GH-36214: [C++] Specify FieldPath::Hash as template parameter where…

… possible (#36222)

### Rationale for this change

Internal specializations of `std::hash<FieldPath>` have caused multiple-definition errors with unity builds.

### What changes are included in this PR?

To avoid exposing a global specialization of `std::hash` (which must be declared in the `std` namespace), this specifies the `FieldPath::Hash` helper as a template parameter to hashable containers instead.

### Are these changes tested?

Yes (covered by existing tests)

### Are there any user-facing changes?

No

* Closes: #36214

Authored-by: benibus <bpharks@gmx.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>

cpp/src/arrow/compute/kernels/vector_sort.cc

@@ -20,9 +20,6 @@
 #include "arrow/compute/kernels/vector_sort_internal.h"
 #include "arrow/compute/registry.h"
 
-template <>
-struct std::hash<arrow::FieldPath> : public arrow::FieldPath::Hash {};
-
 namespace arrow {
 
 using internal::checked_cast;
@@ -1095,7 +1092,7 @@ struct SortFieldPopulator {
   }
 
   std::vector<SortField> sort_fields_;
-  std::unordered_set<FieldPath> seen_;
+  std::unordered_set<FieldPath, FieldPath::Hash> seen_;
   std::vector<int> tmp_indices_;
 };
 

cpp/src/arrow/ipc/dictionary.cc

@@ -35,13 +35,6 @@
 #include "arrow/util/checked_cast.h"
 #include "arrow/util/logging.h"
 
-namespace std {
-template <>
-struct hash<arrow::FieldPath> {
-  size_t operator()(const arrow::FieldPath& path) const { return path.hash(); }
-};
-}  // namespace std
-
 namespace arrow {
 
 using internal::checked_cast;
@@ -54,7 +47,7 @@ using internal::FieldPosition;
 // DictionaryFieldMapper implementation
 
 struct DictionaryFieldMapper::Impl {
-  using FieldPathMap = std::unordered_map<FieldPath, int64_t>;
+  using FieldPathMap = std::unordered_map<FieldPath, int64_t, FieldPath::Hash>;
 
   FieldPathMap field_path_to_id;