size_type now specified by macro CEREAL_SIZE_TYPE

resolves USCiLab#379

include/cereal/details/helpers.hpp

@@ -55,8 +55,10 @@ namespace cereal
   //! The size type used by cereal
   /*! To ensure compatability between 32, 64, etc bit machines, we need to use
       a fixed size type instead of size_t, which may vary from machine to
-      machine. */
-  using size_type = uint64_t;
+      machine.
+
+      The default value for CEREAL_SIZE_TYPE is specified in cereal/macros.hpp */
+  using size_type = CEREAL_SIZE_TYPE;
 
   // forward decls
   class BinaryOutputArchive;

include/cereal/macros.hpp

@@ -56,6 +56,20 @@
 #define CEREAL_THREAD_SAFE 0
 #endif // CEREAL_THREAD_SAFE
 
+#ifndef CEREAL_SIZE_TYPE
+//! Determines the data type used for size_type
+/*! cereal uses size_type to ensure that the serialized size of
+    dynamic containers is compatible across different architectures
+    (e.g. 32 vs 64 bit), which may use different underlying types for
+    std::size_t.
+
+    More information can be found in cereal/details/helpers.hpp.
+
+    If you choose to modify this type, ensure that you use a fixed
+    size type (e.g. uint32_t). */
+#define CEREAL_SIZE_TYPE uint64_t
+#endif // CEREAL_SIZE_TYPE
+
 // ######################################################################
 #ifndef CEREAL_SERIALIZE_FUNCTION_NAME
 //! The serialization/deserialization function name to search for.