C++ port: add C++ equivalent of FSTDocumentKey. (#762)

Also move kDocumentKeyPath to the only point of usage - make it a static
member variable of FieldPath.

1 files changed, 3 insertions, 0 deletions

diff --git a/Firestore/core/src/firebase/firestore/model/field_path.h b/Firestore/core/src/firebase/firestore/model/field_path.h
index ba6e2bd..fdf4918 100644
--- a/Firestore/core/src/firebase/firestore/model/field_path.h
+++ b/Firestore/core/src/firebase/firestore/model/field_path.h
@@ -35,6 +35,9 @@ namespace model {
  */
 class FieldPath : public impl::BasePath<FieldPath> {
  public:
+  /** The field path string that represents the document's key. */
+  static constexpr const char* kDocumentKeyPath = "__name__";
+
   // Note: Xcode 8.2 requires explicit specification of the constructor.
   FieldPath() : impl::BasePath<FieldPath>() {
   }