allow easy default to absl::Hash framework

README.md

@@ -96,8 +96,8 @@ The full types with template parameters can be found in the [parallel_hashmap/ph
 
 ## Changes to Abseil's hashmaps
 
-- The default hash framework is std::hash, not absl::Hash
-- The `erase(iterator)` and `erase(const_iterator)` both return an iterator to the element following the removed element, as the do the std::unordered_map. A non-standard `void _erase(iterator)` is provided in case the return value is not needed.
+- The default hash framework is std::hash, not absl::Hash. However, if you prefer the default to be the Abseil hash framework, include the Abseil headers before `phmap.h` and define the preprocessor macro `PHMAP_USE_ABSL_HASHEQ`.
+- The `erase(iterator)` and `erase(const_iterator)` both return an iterator to the element following the removed element, as does the std::unordered_map. A non-standard `void _erase(iterator)` is provided in case the return value is not needed.
 - No new types, such as `absl::string_view`, are provided. `std::string_view` is supported by phmap tables when built with a stdlib providing this type.
 
 

parallel_hashmap/phmap_fwd_decl.h

@@ -24,11 +24,15 @@ namespace phmap {
         template <class T, class E = void>
         struct HashEq 
         {
+#ifdef PHMAP_USE_ABSL_HASHEQ
+            using Hash = absl::Hash<T>;
+            using Eq   = absl::EqualTo<T>;
+#else
             using Hash = phmap::Hash<T>;
             using Eq   = phmap::EqualTo<T>;
+#endif
         };
 
-
         template <class T>
         using hash_default_hash = typename container_internal::HashEq<T>::Hash;
 
@@ -36,10 +40,12 @@ namespace phmap {
         using hash_default_eq = typename container_internal::HashEq<T>::Eq;
 
         // type alias for std::allocator so we can forward declare without including other headers
-        template <class T>  using Allocator = typename phmap::Allocator<T>;
+        template <class T>  
+        using Allocator = typename phmap::Allocator<T>;
 
         // type alias for std::pair so we can forward declare without including other headers
-        template<class T1, class T2> using Pair = typename phmap::Pair<T1, T2>;
+        template<class T1, class T2> 
+        using Pair = typename phmap::Pair<T1, T2>;
 
     }  // namespace container_internal