mirror of https://github.com/ARMmbed/mbed-os.git
66 lines
2.9 KiB
Markdown
66 lines
2.9 KiB
Markdown
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## C++ support ##
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Mbed OS supports a number of toolchains, and the files here provide support
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to smooth over C++ library differences.
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The current baseline version is C++14, so we hope for full C++14 library
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support.
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Omissions are:
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* areas like chrono and threads, which depend on OS support, where
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retargeting is not complete.
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* atomics and shared pointers, as atomic implementations for ARMv6-M
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are not provided by all toolchains, and the ARMv7-M implementations include
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DMB instructions we do not want/need for non-SMP.
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User code should normally be able to include C++14 headers and get
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most expected functionality. (But bear in mind that many C++ library
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features like streams and STL containers are quite heavy and may
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not be appropriate for small embnedded use).
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However, ARM C 5 has only C++11 language support (at least a large subset), and
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no C++11/14 library. For the headers that are totally new in C++11,
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they are provided here in TOOLCHAIN_ARMC5 under their standard C++11 name.
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(Eg `<array>`, `<cinttypes>`, `<initializer_list>`, `<type_traits>`).
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But for headers that already exist in C++03, extensions are required.
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So, to support ARM C 5, use `#include <mstd_utility>`, rather than
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`#include <utility>` if you need C++11 or later features from that header.
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Each `mstd_` file includes the toolchain's corresponding header file,
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which will provide its facilities in `namespace std`. Any missing
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C++11/C++14 facilities for ARM C 5 are also provided in `namespace std`.
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Then APIs from `namespace std` are added to `namespace mstd`, with adjustment
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if necessary, and APIs being omitted if not suitable for embedded use.
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For example:
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* `std::size_t` (`<cstddef>`) - toolchain's `std::size_t`
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* `mstd::size_t` (`<mstd_cstddef>`) - alias for `std::size_t`
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* `std::swap` (`<utility>`) - toolchain's `std::swap` (not move-capable for ARM C 5)
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* `mstd::swap` (`<mstd_utility>`) - alias for `std::swap` or move-capable ARM C 5 replacement.
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* `std::atomic` (`<atomic>`) - toolchain's `std::atomic` (not implemented for IAR ARMv6)
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* `mstd::atomic` (`<mstd_atomic>`) - custom `mstd::atomic` for all toolchains
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* `std::void_t` (`<type_traits>`) - toolchain's `std::void_t` if available (it's C++17 so likely not)
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* `mstd::void_t` (`<mstd_type_traits>`) - alias for `std::void_t` if available, else local implementation
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* `std::thread` (`<thread>`) - toolchain's `std::thread` - not available or ported
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* `mstd::thread` - doesn't exist - `mstd` APIs only exist if available on all toolchains
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Using `std::xxx` will generally work, but may suffer from toolchain variance. `mstd::xxx` should always be better - it will either be an alias to `std::xxx`, or work better for Mbed OS.
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In portable code, when compiling for non-Mbed OS, the directive `namespace mstd == std` can be used
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to cover the difference:
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```C++
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// my_code.c
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#if TARGET_LIKE_MBED
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#include <mstd_atomic>
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#else
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#include <atomic>
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namespace mstd = std;
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#endif
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mstd::atomic my_atomic;
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```
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