diff options
| author | Abseil Team <absl-team@google.com> | 2019-01-09T17·55-0800 |
|---|---|---|
| committer | Shaindel Schwartz <shaindel@google.com> | 2019-01-09T18·09-0500 |
| commit | 018b4db1d73ec8238e6dc4b17fd9e1fd7468d0ed (patch) | |
| tree | 061373b7238a57c6765f147bbb96cefa5c83230b /absl/container/internal/container_memory.h | |
| parent | 9449ae94397f2fd683851348e25ed8c93f75b3b9 (diff) | |
Export of internal Abseil changes.
-- fd86c60bac6c41f1629ce1ab7dc1c8edff398a59 by Alex Strelnikov <strel@google.com>: Import PR: https://github.com/abseil/abseil-cpp/pull/243 Fix Windows ARM64 intrinsic use. PiperOrigin-RevId: 228535649 -- a0ca663f606a3b31493683e405be2b1cff450894 by CJ Johnson <johnsoncj@google.com>: Fixes issue of mixed signedness comparison PiperOrigin-RevId: 228535623 -- d71aaa1705d7303b43fe02088fe07b153e647796 by Shaindel Schwartz <shaindel@google.com>: Import of CCTZ from GitHub. PiperOrigin-RevId: 228534365 -- c1b49d361aa880198e071f93997724bddbcd4760 by Samuel Benzaquen <sbenza@google.com>: Internal cleanup PiperOrigin-RevId: 228406627 -- 0c4b1c2bed107698e209055b3431771d7a1bdba1 by Dave Walker <dawalker@google.com>: Add comments about the purpose of container_internal::slot_type. PiperOrigin-RevId: 228264537 -- 060aa6077d2f3a0a129149e0644d19f2f521b241 by Abseil Team <absl-team@google.com>: #include <cmath> in hashtablez_sampler.cc Expected to fix the android build. PiperOrigin-RevId: 228222550 GitOrigin-RevId: fd86c60bac6c41f1629ce1ab7dc1c8edff398a59 Change-Id: I26339fd4548c1a81b037cb52c26910d1bd850ea8
Diffstat (limited to 'absl/container/internal/container_memory.h')
| -rw-r--r-- | absl/container/internal/container_memory.h | 27 |
1 files changed, 23 insertions, 4 deletions
diff --git a/absl/container/internal/container_memory.h b/absl/container/internal/container_memory.h index 56c5d2df6731..35b691ce5af3 100644 --- a/absl/container/internal/container_memory.h +++ b/absl/container/internal/container_memory.h @@ -286,11 +286,30 @@ struct IsLayoutCompatible { } // namespace memory_internal -// If kMutableKeys is false, only the value member is accessed. +// The internal storage type for key-value containers like flat_hash_map. // -// If kMutableKeys is true, key is accessed through all slots while value and -// mutable_value are accessed only via INITIALIZED slots. Slots are created and -// destroyed via mutable_value so that the key can be moved later. +// It is convenient for the value_type of a flat_hash_map<K, V> to be +// pair<const K, V>; the "const K" prevents accidental modification of the key +// when dealing with the reference returned from find() and similar methods. +// However, this creates other problems; we want to be able to emplace(K, V) +// efficiently with move operations, and similarly be able to move a +// pair<K, V> in insert(). +// +// The solution is this union, which aliases the const and non-const versions +// of the pair. This also allows flat_hash_map<const K, V> to work, even though +// that has the same efficiency issues with move in emplace() and insert() - +// but people do it anyway. +// +// If kMutableKeys is false, only the value member can be accessed. +// +// If kMutableKeys is true, key can be accessed through all slots while value +// and mutable_value must be accessed only via INITIALIZED slots. Slots are +// created and destroyed via mutable_value so that the key can be moved later. +// +// Accessing one of the union fields while the other is active is safe as +// long as they are layout-compatible, which is guaranteed by the definition of +// kMutableKeys. For C++11, the relevant section of the standard is +// https://timsong-cpp.github.io/cppwp/n3337/class.mem#19 (9.2.19) template <class K, class V> union slot_type { private: |