all non-type template arguments N4268 Clang 3.6 Remove deprecated register storage class P0001R1 Clang 3.8 Remove deprecated bool increment P0002R1 Clang 3.8 Make exception specifications part of the type system P0012R1 Clang 4 __has_include in preprocessor conditionals P0061R1 Yes [[fallthrough]] attribute P0188R1 Clang 3.9 [[nodiscard]] attribute P0189R1 Clang 3.9 P1771R1 (DR) Clang 9 [[maybe_unused]] attribute P0212R1 Clang 3.9 Aggregate initialization of classes with base classes P0017R1 Clang 3.9 constexpr lambda expressions P0170R1 Clang 5 Differing begin and end types in range-based for P0184R0 Clang 3.9 Lambda capture of *this P0018R3 Clang 3.9 Direct-list-initialization of enum s P0138R2 Clang 3.9 Hexadecimal floating-point literals P0245R1 Yes Using attribute namespaces without repetition P0028R4 Clang 3.9 Dynamic memory allocation for over-aligned data P0035R4 Clang 4 Template argument deduction for class templates P0091R3 Clang 5 P0512R0 P0620R0 (DR) Clang 7 P0702R1 (DR) Clang 6 Non-type template parameters with auto type P0127R2 Clang 4 Guaranteed copy elision P0135R1 Clang 4 Stricter expression evaluation order P0145R3 Clang 4 (9) P0400R0 Requirement to ignore unknown attributes P0283R2 Yes constexpr if-statements P0292R2 Clang 3.9 Inline variables P0386R2 Clang 3.9 Structured bindings P0217R3 Clang 4 P0961R1 (DR) Clang 8 P0969R0 (DR) Clang 8 Separate variable and condition for if and switch P0305R1 Clang 3.9 Matching template template parameters to compatible arguments P0522R0 Partial (10) Removing deprecated dynamic exception specifications P0003R5 Clang 4 Pack expansions in using-declarations P0195R2 Clang 4 (8): This is a backwards-incompatible change that is applied to all language versions that allow type deduction from auto (per the request of the C++ committee). In Clang 3.7, a warning is emitted for all cases that would change meaning.

(9): Under the MS ABI, function parameters are destroyed from left to right in the callee. As a result, function parameters in calls to operator<< , operator>> , operator->* , operator&& , operator|| , and operator, functions using expression syntax are no longer guaranteed to be destroyed in reverse construction order in that ABI.

(10): Despite being the resolution to a Defect Report, this feature is disabled by default in all language versions, and can be enabled explicitly with the flag -frelaxed-template-template-args in Clang 4 onwards. The change to the standard lacks a corresponding change for template partial ordering, resulting in ambiguity errors for reasonable and previously-valid code. This issue is expected to be rectified soon.

C++20 implementation status

Clang has support for some of the features of the ISO C++ 2020 Draft International Standard.

You can use Clang in C++20 mode with the -std=c++20 option (use -std=c++2a in Clang 9 and earlier).

List of features and minimum Clang version with support Language Feature C++20 Proposal Available in Clang? Default member initializers for bit-fields P0683R1 Clang 6 const& -qualified pointers to members P0704R1 Clang 6 Allow lambda-capture [=, this] P0409R2 Clang 6 __VA_OPT__ for preprocessor comma elision P0306R4 Clang 6 P1042R1 Clang 9 Designated initializers P0329R4 Clang 10 template-parameter-list for generic lambdas P0428R2 Clang 9 Concepts P0734R0 Clang 10 P0857R0 P1084R2 P1141R2 P0848R3 No P1616R1 Clang 10 P1452R2 P1972R0 No P1980R0 P2103R0 No P2092R0 P2113R0 Range-based for statements with initializer P0614R1 Clang 8 ADL and function templates that are not visible P0846R0 Clang 9 const mismatch with defaulted copy constructor P0641R2 Clang 8 Consistent comparison ( operator<=> ) P0515R3 Clang 10 P0905R1 P1120R0 P1185R2 P1186R3 P1630R1 P1946R0 P1959R0 P2002R1 Partial P2085R0 No Access checking on specializations P0692R1 Partial Default constructible and assignable stateless lambdas P0624R2 Clang 8 Lambdas in unevaluated contexts P0315R4 No [[no_unique_address]] attribute P0840R2 Clang 9 [[likely]] and [[unlikely]] attributes P0479R5 Clang 12 (partial) typename optional in more contexts P0634R3 No Pack expansion in lambda init-capture P0780R2 Clang 9 Class types as non-type template parameters P0732R2 No P1907R1 Destroying operator delete P0722R3 Clang 6 Relaxations of constexpr restrictions P1064R0 Clang 9 P1002R1 Clang 8 P1327R1 Clang 9 P1330R0 P1331R2 Clang 10 P1668R1 P0784R7 Prohibit aggregates with user-declared constructors P1008R1 Clang 8 Feature test macros P0941R2 (see below) explicit(bool) P0892R2 Clang 9 Signed integers are two's complement P1236R1 Clang 9 char8_t P0482R6 Clang 7 (11) Immediate functions ( consteval ) P1073R3 No P1937R2 std::is_constant_evaluated P0595R2 Clang 9 Nested inline namespaces P1094R2 Clang 8 Structured binding extensions P1091R3 Partial P1381R1 Stronger Unicode requirements P1041R4 Yes P1139R2 Parenthesized initialization of aggregates P0960R3 No P1975R0 Modules P1103R3 Partial P1766R1 (DR) Clang 11 P1811R0 No P1703R1 P1874R1 Partial P1979R0 No P1779R3 P1857R3 P2115R0 Partial P1815R2 No Coroutines P0912R5 Partial Deprecate a[b,c] P1161R3 Clang 9 Deprecate some problematic uses of volatile P1152R4 Clang 10 [[nodiscard("with reason")]] P1301R4 Clang 9 using enum P1099R5 No Class template argument deduction for aggregates P1816R0 No P2082R1 Class template argument deduction for alias templates P1814R0 No Permit conversions to arrays of unknown bound P0388R4 No constinit P1143R2 Clang 10 Pseudo-destructors end object lifetimes P0593R6 (DR) Clang 11 (11): Prior to Clang 8, this feature is not enabled by -std=c++20 , but can be enabled with -fchar8_t .

Defect reports

Clang generally aims to implement resolutions to Defect Reports (bug fixes against prior standards) retroactively, in all prior standard versions where the fix is meaningful. Significant Defect Report changes to language features after the publication of the relevant standard are marked (DR) in the above table.

Clang also has a test suite for conformance to resolutions for issues on the C++ core issues list, most of which are considered Defect Reports. Implementation status for C++ core issues based on that test suite is tracked on a separate page.

Technical specifications and standing documents

ISO C++ also publishes a number of documents describing additional language and library features that are not part of standard C++.