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318 lines
12 KiB
318 lines
12 KiB
============
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Using libc++
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============
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.. contents::
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:local:
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Getting Started
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===============
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If you already have libc++ installed you can use it with clang.
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.. code-block:: bash
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$ clang++ -stdlib=libc++ test.cpp
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$ clang++ -std=c++11 -stdlib=libc++ test.cpp
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On OS X and FreeBSD libc++ is the default standard library
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and the ``-stdlib=libc++`` is not required.
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.. _alternate libcxx:
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If you want to select an alternate installation of libc++ you
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can use the following options.
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.. code-block:: bash
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$ clang++ -std=c++11 -stdlib=libc++ -nostdinc++ \
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-I<libcxx-install-prefix>/include/c++/v1 \
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-L<libcxx-install-prefix>/lib \
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-Wl,-rpath,<libcxx-install-prefix>/lib \
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test.cpp
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The option ``-Wl,-rpath,<libcxx-install-prefix>/lib`` adds a runtime library
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search path. Meaning that the systems dynamic linker will look for libc++ in
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``<libcxx-install-prefix>/lib`` whenever the program is run. Alternatively the
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environment variable ``LD_LIBRARY_PATH`` (``DYLD_LIBRARY_PATH`` on OS X) can
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be used to change the dynamic linkers search paths after a program is compiled.
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An example of using ``LD_LIBRARY_PATH``:
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.. code-block:: bash
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$ clang++ -stdlib=libc++ -nostdinc++ \
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-I<libcxx-install-prefix>/include/c++/v1
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-L<libcxx-install-prefix>/lib \
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test.cpp -o
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$ ./a.out # Searches for libc++ in the systems library paths.
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$ export LD_LIBRARY_PATH=<libcxx-install-prefix>/lib
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$ ./a.out # Searches for libc++ along LD_LIBRARY_PATH
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Using ``<filesystem>`` and libc++fs
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====================================
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Libc++ provides the implementation of the filesystem library in a separate
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library. Users of ``<filesystem>`` and ``<experimental/filesystem>`` are
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required to link ``-lc++fs``.
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.. note::
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Prior to libc++ 7.0, users of ``<experimental/filesystem>`` were required
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to link libc++experimental.
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.. warning::
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The Filesystem library is still experimental in nature. As such normal
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guarantees about ABI stability and backwards compatibility do not yet apply
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to it. In the future, this restriction will be removed.
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Using libc++experimental and ``<experimental/...>``
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=====================================================
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Libc++ provides implementations of experimental technical specifications
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in a separate library, ``libc++experimental.a``. Users of ``<experimental/...>``
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headers may be required to link ``-lc++experimental``.
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.. code-block:: bash
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$ clang++ -std=c++14 -stdlib=libc++ test.cpp -lc++experimental
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Libc++experimental.a may not always be available, even when libc++ is already
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installed. For information on building libc++experimental from source see
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:ref:`Building Libc++ <build instructions>` and
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:ref:`libc++experimental CMake Options <libc++experimental options>`.
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Note that as of libc++ 7.0 using the ``<experimental/filesystem>`` requires linking
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libc++fs instead of libc++experimental.
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Also see the `Experimental Library Implementation Status <http://libcxx.llvm.org/ts1z_status.html>`__
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page.
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.. warning::
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Experimental libraries are Experimental.
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* The contents of the ``<experimental/...>`` headers and ``libc++experimental.a``
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library will not remain compatible between versions.
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* No guarantees of API or ABI stability are provided.
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Using libc++ on Linux
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=====================
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On Linux libc++ can typically be used with only '-stdlib=libc++'. However
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some libc++ installations require the user manually link libc++abi themselves.
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If you are running into linker errors when using libc++ try adding '-lc++abi'
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to the link line. For example:
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.. code-block:: bash
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$ clang++ -stdlib=libc++ test.cpp -lc++ -lc++abi -lm -lc -lgcc_s -lgcc
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Alternately, you could just add libc++abi to your libraries list, which in
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most situations will give the same result:
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.. code-block:: bash
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$ clang++ -stdlib=libc++ test.cpp -lc++abi
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Using libc++ with GCC
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---------------------
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GCC does not provide a way to switch from libstdc++ to libc++. You must manually
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configure the compile and link commands.
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In particular you must tell GCC to remove the libstdc++ include directories
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using ``-nostdinc++`` and to not link libstdc++.so using ``-nodefaultlibs``.
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Note that ``-nodefaultlibs`` removes all of the standard system libraries and
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not just libstdc++ so they must be manually linked. For example:
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.. code-block:: bash
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$ g++ -nostdinc++ -I<libcxx-install-prefix>/include/c++/v1 \
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test.cpp -nodefaultlibs -lc++ -lc++abi -lm -lc -lgcc_s -lgcc
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GDB Pretty printers for libc++
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------------------------------
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GDB does not support pretty-printing of libc++ symbols by default. Unfortunately
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libc++ does not provide pretty-printers itself. However there are 3rd
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party implementations available and although they are not officially
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supported by libc++ they may be useful to users.
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Known 3rd Party Implementations Include:
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* `Koutheir's libc++ pretty-printers <https://github.com/koutheir/libcxx-pretty-printers>`_.
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Libc++ Configuration Macros
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===========================
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Libc++ provides a number of configuration macros which can be used to enable
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or disable extended libc++ behavior, including enabling "debug mode" or
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thread safety annotations.
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**_LIBCPP_DEBUG**:
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See :ref:`using-debug-mode` for more information.
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**_LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS**:
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This macro is used to enable -Wthread-safety annotations on libc++'s
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``std::mutex`` and ``std::lock_guard``. By default these annotations are
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disabled and must be manually enabled by the user.
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**_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS**:
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This macro is used to disable all visibility annotations inside libc++.
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Defining this macro and then building libc++ with hidden visibility gives a
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build of libc++ which does not export any symbols, which can be useful when
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building statically for inclusion into another library.
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**_LIBCPP_DISABLE_EXTERN_TEMPLATE**:
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This macro is used to disable extern template declarations in the libc++
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headers. The intended use case is for clients who wish to use the libc++
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headers without taking a dependency on the libc++ library itself.
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**_LIBCPP_ENABLE_TUPLE_IMPLICIT_REDUCED_ARITY_EXTENSION**:
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This macro is used to re-enable an extension in `std::tuple` which allowed
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it to be implicitly constructed from fewer initializers than contained
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elements. Elements without an initializer are default constructed. For example:
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.. code-block:: cpp
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std::tuple<std::string, int, std::error_code> foo() {
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return {"hello world", 42}; // default constructs error_code
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}
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Since libc++ 4.0 this extension has been disabled by default. This macro
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may be defined to re-enable it in order to support existing code that depends
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on the extension. New use of this extension should be discouraged.
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See `PR 27374 <http://llvm.org/PR27374>`_ for more information.
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Note: The "reduced-arity-initialization" extension is still offered but only
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for explicit conversions. Example:
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.. code-block:: cpp
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auto foo() {
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using Tup = std::tuple<std::string, int, std::error_code>;
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return Tup{"hello world", 42}; // explicit constructor called. OK.
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}
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**_LIBCPP_DISABLE_ADDITIONAL_DIAGNOSTICS**:
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This macro disables the additional diagnostics generated by libc++ using the
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`diagnose_if` attribute. These additional diagnostics include checks for:
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* Giving `set`, `map`, `multiset`, `multimap` and their `unordered_`
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counterparts a comparator which is not const callable.
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* Giving an unordered associative container a hasher that is not const
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callable.
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**_LIBCPP_NO_VCRUNTIME**:
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Microsoft's C and C++ headers are fairly entangled, and some of their C++
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headers are fairly hard to avoid. In particular, `vcruntime_new.h` gets pulled
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in from a lot of other headers and provides definitions which clash with
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libc++ headers, such as `nothrow_t` (note that `nothrow_t` is a struct, so
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there's no way for libc++ to provide a compatible definition, since you can't
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have multiple definitions).
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By default, libc++ solves this problem by deferring to Microsoft's vcruntime
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headers where needed. However, it may be undesirable to depend on vcruntime
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headers, since they may not always be available in cross-compilation setups,
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or they may clash with other headers. The `_LIBCPP_NO_VCRUNTIME` macro
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prevents libc++ from depending on vcruntime headers. Consequently, it also
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prevents libc++ headers from being interoperable with vcruntime headers (from
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the aforementioned clashes), so users of this macro are promising to not
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attempt to combine libc++ headers with the problematic vcruntime headers. This
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macro also currently prevents certain `operator new`/`operator delete`
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replacement scenarios from working, e.g. replacing `operator new` and
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expecting a non-replaced `operator new[]` to call the replaced `operator new`.
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**_LIBCPP_ENABLE_NODISCARD**:
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Allow the library to add ``[[nodiscard]]`` attributes to entities not specified
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as ``[[nodiscard]]`` by the current language dialect. This includes
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backporting applications of ``[[nodiscard]]`` from newer dialects and
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additional extended applications at the discretion of the library. All
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additional applications of ``[[nodiscard]]`` are disabled by default.
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See :ref:`Extended Applications of [[nodiscard]] <nodiscard extension>` for
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more information.
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**_LIBCPP_DISABLE_NODISCARD_EXT**:
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This macro prevents the library from applying ``[[nodiscard]]`` to entities
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purely as an extension. See :ref:`Extended Applications of [[nodiscard]] <nodiscard extension>`
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for more information.
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**_LIBCPP_ENABLE_DEPRECATION_WARNINGS**:
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This macro enables warnings when using deprecated components. For example,
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when compiling in C++11 mode, using `std::auto_ptr` with the macro defined
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will trigger a warning saying that `std::auto_ptr` is deprecated. By default,
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this macro is not defined.
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C++17 Specific Configuration Macros
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-----------------------------------
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**_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES**:
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This macro is used to re-enable all the features removed in C++17. The effect
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is equivalent to manually defining each macro listed below.
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**_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS**:
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This macro is used to re-enable the `set_unexpected`, `get_unexpected`, and
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`unexpected` functions, which were removed in C++17.
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**_LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR**:
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This macro is used to re-enable `std::auto_ptr` in C++17.
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C++2a Specific Configuration Macros:
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------------------------------------
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**_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17**:
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This macro can be used to disable diagnostics emitted from functions marked
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``[[nodiscard]]`` in dialects after C++17. See :ref:`Extended Applications of [[nodiscard]] <nodiscard extension>`
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for more information.
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Libc++ Extensions
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=================
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This section documents various extensions provided by libc++, how they're
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provided, and any information regarding how to use them.
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.. _nodiscard extension:
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Extended applications of ``[[nodiscard]]``
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------------------------------------------
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The ``[[nodiscard]]`` attribute is intended to help users find bugs where
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function return values are ignored when they shouldn't be. After C++17 the
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C++ standard has started to declared such library functions as ``[[nodiscard]]``.
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However, this application is limited and applies only to dialects after C++17.
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Users who want help diagnosing misuses of STL functions may desire a more
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liberal application of ``[[nodiscard]]``.
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For this reason libc++ provides an extension that does just that! The
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extension must be enabled by defining ``_LIBCPP_ENABLE_NODISCARD``. The extended
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applications of ``[[nodiscard]]`` takes two forms:
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1. Backporting ``[[nodiscard]]`` to entities declared as such by the
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standard in newer dialects, but not in the present one.
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2. Extended applications of ``[[nodiscard]]``, at the libraries discretion,
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applied to entities never declared as such by the standard.
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Users may also opt-out of additional applications ``[[nodiscard]]`` using
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additional macros.
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Applications of the first form, which backport ``[[nodiscard]]`` from a newer
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dialect may be disabled using macros specific to the dialect it was added. For
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example ``_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17``.
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Applications of the second form, which are pure extensions, may be disabled
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by defining ``_LIBCPP_DISABLE_NODISCARD_EXT``.
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Entities declared with ``_LIBCPP_NODISCARD_EXT``
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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This section lists all extended applications of ``[[nodiscard]]`` to entities
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which no dialect declares as such (See the second form described above).
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* ``get_temporary_buffer``
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