External references for the object API thru a resolver function.

This allows hashed string fields to be used for lookup of any
C++ objects, a pointer to which are then stored in the object
besides the original hash for easy access.

Change-Id: I2247a13c349b905f1c54660becde2c818ad23e97
Tested: on Linux.
Bug: 30204449

docs/source/CppUsage.md

@@ -106,6 +106,25 @@ To use:
     CreateMonster(fbb, monsterobj.get());     // Serialize into new buffer.
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+# External references.
+
+An additional feature of the object API is the ability to allow you to load
+multiple independent FlatBuffers, and have them refer to eachothers objects
+using hashes which are then represented as typed pointers in the object API.
+
+To make this work have a field in the objects you want to referred to which is
+using the string hashing feature (see `hash` attribute in the
+[schema](@ref flatbuffers_guide_writing_schema) documentation). Then you have
+a similar hash in the field referring to it, along with a `cpp_type`
+attribute specifying the C++ type this will refer to (this can be any C++
+type, and will get a `*` added).
+
+Then, in JSON or however you create these buffers, make sure they use the
+same string (or hash).
+
+When you call `UnPack` (or `Create`), you'll need a function that maps from
+hash to the object (see `resolver_function_t` for details).
+
 ## Reflection (& Resizing)
 
 There is experimental support for reflection in FlatBuffers, allowing you to

docs/source/Schemas.md

@@ -304,6 +304,10 @@ Current understood attributes:
 -   `key` (on a field): this field is meant to be used as a key when sorting
     a vector of the type of table it sits in. Can be used for in-place
     binary search.
+-   `hash` (on a field). This is an (un)signed 32/64 bit integer field, whose
+    value during JSON parsing is allowed to be a string, which will then be
+    stored as its hash. The value of attribute is the hashing algorithm to
+    use, one of `fnv1_32` `fnv1_64` `fnv1a_32` `fnv1a_64`.
 
 ## JSON Parsing