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FlatBuffers
An open source project by FPL.
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FlatBuffers
An open source project by FPL.
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The syntax of the schema language (aka IDL, Interface Definition Language) should look quite familiar to users of any of the C family of languages, and also to users of other IDLs. Let's look at an example first:
// example IDL file
namespace MyGame;
attribute "priority";
enum Color : byte { Red = 1, Green, Blue }
union Any { Monster, Weapon, Pickup }
struct Vec3 {
x:float;
y:float;
z:float;
}
table Monster {
pos:Vec3;
mana:short = 150;
hp:short = 100;
name:string;
friendly:bool = false (deprecated, priority: 1);
inventory:[ubyte];
color:Color = Blue;
test:Any;
}
root_type Monster;
(Weapon & Pickup not defined as part of this example).
Tables are the main way of defining objects in FlatBuffers, and consist of a name (here Monster) and a list of fields. Each field has a name, a type, and optionally a default value. If the default value is not specified in the schema, it will be 0 for scalar types, or null for other types. Some languages support setting a scalar's default to null. This makes the scalar optional.
Fields do not have to appear in the wire representation, and you can choose to omit fields when constructing an object. You have the flexibility to add fields without fear of bloating your data. This design is also FlatBuffer's mechanism for forward and backwards compatibility. Note that:
id attribute below.deprecated as in the example above, which will prevent the generation of accessors in the generated C++, as a way to enforce the field not being used any more. (careful: this may break code!).See "Schema evolution examples" below for more on this topic.
Similar to a table, only now none of the fields are optional (so no defaults either), and fields may not be added or be deprecated. Structs may only contain scalars or other structs. Use this for simple objects where you are very sure no changes will ever be made (as quite clear in the example Vec3). Structs use less memory than tables and are even faster to access (they are always stored in-line in their parent object, and use no virtual table).
Built-in scalar types are
byte (int8), ubyte (uint8), boolshort (int16), ushort (uint16)int (int32), uint (uint32), float (float32)long (int64), ulong (uint64), double (float64)The type names in parentheses are alias names such that for example uint8 can be used in place of ubyte, and int32 can be used in place of int without affecting code generation.
Built-in non-scalar types:
[type]). Nesting vectors is not supported, instead you can wrap the inner vector in a table.string, which may only hold UTF-8 or 7-bit ASCII. For other text encodings or general binary data use vectors ([byte] or [ubyte]) instead.You can't change types of fields once they're used, with the exception of same-size data where a reinterpret_cast would give you a desirable result, e.g. you could change a uint to an int if no values in current data use the high bit yet.
Arrays are a convenience short-hand for a fixed-length collection of elements. Arrays can be used to replace the following schema:
struct Vec3 {
x:float;
y:float;
z:float;
}
with the following schema:
struct Vec3 {
v:[float:3];
}
Both representations are binary equivalent.
Arrays are currently only supported in a struct.
There are three, mutually exclusive, reactions to the non-presence of a table's field in the binary data:
null depending on the local language. (In a sense, null is the default value).required tag below.When writing a schema, values are a sequence of digits. Values may be optionally followed by a decimal point (.) and more digits, for float constants, or optionally prefixed by a -. Floats may also be in scientific notation; optionally ending with an e or E, followed by a + or - and more digits. Values can also be the keyword null.
Only scalar values can have defaults, non-scalar (string/vector/table) fields default to null when not present.
You generally do not want to change default values after they're initially defined. Fields that have the default value are not actually stored in the serialized data (see also Gotchas below). Values explicitly written by code generated by the old schema old version, if they happen to be the default, will be read as a different value by code generated with the new schema. This is slightly less bad when converting an optional scalar into a default valued scalar since non-presence would not be overloaded with a previous default value. There are situations, however, where this may be desirable, especially if you can ensure a simultaneous rebuild of all code.
Define a sequence of named constants, each with a given value, or increasing by one from the previous one. The default first value is 0. As you can see in the enum declaration, you specify the underlying integral type of the enum with : (in this case byte), which then determines the type of any fields declared with this enum type.
Only integer types are allowed, i.e. byte, ubyte, short ushort, int, uint, long and ulong.
Typically, enum values should only ever be added, never removed (there is no deprecation for enums). This requires code to handle forwards compatibility itself, by handling unknown enum values.
Unions share a lot of properties with enums, but instead of new names for constants, you use names of tables. You can then declare a union field, which can hold a reference to any of those types, and additionally a field with the suffix _type is generated that holds the corresponding enum value, allowing you to know which type to cast to at runtime.
It's possible to give an alias name to a type union. This way a type can even be used to mean different things depending on the name used:
table PointPosition { x:uint; y:uint; }
table MarkerPosition {}
union Position {
Start:MarkerPosition,
Point:PointPosition,
Finish:MarkerPosition
}
Unions contain a special NONE marker to denote that no value is stored so that name cannot be used as an alias.
Unions are a good way to be able to send multiple message types as a FlatBuffer. Note that because a union field is really two fields, it must always be part of a table, it cannot be the root of a FlatBuffer by itself.
If you have a need to distinguish between different FlatBuffers in a more open-ended way, for example for use as files, see the file identification feature below.
There is an experimental support only in C++ for a vector of unions (and types). In the example IDL file above, use [Any] to add a vector of Any to Monster table. There is also experimental support for other types besides tables in unions, in particular structs and strings. There's no direct support for scalars in unions, but they can be wrapped in a struct at no space cost.
These will generate the corresponding namespace in C++ for all helper code, and packages in Java. You can use . to specify nested namespaces / packages.
You can include other schemas files in your current one, e.g.:
include "mydefinitions.fbs";
This makes it easier to refer to types defined elsewhere. include automatically ensures each file is parsed just once, even when referred to more than once.
When using the flatc compiler to generate code for schema definitions, only definitions in the current file will be generated, not those from the included files (those you still generate separately).
This declares what you consider to be the root table of the serialized data. This is particularly important for parsing JSON data, which doesn't include object type information.
Typically, a FlatBuffer binary buffer is not self-describing, i.e. it needs you to know its schema to parse it correctly. But if you want to use a FlatBuffer as a file format, it would be convenient to be able to have a "magic number" in there, like most file formats have, to be able to do a sanity check to see if you're reading the kind of file you're expecting.
Now, you can always prefix a FlatBuffer with your own file header, but FlatBuffers has a built-in way to add an identifier to a FlatBuffer that takes up minimal space, and keeps the buffer compatible with buffers that don't have such an identifier.
You can specify in a schema, similar to root_type, that you intend for this type of FlatBuffer to be used as a file format:
file_identifier "MYFI";
Identifiers must always be exactly 4 characters long. These 4 characters will end up as bytes at offsets 4-7 (inclusive) in the buffer.
For any schema that has such an identifier, flatc will automatically add the identifier to any binaries it generates (with -b), and generated calls like FinishMonsterBuffer also add the identifier. If you have specified an identifier and wish to generate a buffer without one, you can always still do so by calling FlatBufferBuilder::Finish explicitly.
After loading a buffer, you can use a call like MonsterBufferHasIdentifier to check if the identifier is present.
Note that this is best for open-ended uses such as files. If you simply wanted to send one of a set of possible messages over a network for example, you'd be better off with a union.
Additionally, by default flatc will output binary files as .bin. This declaration in the schema will change that to whatever you want:
file_extension "ext";
You can declare RPC calls in a schema, that define a set of functions that take a FlatBuffer as an argument (the request) and return a FlatBuffer as the response (both of which must be table types):
rpc_service MonsterStorage {
Store(Monster):StoreResponse;
Retrieve(MonsterId):Monster;
}
What code this produces and how it is used depends on language and RPC system used, there is preliminary support for GRPC through the --grpc code generator, see grpc/tests for an example.
May be written as in most C-based languages. Additionally, a triple comment (///) on a line by itself signals that a comment is documentation for whatever is declared on the line after it (table/struct/field/enum/union/element), and the comment is output in the corresponding C++ code. Multiple such lines per item are allowed.
Attributes may be attached to a declaration, behind a field/enum value, or after the name of a table/struct/enum/union. These may either have a value or not. Some attributes like deprecated are understood by the compiler; user defined ones need to be declared with the attribute declaration (like priority in the example above), and are available to query if you parse the schema at runtime. This is useful if you write your own code generators/editors etc., and you wish to add additional information specific to your tool (such as a help text).
Current understood attributes:
id: n (on a table field): manually set the field identifier to n. If you use this attribute, you must use it on ALL fields of this table, and the numbers must be a contiguous range from 0 onwards. Additionally, since a union type effectively adds two fields, its id must be that of the second field (the first field is the type field and not explicitly declared in the schema). For example, if the last field before the union field had id 6, the union field should have id 8, and the unions type field will implicitly be 7. IDs allow the fields to be placed in any order in the schema. When a new field is added to the schema it must use the next available ID.deprecated (on a field): do not generate accessors for this field anymore, code should stop using this data. Old data may still contain this field, but it won't be accessible anymore by newer code. Note that if you deprecate a field that was previous required, old code may fail to validate new data (when using the optional verifier).required (on a non-scalar table field): this field must always be set. By default, fields do not need to be present in the binary. This is desirable, as it helps with forwards/backwards compatibility, and flexibility of data structures. By specifying this attribute, you make non- presence in an error for both reader and writer. The reading code may access the field directly, without checking for null. If the constructing code does not initialize this field, they will get an assert, and also the verifier will fail on buffers that have missing required fields. Both adding and removing this attribute may be forwards/backwards incompatible as readers will be unable read old or new data, respectively, unless the data happens to always have the field set.force_align: size (on a struct): force the alignment of this struct to be something higher than what it is naturally aligned to. Causes these structs to be aligned to that amount inside a buffer, IF that buffer is allocated with that alignment (which is not necessarily the case for buffers accessed directly inside a FlatBufferBuilder). Note: currently not guaranteed to have an effect when used with --object-api, since that may allocate objects at alignments less than what you specify with force_align.force_align: size (on a vector): force the alignment of this vector to be something different than what the element size would normally dictate. Note: Now only work for generated C++ code.bit_flags (on an unsigned enum): the values of this field indicate bits, meaning that any unsigned value N specified in the schema will end up representing 1<<N, or if you don't specify values at all, you'll get the sequence 1, 2, 4, 8, ...nested_flatbuffer: "table_name" (on a field): this indicates that the field (which must be a vector of ubyte) contains flatbuffer data, for which the root type is given by table_name. The generated code will then produce a convenient accessor for the nested FlatBuffer.flexbuffer (on a field): this indicates that the field (which must be a vector of ubyte) contains flexbuffer data. The generated code will then produce a convenient accessor for the FlexBuffer root.key (on a field): this field is meant to be used as a key when sorting a vector of the type of table it sits in. Can be used for in-place binary search.hash (on a field). This is an (un)signed 32/64 bit integer field, whose value during JSON parsing is allowed to be a string, which will then be stored as its hash. The value of attribute is the hashing algorithm to use, one of fnv1_32 fnv1_64 fnv1a_32 fnv1a_64.original_order (on a table): since elements in a table do not need to be stored in any particular order, they are often optimized for space by sorting them to size. This attribute stops that from happening. There should generally not be any reason to use this flag.