Mutable FlatBuffers: in-place updates.

This commit contains the first step in providing mutable FlatBuffers,
non-const accessors and mutation functions for existing fields generated
from --gen-mutable.

Change-Id: Iebee3975f05c1001f8e22824725edeaa6d85fbee
Tested: on Linux.
Bug: 15777024

include/flatbuffers/flatbuffers.h

@@ -289,11 +289,21 @@ public:
   iterator end() { return iterator(Data(), length_); }
   const_iterator end() const { return const_iterator(Data(), length_); }
 
+  // Change elements if you have a non-const pointer to this object.
+  void Mutate(uoffset_t i, T val) {
+    assert(i < size());
+    WriteScalar(Data() + i * sizeof(T), val);
+  }
+
   // The raw data in little endian format. Use with care.
   const uint8_t *Data() const {
     return reinterpret_cast<const uint8_t *>(&length_ + 1);
   }
 
+  uint8_t *Data() {
+    return reinterpret_cast<uint8_t *>(&length_ + 1);
+  }
+
   template<typename K> return_type LookupByKey(K key) const {
     auto span = size();
     uoffset_t start = 0;
@@ -806,11 +816,15 @@ class FlatBufferBuilder FLATBUFFERS_FINAL_CLASS {
   bool force_defaults_;  // Serialize values equal to their defaults anyway.
 };
 
-// Helper to get a typed pointer to the root object contained in the buffer.
-template<typename T> const T *GetRoot(const void *buf) {
+// Helpers to get a typed pointer to the root object contained in the buffer.
+template<typename T> T *GetMutableRoot(void *buf) {
   EndianCheck();
-  return reinterpret_cast<const T *>(reinterpret_cast<const uint8_t *>(buf) +
-    EndianScalar(*reinterpret_cast<const uoffset_t *>(buf)));
+  return reinterpret_cast<T *>(reinterpret_cast<uint8_t *>(buf) +
+    EndianScalar(*reinterpret_cast<uoffset_t *>(buf)));
+}
+
+template<typename T> const T *GetRoot(const void *buf) {
+  return GetMutableRoot<T>(const_cast<void *>(buf));
 }
 
 // Helper to see if the identifier in a buffer has the expected value.
@@ -978,7 +992,7 @@ class Table {
     return field_offset ? ReadScalar<T>(data_ + field_offset) : defaultval;
   }
 
-  template<typename P> P GetPointer(voffset_t field) const {
+  template<typename P> P GetPointer(voffset_t field) {
     auto field_offset = GetOptionalFieldOffset(field);
     auto p = data_ + field_offset;
     return field_offset
@@ -986,18 +1000,21 @@ class Table {
       : nullptr;
   }
 
-  template<typename P> P GetStruct(voffset_t field) const {
-    auto field_offset = GetOptionalFieldOffset(field);
-    return field_offset ? reinterpret_cast<P>(data_ + field_offset) : nullptr;
+  template<typename P> P GetPointer(voffset_t field) const {
+    return const_cast<Table *>(this)->GetPointer<P>(field);
   }
 
-  template<typename T> void SetField(voffset_t field, T val) {
+  template<typename P> P GetStruct(voffset_t field) const {
     auto field_offset = GetOptionalFieldOffset(field);
-    // If this asserts, you're trying to set a field that's not there
-    // (or should we return a bool instead?).
-    // check if it exists first using CheckField()
-    assert(field_offset);
+    auto p = const_cast<u_int8_t *>(data_ + field_offset);
+    return field_offset ? reinterpret_cast<P>(p) : nullptr;
+  }
+
+  template<typename T> bool SetField(voffset_t field, T val) {
+    auto field_offset = GetOptionalFieldOffset(field);
+    if (!field_offset) return false;
     WriteScalar(data_ + field_offset, val);
+    return true;
   }
 
   bool CheckField(voffset_t field) const {

include/flatbuffers/idl.h

@@ -395,6 +395,7 @@ struct GeneratorOptions {
   bool output_enum_identifiers;
   bool prefixed_enums;
   bool include_dependence_headers;
+  bool mutable_buffer;
 
   // Possible options for the more general generator below.
   enum Language { kJava, kCSharp, kGo, kMAX };
@@ -404,6 +405,7 @@ struct GeneratorOptions {
   GeneratorOptions() : strict_json(false), indent_step(2),
                        output_enum_identifiers(true), prefixed_enums(true),
                        include_dependence_headers(false),
+                       mutable_buffer(false),
                        lang(GeneratorOptions::kJava) {}
 };