Using libc++

Usually, libc++ is packaged and shipped by a vendor through some delivery vehicle (operating system distribution, SDK, toolchain, etc) and users don’t need to do anything special in order to use the library.

This page contains information about configuration knobs that can be used by users when they know libc++ is used by their toolchain, and how to use libc++ when it is not the default library used by their toolchain.

Using a different version of the C++ Standard

Libc++ implements the various versions of the C++ Standard. Changing the version of the standard can be done by passing -std=c++XY to the compiler. Libc++ will automatically detect what Standard is being used and will provide functionality that matches that Standard in the library.

$ clang++ -std=c++17 test.cpp


Using -std=c++XY with a version of the Standard that has not been ratified yet is considered unstable. Libc++ reserves the right to make breaking changes to the library until the standard has been ratified.

Enabling experimental C++ Library features

Libc++ provides implementations of some experimental features. Experimental features are either Technical Specifications (TSes) or official features that were voted to the Standard but whose implementation is not complete or stable yet in libc++. Those are disabled by default because they are neither API nor ABI stable. However, the -fexperimental-library compiler flag can be defined to turn those features on.

The following features are currently considered experimental and are only provided when -fexperimental-library is passed:

  • The parallel algorithms library (<execution> and the associated algorithms)

  • std::stop_token, std::stop_source and std::stop_callback

  • std::jthread

  • std::chrono::tzdb and related time zone functionality


Experimental libraries are experimental.
  • The contents of the <experimental/...> headers and the associated static library will not remain compatible between versions.

  • No guarantees of API or ABI stability are provided.

  • When the standardized version of an experimental feature is implemented, the experimental feature is removed two releases after the non-experimental version has shipped. The full policy is explained here.


On compilers that do not support the -fexperimental-library flag, users can define the _LIBCPP_ENABLE_EXPERIMENTAL macro and manually link against the appropriate static library (usually shipped as libc++experimental.a) to get access to experimental library features.

Using libc++ when it is not the system default

On systems where libc++ is provided but is not the default, Clang provides a flag called -stdlib= that can be used to decide which standard library is used. Using -stdlib=libc++ will select libc++:

$ clang++ -stdlib=libc++ test.cpp

On systems where libc++ is the library in use by default such as macOS and FreeBSD, this flag is not required.

Using a custom built libc++

Most compilers provide a way to disable the default behavior for finding the standard library and to override it with custom paths. With Clang, this can be done with:

$ clang++ -nostdinc++ -nostdlib++           \
          -isystem <install>/include/c++/v1 \
          -L <install>/lib                  \
          -Wl,-rpath,<install>/lib          \
          -lc++                             \

The option -Wl,-rpath,<install>/lib adds a runtime library search path, which causes the system’s dynamic linker to look for libc++ in <install>/lib whenever the program is loaded.

GCC does not support the -nostdlib++ flag, so one must use -nodefaultlibs instead. Since that removes all the standard system libraries and not just libc++, the system libraries must be re-added manually. For example:

$ g++ -nostdinc++ -nodefaultlibs           \
      -isystem <install>/include/c++/v1    \
      -L <install>/lib                     \
      -Wl,-rpath,<install>/lib             \
      -lc++ -lc++abi -lm -lc -lgcc_s -lgcc \

GDB Pretty printers for libc++

GDB does not support pretty-printing of libc++ symbols by default. However, libc++ does provide pretty-printers itself. Those can be used as:

$ gdb -ex "source <libcxx>/utils/gdb/libcxx/" \
      -ex "python register_libcxx_printer_loader()" \

include-what-you-use (IWYU)

libc++ provides an IWYU mapping file, which drastically improves the accuracy of the tool when using libc++. To use the mapping file with IWYU, you should run the tool like so:

$ include-what-you-use -Xiwyu --mapping_file=/path/to/libcxx/include/libcxx.imp file.cpp

If you would prefer to not use that flag, then you can replace /path/to/include-what-you-use/share/libcxx.imp file with the libc++-provided libcxx.imp file.

Libc++ Configuration Macros

Libc++ provides a number of configuration macros which can be used to enable or disable extended libc++ behavior, including enabling hardening or thread safety annotations.


This macro is used to enable -Wthread-safety annotations on libc++’s std::mutex and std::lock_guard. By default, these annotations are disabled and must be manually enabled by the user.


This macro is used to choose the hardening mode.


This macro is used to disable all visibility annotations inside libc++. Defining this macro and then building libc++ with hidden visibility gives a build of libc++ which does not export any symbols, which can be useful when building statically for inclusion into another library.


Microsoft’s C and C++ headers are fairly entangled, and some of their C++ headers are fairly hard to avoid. In particular, vcruntime_new.h gets pulled in from a lot of other headers and provides definitions which clash with libc++ headers, such as nothrow_t (note that nothrow_t is a struct, so there’s no way for libc++ to provide a compatible definition, since you can’t have multiple definitions).

By default, libc++ solves this problem by deferring to Microsoft’s vcruntime headers where needed. However, it may be undesirable to depend on vcruntime headers, since they may not always be available in cross-compilation setups, or they may clash with other headers. The _LIBCPP_NO_VCRUNTIME macro prevents libc++ from depending on vcruntime headers. Consequently, it also prevents libc++ headers from being interoperable with vcruntime headers (from the aforementioned clashes), so users of this macro are promising to not attempt to combine libc++ headers with the problematic vcruntime headers. This macro also currently prevents certain operator new/operator delete replacement scenarios from working, e.g. replacing operator new and expecting a non-replaced operator new[] to call the replaced operator new.


This macro disables warnings when using deprecated components. For example, using std::auto_ptr when compiling in C++11 mode will normally trigger a warning saying that std::auto_ptr is deprecated. If the macro is defined, no warning will be emitted. By default, this macro is not defined.

C++17 Specific Configuration Macros


This macro is used to re-enable auto_ptr.


This macro is used to re-enable the binder1st, binder2nd, pointer_to_unary_function, pointer_to_binary_function, mem_fun_t, mem_fun1_t, mem_fun_ref_t, mem_fun1_ref_t, const_mem_fun_t, const_mem_fun1_t, const_mem_fun_ref_t, and const_mem_fun1_ref_t class templates, and the bind1st, bind2nd, mem_fun, mem_fun_ref, and ptr_fun functions.


This macro is used to re-enable the random_shuffle algorithm.


This macro is used to re-enable set_unexpected, get_unexpected, and unexpected.

C++20 Specific Configuration Macros


This macro is used to re-enable the function std::shared_ptr<...>::unique().


This macro is used to re-enable the argument_type, result_type, first_argument_type, and second_argument_type members of class templates such as plus, logical_not, hash, and owner_less.


This macro is used to re-enable not1, not2, unary_negate, and binary_negate.


This macro is used to re-enable raw_storage_iterator.


This macro is used to re-enable is_literal_type, is_literal_type_v, result_of and result_of_t.

C++26 Specific Configuration Macros


This macro is used to re-enable all named declarations in <codecvt>.


This macro is used to re-enable the function std::basic_string<...>::reserve().


This macro is used to re-enable redundant member of allocator<T>::is_always_equal.


This macro is used to re-enable all named declarations in <strstream>.


This macro is used to re-enable the wstring_convert and wbuffer_convert in <locale>.

Libc++ Extensions

This section documents various extensions provided by libc++, how they’re provided, and any information regarding how to use them.

Extended integral type support

Several platforms support types that are not specified in the Standard, such as the 128-bit integral types __int128_t and __uint128_t. As an extension, libc++ does a best-effort attempt to support these types like other integral types, by supporting them notably in:

  • <bits>

  • <charconv>

  • <functional>

  • <type_traits>

  • <format>

  • <random>

Additional types supported in random distributions

The C++ Standard mentions that instantiating several random number distributions with types other than short, int, long, long long, and their unsigned versions is undefined. As an extension, libc++ supports instantiating binomial_distribution, discrete_distribution, geometric_distribution, negative_binomial_distribution, poisson_distribution, and uniform_int_distribution with int8_t, __int128_t and their unsigned versions.

Extensions to <format>

The exposition only type basic-format-string and its typedefs format-string and wformat-string became basic_format_string, format_string, and wformat_string in C++23. Libc++ makes these types available in C++20 as an extension.

For padding Unicode strings the format library relies on the Unicode Standard. Libc++ retroactively updates the Unicode Standard in older C++ versions. This allows the library to have better estimates for newly introduced Unicode code points, without requiring the user to use the latest C++ version in their code base.

In C++26 formatting pointers gained a type P and allows to use zero-padding. These options have been retroactively applied to C++20.

Extensions to the C++23 modules std and std.compat

Like other major implementations, libc++ provides C++23 modules std and std.compat in C++20 as an extension”

Constant-initialized std::string

As an implementation-specific optimization, std::basic_string (std::string, std::wstring, etc.) may either store the string data directly in the object, or else store a pointer to heap-allocated memory, depending on the length of the string.

As of C++20, the constructors are now declared constexpr, which permits strings to be used during constant-evaluation time. In libc++, as in other common implementations, it is also possible to constant-initialize a string object (e.g. via declaring a variable with constinit or constexpr), but, only if the string is short enough to not require a heap allocation. Reliance upon this should be discouraged in portable code, as the allowed length differs based on the standard-library implementation and also based on whether the platform uses 32-bit or 64-bit pointers.

// Non-portable: 11-char string works on 64-bit libc++, but not on 32-bit.
constinit std::string x = "hello world";

// Prefer to use string_view, or remove constinit/constexpr from the variable definition:
constinit std::string_view x = "hello world";
std::string_view y = "hello world";

Turning off ASan annotation in containers

__asan_annotate_container_with_allocator is a customization point to allow users to disable Address Sanitizer annotations for containers for specific allocators. This may be necessary for allocators that access allocated memory. This customization point exists only when _LIBCPP_HAS_ASAN_CONTAINER_ANNOTATIONS_FOR_ALL_ALLOCATORS Feature Test Macro is defined.

For allocators not running destructors, it is also possible to bulk-unpoison memory instead of disabling annotations altogether.

The struct may be specialized for user-defined allocators. It is a Cpp17UnaryTypeTrait with a base characteristic of true_type if the container is allowed to use annotations and false_type otherwise.

The annotations for a user_allocator can be disabled like this:

template <class T>
struct std::__asan_annotate_container_with_allocator<user_allocator<T>> : std::false_type {};

Why may I want to turn it off?

There are a few reasons why you may want to turn off annotations for an allocator. Unpoisoning may not be an option, if (for example) you are not maintaining the allocator.

  • You are using allocator, which does not call destructor during deallocation.

  • You are aware that memory allocated with an allocator may be accessed, even when unused by container.

Platform specific behavior


The stdout, stderr, and stdin file streams can be placed in Unicode mode by a suitable call to _setmode(). When in this mode, the sequence of bytes read from, or written to, these streams is interpreted as a sequence of little-endian wchar_t elements. Thus, use of std::cout, std::cerr, or std::cin with streams in Unicode mode will not behave as they usually do since bytes read or written won’t be interpreted as individual char elements. However, std::wcout, std::wcerr, and std::wcin will behave as expected.

Wide character stream such as std::wcin or std::wcout imbued with a locale behave differently than they otherwise do. By default, wide character streams don’t convert wide characters but input/output them as is. If a specific locale is imbued, the IO with the underlying stream happens with regular char elements, which are converted to/from wide characters according to the locale. Note that this doesn’t behave as expected if the stream has been set in Unicode mode.