C++ support with the Android NDK
================================


The Android platform provides a very minimal C++ runtime support library
(/system/lib/libstdc++) and corresponding headers for it in the NDK.

By default, this 'system' runtime does *not* provide the following:

  - Standard C++ Library support (except a few trivial headers).
  - C++ exceptions support
  - RTTI support

However, the NDK provides various "helper C++ runtimes" which can provide them,
or a subset of these features.

To select the runtime you want to use, define APP_STL inside your
Application.mk to one of the following values:

        system          -> Use the default minimal system C++ runtime library.
        gabi++_static   -> Use the GAbi++ runtime as a static library.
        gabi++_shared   -> Use the GAbi++ runtime as a shared library.
        stlport_static  -> Use the STLport runtime as a static library.
        stlport_shared  -> Use the STLport runtime as a shared library.
        gnustl_static   -> Use the GNU STL as a static library.
        gnustl_shared   -> Use the GNU STL as a shared library.
        c++_static      -> Use the LLVM libc++ as a static library.
        c++_shared      -> Use the LLVM libc++ as a shared library.

The 'system' runtime is the default if there is no APP_STL definition in
your Application.mk. As an example, to use the static GNU STL, add a line like:

        APP_STL := gnustl_static

To your Application.mk. You can only select a single C++ runtime that all
your code will depend on. It is not possible to mix shared libraries compiled
against different C++ runtimes.

IMPORTANT: Defining APP_STL in Android.mk has no effect!

If you are not using the NDK build system, you can still use on of
STLport, libc++ or GNU STL via "make-standalone-toolchain.sh --stl=".
see docs/STANDALONE-TOOLCHAIN.html for more details.

The capabilities of the various runtimes vary. See this table:

                    C++       C++   Standard
                  Exceptions  RTTI    Library

        system        no       no        no
        gabi++       yes      yes        no
        stlport      yes      yes       yes
        gnustl       yes      yes       yes
        libc++       yes      yes       yes

#

- - - -
I. Runtimes overview:
---------------------

### I.1. System runtime:

The system runtime only provides a very small number of C++ standard headers.

This corresponds to the actual C++ runtime provided by the Android platform.
If you use it, your C++ binaries will automatically be linked against the
system libstdc++.

The only headers provided here are the following:

        cassert cctype cerrno cfloat climits cmath csetjmp csignal cstddef
        cstdint cstdio cstdlib cstring ctime cwchar new stl_pair.h typeinfo
        utility

Anything else is _not_ supported, including std::string or std::vector.


### I.2. GAbi++ runtime:

This is a new minimalistic runtime that provides the same headers than
the system one, with the addition of RTTI (RunTime Type Information) and
exception handling support.

If you insist on using it, read the "RTTI Support" and
"Static runtime considerations" sections below.


### I.3. STLport runtime:

This is a port of STLport (http://www.stlport.org) that can be used on
Android. It will provide you with a complete set of C++ standard library
headers, with RTTI and exception handling support.

That's because the library embeds its own copy of GAbi++.

Available as both both static and shared libraries. To use it, use either
one of these two lines in your Application.mk:

      APP_STL := stlport_shared
      APP_STL := stlport_static

Note that 'stlport_shared' is preferred, for reasons explained in
"Static runtime considerations".  The shared library file is named
libstlport_shared.so instead of "libstdc++.so" as on other platforms.


### I.4. GNU STL runtime:

This is the GNU Standard C++ Library (a.k.a. libstdc++-v3), providing the
more features. Note that the shared library file is named "libgnustl_shared.so".


### I.5. libC++ runtime:

This is a port of LLVM libc++: http://libcxx.llvm.org/.  Note that the shared library
file is named "libc++_shared.so".


Please read the "C++ Exceptions support", "RTTI Support" and
"Static runtime considerations" sections below.


- - - -
II. Important Considerations:
-----------------------------

### II.1. C++ Exceptions support:

The NDK toolchain supports C++ exceptions, since NDK r5, however all C++
sources are compiled with -fno-exceptions support by default, for
compatibility reasons with previous releases.

To enable it, use the new LOCAL_CPP_FEATURES variable in your Android.mk,
as in:

        LOCAL_CPP_FEATURES += exceptions

See docs/ANDROID-MK.html for more details about this variable.

Another way to do the same is to define it in your LOCAL_CPPFLAGS definition
(but using LOCAL_CPP_FEATURES is preferred), as in:

        LOCAL_CPPFLAGS += -fexceptions

More simply, add a single line to your Application.mk, the setting will
automatically apply to all your project's NDK modules:

        APP_CPPFLAGS += -fexceptions

IMPORTANT: You *will* have to select a C++ runtime that supports
           exceptions to be able to link / run your code.


### II.2. RTTI support:

Similarly, the NDK toolchain supports C++ RTTI (RunTime Type Information)
since NDK r5, but all C++ sources are built with -fno-rtti by default for
compatibility reasons. To enable it, add the following to your module
declarations:

        LOCAL_CPP_FEATURES += rtti

This will be equivalent to:

        LOCAL_CPPFLAGS += -frtti

Or more simply to your Application.mk:

        APP_CPPFLAGS += -frtti


### II.3. Static runtimes:

Please keep in mind that the static library variant of a given C++ runtime
SHALL ONLY BE LINKED INTO A SINGLE BINARY for optimal conditions.

What this means is that if your project consists of a single shared
library, you can link against, e.g., stlport_static, and everything will
work correctly.

On the other hand, if you have two shared libraries in your project
(e.g. libfoo.so and libbar.so) which both link against the same static
runtime, each one of them will  include a copy of the runtime's code in
its final binary image. This is problematic because certain global
variables used/provided internally by the runtime are duplicated.

This is likely to result in code that doesn't work correctly, for example:

  - memory allocated in one library, and freed in the other would leak
    or even corrupt the heap.

  - exceptions raised in libfoo.so cannot be caught in libbar.so (and may
    simply crash the program).

  - the buffering of std::cout not working properly

This problem also happens if you want to link an executable and a shared
library to the same static library.

In other words, if your project requires several shared library modules,
then use the shared library variant of your C++ runtime.


### II.4. Shared runtimes:

If you use the shared library variant of a given C++ runtime, keep in mind
that you must load it before any library that depends on it when your
application starts.

As an example, let's consider the case where we have the following modules

  * libfoo.so
  * libbar.so which is used by libfoo.so
  * libstlport_shared.so, used by both libfoo and libbar

You will need to load the libraries in reverse dependency order, as in:

        static {
          System.loadLibrary("stlport_shared");
          System.loadLibrary("bar");
          System.loadLibrary("foo");
        }

Note that you shouldn't use the 'lib' prefix when calling
System.loadLibrary(), unless you specify the full path as in:

    System.loadLibrary("/path/to/libstlport_shared.so")

Which is not recommended, since this hard-codes the path in your code.

- - - -
III. EXTRAS:
------------

### III.1. STLport-specific issues:

This NDK provides prebuilt static and shared libraries for STLport,
but you can force it to be rebuilt from sources by defining the following
in your environment or your Application.mk before building:

        STLPORT_FORCE_REBUILD := true

STLport is licensed under a BSD-style open-source license. See
sources/cxx-stl/stlport/README for more details about the library.


### III.2. GNU libstdc++ license is GPLv3 + linking exception!

Be aware that the GNU libstdc++ is covered by the GPLv3 license (and *not*
the LGPLv2 or LGPLv3 as some assume), full details available here:

        http://gcc.gnu.org/onlinedocs/libstdc++/manual/license.html

Be sure that you comply with all clauses of this license.


### III.3. libc++-specific issues:

"-std=c++11" is turned on by default.

Similiar to GNU libstdc++, you need to explicitly turns on exceptions or rtti
support in "LOCAL_CPP_FEATURES" if you wish.

It's likely that you need libatomic if you #include <atomic>.  Add
"LOCAL_LDLIBS += -latomic" for ndk-build, and "-latomic" for standalone toolchain.
Note that -latomic is only available in gcc4.8, not gcc4.6.  Clang3.4/3.3 use gcc4.8's
as/ld/headers/libraries so they get -latomic too.  The version of libatomic in gcc4.8
*may* work for gcc4.6, although it's not tested and you have to copy them manually.

This NDK provides prebuilt static and shared libraries for libc++
compiled by clang, but you can force it to be rebuilt from sources by defining
the following in your environment or your Application.mk before building:

        LIBCXX_FORCE_REBUILD := true

Around 98% of current 4645 tests passes when compiling libc++ with
clang for all supported ABIs. The remaining fails are mostly in the areas
of wchar_t and locale Android bionic don't support.  Switching locale
from the default produces the following warning in logcat

        newlocale() WARNING: Trying to set locale to en_US.UTF-8 other than "", "C" or "POSIX"

Using gabi++ as run-time also causes some issues in the nested exception and propagation too,
which will be addressed when libc++ switches to use libc++abi in the future.

More fails when compiling libc++ with gcc4.8 mostly because the lack of support
in _Atomic (ETA: gcc 4.9).  See platform/ndk.git
29d9f88ef5a33cd65b4b9977aed628bc195facf3 for details

Using libc++ with gcc4.6 is not recommended (at least not tested at
this moment) because its c++11 support isn't great

See "black_list*" in
tests/device/test-libc++-shared-full/jni/Android.mk for tests which fail to
compile, and tests/device/test-libc++-shared-full/BROKEN_RUN for tests
which fails to run correctly.

### IV. Future Plans:

  - uSTL support?