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Add non scalar default support to BFBS files #8671

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1f62fcb
add non scalar default bfbs support
jtdavis777 Dec 6, 2025
25ea3ab
update generated code -- seems to be a bug in ts that I need to chase…
jtdavis777 Dec 6, 2025
bf03023
regen code
jtdavis777 Dec 6, 2025
6c971ad
Merge branch 'master' into feature/non_scalar_default_bfbs_support
jtdavis777 Dec 20, 2025
305a109
Merge branch 'master' into feature/non_scalar_default_bfbs_support
dbaileychess Dec 21, 2025
d28ff3e
Merge branch 'master' into feature/non_scalar_default_bfbs_support
jtdavis777 Dec 22, 2025
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3 changes: 3 additions & 0 deletions include/flatbuffers/idl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -227,9 +227,12 @@ struct Type {
struct Value {
Value()
: constant("0"),
has_non_scalar_constant(false),
offset(static_cast<voffset_t>(~(static_cast<voffset_t>(0U)))) {}
Type type;
std::string constant;
// not the most elegant solution, but much better than a full refactor.
bool has_non_scalar_constant;
voffset_t offset;
};

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22 changes: 18 additions & 4 deletions include/flatbuffers/reflection_generated.h
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Original file line number Diff line number Diff line change
Expand Up @@ -621,7 +621,8 @@ struct Field FLATBUFFERS_FINAL_CLASS : private ::flatbuffers::Table {
VT_DOCUMENTATION = 24,
VT_OPTIONAL = 26,
VT_PADDING = 28,
VT_OFFSET64 = 30
VT_OFFSET64 = 30,
VT_DEFAULT_NON_SCALAR = 32
};
const ::flatbuffers::String *name() const {
return GetPointer<const ::flatbuffers::String *>(VT_NAME);
Expand Down Expand Up @@ -679,6 +680,9 @@ struct Field FLATBUFFERS_FINAL_CLASS : private ::flatbuffers::Table {
bool offset64() const {
return GetField<uint8_t>(VT_OFFSET64, 0) != 0;
}
const ::flatbuffers::String *default_non_scalar() const {
return GetPointer<const ::flatbuffers::String *>(VT_DEFAULT_NON_SCALAR);
}
template <bool B = false>
bool Verify(::flatbuffers::VerifierTemplate<B> &verifier) const {
return VerifyTableStart(verifier) &&
Expand All @@ -702,6 +706,8 @@ struct Field FLATBUFFERS_FINAL_CLASS : private ::flatbuffers::Table {
VerifyField<uint8_t>(verifier, VT_OPTIONAL, 1) &&
VerifyField<uint16_t>(verifier, VT_PADDING, 2) &&
VerifyField<uint8_t>(verifier, VT_OFFSET64, 1) &&
VerifyOffset(verifier, VT_DEFAULT_NON_SCALAR) &&
verifier.VerifyString(default_non_scalar()) &&
verifier.EndTable();
}
};
Expand Down Expand Up @@ -752,6 +758,9 @@ struct FieldBuilder {
void add_offset64(bool offset64) {
fbb_.AddElement<uint8_t>(Field::VT_OFFSET64, static_cast<uint8_t>(offset64), 0);
}
void add_default_non_scalar(::flatbuffers::Offset<::flatbuffers::String> default_non_scalar) {
fbb_.AddOffset(Field::VT_DEFAULT_NON_SCALAR, default_non_scalar);
}
explicit FieldBuilder(::flatbuffers::FlatBufferBuilder &_fbb)
: fbb_(_fbb) {
start_ = fbb_.StartTable();
Expand Down Expand Up @@ -780,10 +789,12 @@ inline ::flatbuffers::Offset<Field> CreateField(
::flatbuffers::Offset<::flatbuffers::Vector<::flatbuffers::Offset<::flatbuffers::String>>> documentation = 0,
bool optional = false,
uint16_t padding = 0,
bool offset64 = false) {
bool offset64 = false,
::flatbuffers::Offset<::flatbuffers::String> default_non_scalar = 0) {
FieldBuilder builder_(_fbb);
builder_.add_default_real(default_real);
builder_.add_default_integer(default_integer);
builder_.add_default_non_scalar(default_non_scalar);
builder_.add_documentation(documentation);
builder_.add_attributes(attributes);
builder_.add_type(type);
Expand Down Expand Up @@ -814,10 +825,12 @@ inline ::flatbuffers::Offset<Field> CreateFieldDirect(
const std::vector<::flatbuffers::Offset<::flatbuffers::String>> *documentation = nullptr,
bool optional = false,
uint16_t padding = 0,
bool offset64 = false) {
bool offset64 = false,
const char *default_non_scalar = nullptr) {
auto name__ = name ? _fbb.CreateString(name) : 0;
auto attributes__ = attributes ? _fbb.CreateVectorOfSortedTables<reflection::KeyValue>(attributes) : 0;
auto documentation__ = documentation ? _fbb.CreateVector<::flatbuffers::Offset<::flatbuffers::String>>(*documentation) : 0;
auto default_non_scalar__ = default_non_scalar ? _fbb.CreateString(default_non_scalar) : 0;
return reflection::CreateField(
_fbb,
name__,
Expand All @@ -833,7 +846,8 @@ inline ::flatbuffers::Offset<Field> CreateFieldDirect(
documentation__,
optional,
padding,
offset64);
offset64,
default_non_scalar__);
}

struct Object FLATBUFFERS_FINAL_CLASS : private ::flatbuffers::Table {
Expand Down
12 changes: 9 additions & 3 deletions java/src/main/java/com/google/flatbuffers/reflection/Field.java
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Original file line number Diff line number Diff line change
Expand Up @@ -58,6 +58,9 @@ public final class Field extends com.google.flatbuffers.Table {
* If the field uses 64-bit offsets.
*/
public boolean offset64() { int o = __offset(30); return o != 0 ? 0!=bb.get(o + bb_pos) : false; }
public String defaultNonScalar() { int o = __offset(32); return o != 0 ? __string(o + bb_pos) : null; }
public ByteBuffer defaultNonScalarAsByteBuffer() { return __vector_as_bytebuffer(32, 1); }
public ByteBuffer defaultNonScalarInByteBuffer(ByteBuffer _bb) { return __vector_in_bytebuffer(_bb, 32, 1); }

public static int createField(FlatBufferBuilder builder,
int nameOffset,
Expand All @@ -73,10 +76,12 @@ public static int createField(FlatBufferBuilder builder,
int documentationOffset,
boolean optional,
int padding,
boolean offset64) {
builder.startTable(14);
boolean offset64,
int defaultNonScalarOffset) {
builder.startTable(15);
Field.addDefaultReal(builder, defaultReal);
Field.addDefaultInteger(builder, defaultInteger);
Field.addDefaultNonScalar(builder, defaultNonScalarOffset);
Field.addDocumentation(builder, documentationOffset);
Field.addAttributes(builder, attributesOffset);
Field.addType(builder, typeOffset);
Expand All @@ -92,7 +97,7 @@ public static int createField(FlatBufferBuilder builder,
return Field.endField(builder);
}

public static void startField(FlatBufferBuilder builder) { builder.startTable(14); }
public static void startField(FlatBufferBuilder builder) { builder.startTable(15); }
public static void addName(FlatBufferBuilder builder, int nameOffset) { builder.addOffset(nameOffset); builder.slot(0); }
public static void addType(FlatBufferBuilder builder, int typeOffset) { builder.addOffset(1, typeOffset, 0); }
public static void addId(FlatBufferBuilder builder, int id) { builder.addShort(2, (short) id, (short) 0); }
Expand All @@ -111,6 +116,7 @@ public static int createField(FlatBufferBuilder builder,
public static void addOptional(FlatBufferBuilder builder, boolean optional) { builder.addBoolean(11, optional, false); }
public static void addPadding(FlatBufferBuilder builder, int padding) { builder.addShort(12, (short) padding, (short) 0); }
public static void addOffset64(FlatBufferBuilder builder, boolean offset64) { builder.addBoolean(13, offset64, false); }
public static void addDefaultNonScalar(FlatBufferBuilder builder, int defaultNonScalarOffset) { builder.addOffset(14, defaultNonScalarOffset, 0); }
public static int endField(FlatBufferBuilder builder) {
int o = builder.endTable();
builder.required(o, 4); // name
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15 changes: 14 additions & 1 deletion python/flatbuffers/reflection/Field.py
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Expand Up @@ -163,8 +163,15 @@ def Offset64(self):
return bool(self._tab.Get(flatbuffers.number_types.BoolFlags, o + self._tab.Pos))
return False

# Field
def DefaultNonScalar(self):
o = flatbuffers.number_types.UOffsetTFlags.py_type(self._tab.Offset(32))
if o != 0:
return self._tab.String(o + self._tab.Pos)
return None

def FieldStart(builder):
builder.StartObject(14)
builder.StartObject(15)

def Start(builder):
FieldStart(builder)
Expand Down Expand Up @@ -265,6 +272,12 @@ def FieldAddOffset64(builder, offset64):
def AddOffset64(builder, offset64):
FieldAddOffset64(builder, offset64)

def FieldAddDefaultNonScalar(builder, defaultNonScalar):
builder.PrependUOffsetTRelativeSlot(14, flatbuffers.number_types.UOffsetTFlags.py_type(defaultNonScalar), 0)

def AddDefaultNonScalar(builder, defaultNonScalar):
FieldAddDefaultNonScalar(builder, defaultNonScalar)

def FieldEnd(builder):
return builder.EndObject()

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1 change: 1 addition & 0 deletions reflection/reflection.fbs
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Expand Up @@ -88,6 +88,7 @@ table Field {
padding:uint16 = 0;
/// If the field uses 64-bit offsets.
offset64:bool = false;
default_non_scalar:string;
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@dbaileychess dbaileychess Dec 21, 2025

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I feel like a better long term solution is to have a default_value field that is of type Union (long, string, short, etc..)

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@jtdavis777 jtdavis777 Dec 21, 2025

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Would you want me to deprecate the old constant field to do so?

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@jtdavis777 jtdavis777 Dec 21, 2025

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This would also have to be a union of struct of primitive, since unions of primitives aren't a thing

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@aardappel aardappel Dec 21, 2025

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Probably not great to remove the existing fields.. so we may as well continue with the current pattern?

Just not great to merge strings and vectors into 1 field. Better probably to have default:string (which at least in FlatBuffers can be detected as not-set vs empty) and default_vector:bool` ?

In terms of the variable constant in the parser.. I actually like your idea of making it contain the string including extra "": this would make a lot of sense as way to represent different types of values inside a string.

The long term solution would be to replace constant with a variant<int64_t,double,string,bool> or similar? But that will likely cause a LOT of code churn.

}

table Object { // Used for both tables and structs.
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6 changes: 6 additions & 0 deletions src/binary_annotator.cpp
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Expand Up @@ -428,6 +428,12 @@ BinaryAnnotator::VTable* BinaryAnnotator::GetOrBuildVTable(
? std::to_string(field->default_real())
: std::to_string(field->default_integer());
default_label += "> (";
} else if ((field->type()->base_type() == reflection::String ||
field->type()->base_type() == reflection::Vector) &&
field->default_non_scalar() != nullptr) {
default_label += "<defaults to \"";
default_label += field->default_non_scalar()->str();
default_label += "\"> (";
} else {
default_label += "<null> (";
}
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10 changes: 9 additions & 1 deletion src/idl_parser.cpp
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Expand Up @@ -1016,6 +1016,7 @@ CheckedError Parser::ParseField(StructDef& struct_def) {
"Default values for strings and vectors are not supported in one "
"of the specified programming languages");
}
field->value.has_non_scalar_constant = true;
}

if (IsVector(type) && field->value.constant != "0" &&
Expand Down Expand Up @@ -4136,6 +4137,8 @@ Offset<reflection::Field> FieldDef::Serialize(FlatBufferBuilder* builder,
auto docs__ = parser.opts.binary_schema_comments && !doc_comment.empty()
? builder->CreateVectorOfStrings(doc_comment)
: 0;
auto default_non_scalar__ =
value.has_non_scalar_constant ? builder->CreateString(value.constant) : 0;
double d;
StringToNumber(value.constant.c_str(), &d);
return reflection::CreateField(
Expand All @@ -4144,7 +4147,8 @@ Offset<reflection::Field> FieldDef::Serialize(FlatBufferBuilder* builder,
IsInteger(value.type.base_type) ? StringToInt(value.constant.c_str()) : 0,
// result may be platform-dependent if underlying is float (not double)
IsFloat(value.type.base_type) ? d : 0.0, deprecated, IsRequired(), key,
attr__, docs__, IsOptional(), static_cast<uint16_t>(padding), offset64);
attr__, docs__, IsOptional(), static_cast<uint16_t>(padding), offset64,
default_non_scalar__);
// TODO: value.constant is almost always "0", we could save quite a bit of
// space by sharing it. Same for common values of value.type.
}
Expand All @@ -4158,6 +4162,10 @@ bool FieldDef::Deserialize(Parser& parser, const reflection::Field* field) {
value.constant = NumToString(field->default_integer());
} else if (IsFloat(value.type.base_type)) {
value.constant = FloatToString(field->default_real(), 17);
} else if (IsString(value.type) || IsVector(value.type)) {
value.has_non_scalar_constant = field->default_non_scalar();
value.constant =
value.has_non_scalar_constant ? field->default_non_scalar()->str() : "";
}
presence = FieldDef::MakeFieldPresence(field->optional(), field->required());
padding = field->padding();
Expand Down
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