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

tests/test.cpp

@@ -127,7 +127,7 @@ flatbuffers::unique_ptr_t CreateFlatBufferTest(std::string &buffer) {
 }
 
 //  example of accessing a buffer loaded in memory:
-void AccessFlatBufferTest(const uint8_t *flatbuf, const std::size_t length) {
+void AccessFlatBufferTest(const uint8_t *flatbuf, size_t length) {
 
   // First, verify the buffers integrity (optional)
   flatbuffers::Verifier verifier(flatbuf, length);
@@ -159,6 +159,8 @@ void AccessFlatBufferTest(const uint8_t *flatbuf, const std::size_t length) {
   for (auto it = inventory->begin(); it != inventory->end(); ++it)
     TEST_EQ(*it, inv_data[it - inventory->begin()]);
 
+  TEST_EQ(monster->color(), Color_Blue);
+
   // Example of accessing a union:
   TEST_EQ(monster->test_type(), Any_Monster);  // First make sure which it is.
   auto monster2 = reinterpret_cast<const Monster *>(monster->test());
@@ -200,7 +202,39 @@ void AccessFlatBufferTest(const uint8_t *flatbuf, const std::size_t length) {
   for (auto it = tests->begin(); it != tests->end(); ++it) {
     TEST_EQ(it->a() == 10 || it->a() == 30, true);  // Just testing iterators.
   }
+}
 
+// Change a FlatBuffer in-place, after it has been constructed.
+void MutateFlatBuffersTest(uint8_t *flatbuf, std::size_t length) {
+  // Get non-const pointer to root.
+  auto monster = GetMutableMonster(flatbuf);
+
+  // Each of these tests mutates, then tests, then set back to the original,
+  // so we can test that the buffer in the end still passes our original test.
+  auto hp_ok = monster->mutate_hp(10);
+  TEST_EQ(hp_ok, true);  // Field was present.
+  TEST_EQ(monster->hp(), 10);
+  monster->mutate_hp(80);
+
+  auto mana_ok = monster->mutate_mana(10);
+  TEST_EQ(mana_ok, false);  // Field was NOT present, because default value.
+
+  // Mutate structs.
+  auto pos = monster->mutable_pos();
+  auto test3 = pos->mutable_test3();  // Struct inside a struct.
+  test3.mutate_a(50);                 // Struct fields never fail.
+  TEST_EQ(test3.a(), 50);
+  test3.mutate_a(10);
+
+  // Mutate vectors.
+  auto inventory = monster->mutable_inventory();
+  inventory->Mutate(9, 100);
+  TEST_EQ(inventory->Get(9), 100);
+  inventory->Mutate(9, 9);
+
+  // Run the verifier and the regular test to make sure we didn't trample on
+  // anything.
+  AccessFlatBufferTest(flatbuf, length);
 }
 
 // example of parsing text straight into a buffer, and generating
@@ -626,6 +660,8 @@ int main(int /*argc*/, const char * /*argv*/[]) {
                        rawbuf.length());
   AccessFlatBufferTest(flatbuf.get(), rawbuf.length());
 
+  MutateFlatBuffersTest(flatbuf.get(), rawbuf.length());
+
   #ifndef __ANDROID__  // requires file access
   ParseAndGenerateTextTest();
   ParseProtoTest();