Google Git
Sign in
android / platform / system / libbase / 5284bc2af27a41a11bf3952b8b3422bb7dc5357b
commit5284bc2af27a41a11bf3952b8b3422bb7dc5357b[log] [tgz]
authorRyan Prichard <rprichard@google.com>Wed Mar 06 23:59:57 2024 -0800
committerRyan Prichard <rprichard@google.com>Thu Mar 21 23:59:36 2024 -0700
treee065fc681eb634d584ea748ed57add344214cd9b
parent398ff505b09500a6f1bfb08024ea7d0b3ff85acc [diff]
Update base::expected using current C++ spec

The current base::expected is based on p0323r7.

Fix bugs:

 * Make the unexpected<E> from Err constructor explicit. This
   constructor is explicit even in p0323r7. Allowing implicit
   conversions of any type to unexpected<E> is hazardous, because it
   could turn an intended successful value into an error value.

 * In the copy/move expected<T, E> constructors that convert from a
   different expected<U, G> type, use member initialization syntax,
   var_(...), rather than assigning to var_ in the body. The value and
   errors type T and E might not have default constructors, and their
   copy/move constructors should be called, not their assignment
   operators. Using assignment operators also broke the U to T and the
   G to E conversions when it required calling an explicit constructor.

 * Non-swappable value and error types didn't always work. Previously,
   expected<void, E> could not be instantiated with a non-swappable E
   because the member swap function wasn't using SFINAE properly.
   Previously, it was also not possible to instantiate
   std::is_swappable_v<expected<T, E>> for non-swappable T or E because
   the namespace-scope swap function wasn't disabled when the member
   swap function was disabled.

 * unexpected::swap was calling std::swap. It should instead use the
   standard idiom of `using std::swap; swap(...);` so that
   namespace-scope swap functions are called for user-declared classes.

Move towards the current C++ draft at https://eel.is/c++draft:

 * Remove unexpected<Err> to unexpected<E> conversions where Err != E.
   Converting an Err error value to E for expected<T, E> now happens
   only in the expected class's constructors and assignment operators.

 * Rename unexpected<E>::value to unexpected<E>::error to be consistent
   with expected<T, E>::error.

 * Include the special bool-related constraints for wg21.link/LWG3836.
   Ensure that converting expected<bool, E> to expected<bool, G> works
   as expected by transferring either the success or the error value.

Other changes:

 * Always be explicit about which std::variant alternative is being
   initialized or assigned. Avoid ambiguity when a value could be
   converted to either alternative.

 * Add static_asserts to prevent invalid T and E types.

 * Mark everything constexpr and provide reasonable noexcept
   conditions.

 * Change the float literals in expected_test.cpp to double literals.
   The use of float literals breaks this line:

       exp_double::unexpected_type unexp2 = unexpected(10.5f);

   This line attempts to implicitly convert unexpected<float> to
   unexpected<double>, which is now a compile-time error.

 * Remove the bugprone-forwarding-reference-overload lint lines.
   clang-tidy recognizes this use of std::enable_if in LLVM 14 and up.

Bug: b/175635923
Bug: b/328549318
Test: libbase_test
Change-Id: Id886db72c0b0103ff2c570ca650cee6d5c463721
2 files changed
tree: e065fc681eb634d584ea748ed57add344214cd9b
  1. include/
  2. tidy/
  3. .clang-format ⇨ ../../build/soong/scripts/system-clang-format-2
  4. abi_compatibility.cpp
  5. Android.bp
  6. chrono_utils.cpp
  7. chrono_utils_test.cpp
  8. cmsg.cpp
  9. cmsg_test.cpp
  10. CPPLINT.cfg
  11. endian_test.cpp
  12. errors_test.cpp
  13. errors_unix.cpp
  14. errors_windows.cpp
  15. expected_test.cpp
  16. file.cpp
  17. file_benchmark.cpp
  18. file_test.cpp
  19. format_benchmark.cpp
  20. function_ref_benchmark.cpp
  21. function_ref_test.cpp
  22. hex.cpp
  23. hex_test.cpp
  24. logging.cpp
  25. logging_splitters.h
  26. logging_splitters_test.cpp
  27. logging_test.cpp
  28. macros_test.cpp
  29. mapped_file.cpp
  30. mapped_file_test.cpp
  31. no_destructor_test.cpp
  32. NOTICE
  33. OWNERS
  34. parsebool.cpp
  35. parsebool_test.cpp
  36. parsedouble_test.cpp
  37. parseint_test.cpp
  38. parsenetaddress.cpp
  39. parsenetaddress_fuzzer.cpp
  40. parsenetaddress_fuzzer.dict
  41. parsenetaddress_test.cpp
  42. posix_strerror_r.cpp
  43. PREUPLOAD.cfg
  44. process.cpp
  45. process_test.cpp
  46. properties.cpp
  47. properties_test.cpp
  48. README.md
  49. result_test.cpp
  50. result_test_constraint.cpp
  51. scopeguard_test.cpp
  52. stringprintf.cpp
  53. stringprintf_test.cpp
  54. strings.cpp
  55. strings_test.cpp
  56. test_main.cpp
  57. TEST_MAPPING
  58. test_utils.cpp
  59. test_utils_test.cpp
  60. threads.cpp
  61. utf8.cpp
  62. utf8_test.cpp
README.md

libbase

Who is this library for?

This library is a collection of convenience functions to make common tasks easier and less error-prone.

In this context, “error-prone” covers both “hard to do correctly” and “hard to do with good performance”, but as a general purpose library, libbase's primary focus is on making it easier to do things easily and correctly when a compromise has to be made between “simplest API” on the one hand and “fastest implementation” on the other. Though obviously the ideal is to have both.

Should my routine be added?

The intention is to cover the 80% use cases, not be all things to all users.

If you have a routine that‘s really useful in your project, congratulations. But that doesn’t mean it should be here rather than just in your project.

The question for libbase is “should everyone be doing this?”/“does this make everyone's code cleaner/safer?”. Historically we've considered the bar for inclusion to be “are there at least three unrelated projects that would be cleaned up by doing so”.

If your routine is actually something from a future C++ standard (that isn‘t yet in libc++), or it’s widely used in another library, that helps show that there's precedent. Being able to say “so-and-so has used this API for n years” is a good way to reduce concerns about API choices.

Any other restrictions?

Unlike most Android code, code in libbase has to build for Mac and Windows too.

Code here is also expected to have good test coverage.

By its nature, it‘s difficult to change libbase API. It’s often best to start using your routine just in your project, and let it “graduate” after you're certain that the API is solid.