# 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. This is not fully supported during constant expression evaluation until Clang 12.
(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 | Partial
P1084R2 | Clang 10
P1141R2
P0848R3 | No
P1616R1 | Clang 10
P1452R2
P1972R0 | No
P1980R0
P2103R0
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
`typename` optional in more contexts | P0634R3 | No
Pack expansion in lambda init-capture | P0780R2 | Clang 9
Class types as non-type template parameters | P0732R2 | Clang 12
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++.

List of features and minimum Clang version with support

Document | Latest draft | Compiler flag | Available in Clang?
--- | --- | --- | ---
SD-6: SG10 feature test recommendations | SD-6 | N/A | Clang 3.4 (N3745)
Clang 3.6 (N4200)
Clang 4 (P0096R3)
Clang 5 (P0096R4)
Clang 7 (P0096R5)
Clang 9 (P1353R0)
Clang 10 (P1902R1)
[TS] Concepts | P0121R0 |  | Superseded by P0734R0
[TS] Coroutines | N4663 | `-fcoroutines-ts -stdlib=libc++` | Clang 5
`-std=c++20 -stdlib=libc++` | Superseded by P0912R5
[TS] Library Fundamentals, Version 1 (invocation type traits) | N4480 | N/A | No
[TS] Library Fundamentals, Version 2 (`source_location`) | N4617 | N/A | Clang 9 (documentation)
[TS] Modules | N4720 | `-fmodules-ts` | Superseded by P1103R3
[DRAFT TS] Reflection | N4818 |  | No
[TS] Transactional Memory | N4514 |  | No