diff options
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 56c5d2df..35b691ce 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: |