Does clang use libc ++ by default?

 

Using libc++

• Getting Started
• Using libc++experimental and <experimental/...>
• Using libc++ on Linux
  • Using libc++ with GCC
  • GDB Pretty printers for libc++
• Libc++ Configuration Macros
  • C++17 Specific Configuration Macros

Getting Started

If you already have libc++ installed you can use it with clang.

$ clang++ -stdlib=libc++ test.cpp
$ clang++ -std=c++11 -stdlib=libc++ test.cpp

On OS X and FreeBSD libc++ is the default standard library and the -stdlib=libc++ is not required.

If you want to select an alternate installation of libc++ you can use the following options.

$ clang++ -std=c++11 -stdlib=libc++ -nostdinc++ \
          -I<libcxx-install-prefix>/include/c++/v1 \
          -L<libcxx-install-prefix>/lib \
          -Wl,-rpath,<libcxx-install-prefix>/lib \
          test.cpp

The option -Wl,-rpath,<libcxx-install-prefix>/lib adds a runtime library search path. Meaning that the systems dynamic linker will look for libc++ in <libcxx-install-prefix>/lib whenever the program is run. Alternatively the environment variable LD_LIBRARY_PATH (DYLD_LIBRARY_PATH on OS X) can be used to change the dynamic linkers search paths after a program is compiled.

An example of using LD_LIBRARY_PATH:

$ clang++ -stdlib=libc++ -nostdinc++ \
          -I<libcxx-install-prefix>/include/c++/v1
          -L<libcxx-install-prefix>/lib \
          test.cpp -o
$ ./a.out # Searches for libc++ in the systems library paths.
$ export LD_LIBRARY_PATH=<libcxx-install-prefix>/lib
$ ./a.out # Searches for libc++ along LD_LIBRARY_PATH

Using libc++experimental and <experimental/...>

Libc++ provides implementations of experimental technical specifications in a separate library, libc++experimental.a. Users of <experimental/...> headers may be required to link -lc++experimental.

$ clang++ -std=c++14 -stdlib=libc++ test.cpp -lc++experimental

Libc++experimental.a may not always be available, even when libc++ is already installed. For information on building libc++experimental from source see Building Libc++ and libc++experimental CMake Options.

Also see the Experimental Library Implementation Status page.

Warning

Experimental libraries are Experimental.

• The contents of the <experimental/...> headers and libc++experimental.a library will not remain compatible between versions.
• No guarantees of API or ABI stability are provided.

Using libc++ on Linux

On Linux libc++ can typically be used with only ‘-stdlib=libc++’. However some libc++ installations require the user manually link libc++abi themselves. If you are running into linker errors when using libc++ try adding ‘-lc++abi’ to the link line. For example:

$ clang++ -stdlib=libc++ test.cpp -lc++ -lc++abi -lm -lc -lgcc_s -lgcc

Alternately, you could just add libc++abi to your libraries list, which in most situations will give the same result:

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

Using libc++ with GCC

GCC does not provide a way to switch from libstdc++ to libc++. You must manually configure the compile and link commands.

In particular you must tell GCC to remove the libstdc++ include directories using -nostdinc++ and to not link libstdc++.so using -nodefaultlibs.

Note that -nodefaultlibs removes all of the standard system libraries and not just libstdc++ so they must be manually linked. For example:

$ g++ -nostdinc++ -I<libcxx-install-prefix>/include/c++/v1 \
       test.cpp -nodefaultlibs -lc++ -lc++abi -lm -lc -lgcc_s -lgcc

GDB Pretty printers for libc++

GDB does not support pretty-printing of libc++ symbols by default. Unfortunately libc++ does not provide pretty-printers itself. However there are 3rd party implementations available and although they are not officially supported by libc++ they may be useful to users.

Known 3rd Party Implementations Include:

• Koutheir’s libc++ pretty-printers.

Libc++ Configuration Macros

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

_LIBCPP_DEBUG:

See Using Debug Mode for more information.

_LIBCPP_ENABLE_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.

_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS:

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.

_LIBCPP_DISABLE_EXTERN_TEMPLATE:

This macro is used to disable extern template declarations in the libc++ headers. The intended use case is for clients who wish to use the libc++ headers without taking a dependency on the libc++ library itself.

_LIBCPP_ENABLE_TUPLE_IMPLICIT_REDUCED_ARITY_EXTENSION:

This macro is used to re-enable an extension in std::tuple which allowed it to be implicitly constructed from fewer initializers than contained elements. Elements without an initializer are default constructed. For example:

std::tuple<std::string, int, std::error_code> foo() {
  return {"hello world", 42}; // default constructs error_code
}

Since libc++ 4.0 this extension has been disabled by default. This macro may be defined to re-enable it in order to support existing code that depends on the extension. New use of this extension should be discouraged. See PR 27374 for more information.

Note: The “reduced-arity-initialization” extension is still offered but only for explicit conversions. Example:

auto foo() {
  using Tup = std::tuple<std::string, int, std::error_code>;
  return Tup{"hello world", 42}; // explicit constructor called. OK.
}

_LIBCPP_DISABLE_ADDITIONAL_DIAGNOSTICS:

This macro disables the additional diagnostics generated by libc++ using the diagnose_if attribute. These additional diagnostics include checks for:

• Giving set, map, multiset, multimap a comparator which is not const callable.

C++17 Specific Configuration Macros

_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES:

This macro is used to re-enable all the features removed in C++17. The effect is equivalent to manually defining each macro listed below.

_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS:

This macro is used to re-enable the set_unexpected, get_unexpected, and unexpected functions, which were removed in C++17.

_LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR:

This macro is used to re-enable std::auto_ptr in C++17.