diff --git a/lib/cpp/src/TReflectionLocal.h b/lib/cpp/src/TReflectionLocal.h
index df6e706..3fe7c63 100644
--- a/lib/cpp/src/TReflectionLocal.h
+++ b/lib/cpp/src/TReflectionLocal.h
@@ -8,18 +8,25 @@
 #define _THRIFT_TREFLECTIONLOCAL_H_ 1
 
 #include <stdint.h>
+#include <cstring>
 #include <protocol/TProtocol.h>
 
+/**
+ * Local Reflection is a blanket term referring to the the structure
+ * and generation of this particular representation of Thrift types.
+ * (It is called local because it cannot be serialized by Thrift).
+ *
+ * @author David Reiss <dreiss@facebook.com>
+ */
+
 namespace facebook { namespace thrift { namespace reflection { namespace local {
 
 using facebook::thrift::protocol::TType;
 
-/**
- * A local reflection is a representation of a Thrift structure.
- * (It is called local because it cannot be serialized by Thrift).
- *
- * @author David Reiss <dreiss@facebook.com>
- */
+// We include this many bytes of the structure's fingerprint when serializing
+// a top-level structure.  Long enough to make collisions unlikely, short
+// enough to not significantly affect the amount of memory used.
+const int FP_PREFIX_LEN = 4;
 
 struct FieldMeta {
   int16_t tag;
@@ -27,16 +34,15 @@
 };
 
 struct TypeSpec {
+  TType ttype;
+  uint8_t    fp_prefix[FP_PREFIX_LEN];
+
   // Use an anonymous union here so we can fit two TypeSpecs in one cache line.
   union {
     struct {
       // Use parallel arrays here for denser packing (of the arrays).
       FieldMeta* metas;
       TypeSpec** specs;
-      // n_fields is only used for debugging, but it should only add
-      // a minimimal amount to the effective size of this structure
-      // because of alignment restrictions.
-      int        n_fields;
     } tstruct;
     struct {
       TypeSpec *subtype1;
@@ -44,21 +50,21 @@
     } tcontainer;
   };
 
-  // Put this at the end so the 32-bit enum can be crammed up next to the
-  // 32-bit int (n_fields).
-  TType ttype;
-
-
   // Static initialization of unions isn't really possible,
   // so take the plunge and use constructors.
   // Hopefully they'll be evaluated at compile time.
 
-  TypeSpec(TType ttype) : ttype(ttype) {}
+  TypeSpec(TType ttype) : ttype(ttype) {
+    std::memset(fp_prefix, 0, FP_PREFIX_LEN);
+  }
 
-  TypeSpec(TType ttype, int n_fields, FieldMeta* metas, TypeSpec** specs) :
+  TypeSpec(TType ttype,
+           uint8_t* fingerprint,
+           FieldMeta* metas,
+           TypeSpec** specs) :
     ttype(ttype)
   {
-    tstruct.n_fields = n_fields;
+    std::memcpy(fp_prefix, fingerprint, FP_PREFIX_LEN);
     tstruct.metas = metas;
     tstruct.specs = specs;
   }
@@ -66,6 +72,7 @@
   TypeSpec(TType ttype, TypeSpec* subtype1, TypeSpec* subtype2) :
     ttype(ttype)
   {
+    std::memset(fp_prefix, 0, FP_PREFIX_LEN);
     tcontainer.subtype1 = subtype1;
     tcontainer.subtype2 = subtype2;
   }