[C++] Add GetMutableSizePrefixedRoot and generate GetMutableSizePrefixedXxx functions (#6815)

* flattests_cpp17 doesn't compile with Visual Studio 2017: warning C4100: 'indent': unreferenced formal parameter

stringify_util.h(127): error C2220: warning treated as error - no 'object' file generated
stringify_util.h(127): warning C4100: 'indent': unreferenced formal parameter
stringify_util.h(85): warning C4100: 'indent': unreferenced formal parameter
stringify_util.h(85): warning C4100: 'fbs': unreferenced formal parameter

* [C++] Add GetMutableSizePrefixedRoot() and generate a GetMutableSizePrefixed function
When using the mutable API together with size prefixed buffers these functions should be present.

* clang-format

* Cleanup branch for PR
Revert "flattests_cpp17 doesn't compile with Visual Studio 2017: warning C4100: 'indent': unreferenced formal parameter"

This reverts commit a92055203e.

include/flatbuffers/flatbuffers.h

@@ -2327,6 +2327,11 @@ template<typename T> T *GetMutableRoot(void *buf) {
       EndianScalar(*reinterpret_cast<uoffset_t *>(buf)));
 }
 
+template<typename T> T *GetMutableSizePrefixedRoot(void *buf) {
+  return GetMutableRoot<T>(reinterpret_cast<uint8_t *>(buf) +
+                           sizeof(uoffset_t));
+}
+
 template<typename T> const T *GetRoot(const void *buf) {
   return GetMutableRoot<T>(const_cast<void *>(buf));
 }

samples/monster_generated.h

@@ -834,6 +834,10 @@ inline Monster *GetMutableMonster(void *buf) {
   return flatbuffers::GetMutableRoot<Monster>(buf);
 }
 
+inline MyGame::Sample::Monster *GetMutableSizePrefixedMonster(void *buf) {
+  return flatbuffers::GetMutableSizePrefixedRoot<MyGame::Sample::Monster>(buf);
+}
+
 inline bool VerifyMonsterBuffer(
     flatbuffers::Verifier &verifier) {
   return verifier.VerifyBuffer<MyGame::Sample::Monster>(nullptr);

src/idl_gen_cpp.cpp

@@ -505,6 +505,17 @@ class CppGenerator : public BaseGenerator {
         code_ += "  return flatbuffers::GetMutableRoot<{{STRUCT_NAME}}>(buf);";
         code_ += "}";
         code_ += "";
+
+        code_ += "inline \\";
+        code_ +=
+            "{{CPP_NAME}} "
+            "*{{NULLABLE_EXT}}GetMutableSizePrefixed{{STRUCT_NAME}}(void "
+            "*buf) {";
+        code_ +=
+            "  return "
+            "flatbuffers::GetMutableSizePrefixedRoot<{{CPP_NAME}}>(buf);";
+        code_ += "}";
+        code_ += "";
       }
 
       if (parser_.file_identifier_.length()) {

tests/arrays_test_generated.h

@@ -457,6 +457,10 @@ inline ArrayTable *GetMutableArrayTable(void *buf) {
   return flatbuffers::GetMutableRoot<ArrayTable>(buf);
 }
 
+inline MyGame::Example::ArrayTable *GetMutableSizePrefixedArrayTable(void *buf) {
+  return flatbuffers::GetMutableSizePrefixedRoot<MyGame::Example::ArrayTable>(buf);
+}
+
 inline const char *ArrayTableIdentifier() {
   return "ARRT";
 }

tests/cpp17/generated_cpp17/monster_test_generated.h

@@ -3677,6 +3677,10 @@ inline Monster *GetMutableMonster(void *buf) {
   return flatbuffers::GetMutableRoot<Monster>(buf);
 }
 
+inline MyGame::Example::Monster *GetMutableSizePrefixedMonster(void *buf) {
+  return flatbuffers::GetMutableSizePrefixedRoot<MyGame::Example::Monster>(buf);
+}
+
 inline const char *MonsterIdentifier() {
   return "MONS";
 }

tests/cpp17/generated_cpp17/optional_scalars_generated.h

@@ -933,6 +933,10 @@ inline ScalarStuff *GetMutableScalarStuff(void *buf) {
   return flatbuffers::GetMutableRoot<ScalarStuff>(buf);
 }
 
+inline optional_scalars::ScalarStuff *GetMutableSizePrefixedScalarStuff(void *buf) {
+  return flatbuffers::GetMutableSizePrefixedRoot<optional_scalars::ScalarStuff>(buf);
+}
+
 inline const char *ScalarStuffIdentifier() {
   return "NULL";
 }

tests/cpp17/generated_cpp17/union_vector_generated.h

@@ -798,6 +798,10 @@ inline Movie *GetMutableMovie(void *buf) {
   return flatbuffers::GetMutableRoot<Movie>(buf);
 }
 
+inline Movie *GetMutableSizePrefixedMovie(void *buf) {
+  return flatbuffers::GetMutableSizePrefixedRoot<Movie>(buf);
+}
+
 inline const char *MovieIdentifier() {
   return "MOVI";
 }

tests/monster_extra_generated.h

@@ -349,6 +349,10 @@ inline MonsterExtra *GetMutableMonsterExtra(void *buf) {
   return flatbuffers::GetMutableRoot<MonsterExtra>(buf);
 }
 
+inline MyGame::MonsterExtra *GetMutableSizePrefixedMonsterExtra(void *buf) {
+  return flatbuffers::GetMutableSizePrefixedRoot<MyGame::MonsterExtra>(buf);
+}
+
 inline const char *MonsterExtraIdentifier() {
   return "MONE";
 }

tests/monster_test_generated.h

@@ -3648,6 +3648,10 @@ inline Monster *GetMutableMonster(void *buf) {
   return flatbuffers::GetMutableRoot<Monster>(buf);
 }
 
+inline MyGame::Example::Monster *GetMutableSizePrefixedMonster(void *buf) {
+  return flatbuffers::GetMutableSizePrefixedRoot<MyGame::Example::Monster>(buf);
+}
+
 inline const char *MonsterIdentifier() {
   return "MONS";
 }

tests/native_type_test_generated.h

@@ -288,6 +288,10 @@ inline ApplicationData *GetMutableApplicationData(void *buf) {
   return flatbuffers::GetMutableRoot<ApplicationData>(buf);
 }
 
+inline Geometry::ApplicationData *GetMutableSizePrefixedApplicationData(void *buf) {
+  return flatbuffers::GetMutableSizePrefixedRoot<Geometry::ApplicationData>(buf);
+}
+
 inline bool VerifyApplicationDataBuffer(
     flatbuffers::Verifier &verifier) {
   return verifier.VerifyBuffer<Geometry::ApplicationData>(nullptr);

tests/optional_scalars_generated.h

@@ -893,6 +893,10 @@ inline ScalarStuff *GetMutableScalarStuff(void *buf) {
   return flatbuffers::GetMutableRoot<ScalarStuff>(buf);
 }
 
+inline optional_scalars::ScalarStuff *GetMutableSizePrefixedScalarStuff(void *buf) {
+  return flatbuffers::GetMutableSizePrefixedRoot<optional_scalars::ScalarStuff>(buf);
+}
+
 inline const char *ScalarStuffIdentifier() {
   return "NULL";
 }

tests/union_vector/union_vector_generated.h

@@ -829,6 +829,10 @@ inline Movie *GetMutableMovie(void *buf) {
   return flatbuffers::GetMutableRoot<Movie>(buf);
 }
 
+inline Movie *GetMutableSizePrefixedMovie(void *buf) {
+  return flatbuffers::GetMutableSizePrefixedRoot<Movie>(buf);
+}
+
 inline const char *MovieIdentifier() {
   return "MOVI";
 }