Added an IsFieldPresent helper function.

This is implemented as a template function, since Table::CheckField
is not reachable by subclasses of Table (private base class).

Change-Id: I1ed4d47ce7cb672460ccab61cf7442eb9136b0f1
Tested: on Linux.
Bug: 26273432

include/flatbuffers/flatbuffers.h

@@ -1187,6 +1187,18 @@ class Table {
   uint8_t data_[1];
 };
 
+// Helper function to test if a field is present, using any of the field
+// enums in the generated code.
+// `table` must be a generated table type. Since this is a template parameter,
+// this is not typechecked to be a subclass of Table, so beware!
+// Note: this function will return false for fields equal to the default
+// value, since they're not stored in the buffer (unless force_defaults was
+// used).
+template<typename T> bool IsFieldPresent(const T *table, voffset_t field) {
+  // Cast, since Table is a private baseclass of any table types.
+  return reinterpret_cast<const Table *>(table)->CheckField(field);
+}
+
 // Utility function for reverse lookups on the EnumNames*() functions
 // (in the generated C++ code)
 // names must be NULL terminated.

tests/test.cpp

@@ -221,6 +221,10 @@ void AccessFlatBufferTest(const uint8_t *flatbuf, size_t length) {
   for (auto it = tests->begin(); it != tests->end(); ++it) {
     TEST_EQ(it->a() == 10 || it->a() == 30, true);  // Just testing iterators.
   }
+
+  // Checking for presence of fields:
+  TEST_EQ(flatbuffers::IsFieldPresent(monster, Monster::VT_HP), true);
+  TEST_EQ(flatbuffers::IsFieldPresent(monster, Monster::VT_MANA), false);
 }
 
 // Change a FlatBuffer in-place, after it has been constructed.