diff options
Diffstat (limited to 'absl/container/flat_hash_map.h')
| -rw-r--r-- | absl/container/flat_hash_map.h | 15 |
1 files changed, 14 insertions, 1 deletions
diff --git a/absl/container/flat_hash_map.h b/absl/container/flat_hash_map.h index 2a1ad0fb..a33c794f 100644 --- a/absl/container/flat_hash_map.h +++ b/absl/container/flat_hash_map.h @@ -62,7 +62,7 @@ struct FlatHashMapPolicy; // * Requires values that are MoveConstructible // * Supports heterogeneous lookup, through `find()`, `operator[]()` and // `insert()`, provided that the map is provided a compatible heterogeneous -// hashing function and equality operator. +// hashing function and equality operator. See below for details. // * Invalidates any references and pointers to elements within the table after // `rehash()` and when the table is moved. // * Contains a `capacity()` member function indicating the number of element @@ -80,6 +80,19 @@ struct FlatHashMapPolicy; // libraries (e.g. .dll, .so) is unsupported due to way `absl::Hash` values may // be randomized across dynamically loaded libraries. // +// To achieve heterogeneous lookup for custom types either `Hash` and `Eq` type +// parameters can be used or `T` should have public inner types +// `absl_container_hash` and (optionally) `absl_container_eq`. In either case, +// `typename Hash::is_transparent` and `typename Eq::is_transparent` should be +// well-formed. Both types are basically functors: +// * `Hash` should support `size_t operator()(U val) const` that returns a hash +// for the given `val`. +// * `Eq` should support `bool operator()(U lhs, V rhs) const` that returns true +// if `lhs` is equal to `rhs`. +// +// In most cases `T` needs only to provide the `absl_container_hash`. In this +// case `std::equal_to<void>` will be used instead of `eq` part. +// // NOTE: A `flat_hash_map` stores its value types directly inside its // implementation array to avoid memory indirection. Because a `flat_hash_map` // is designed to move data when rehashed, map values will not retain pointer |