80 KiB
Tutorial
Overview
This tutorial provides a basic example of how to work with [FlatBuffers](@ref flatbuffers_overview). We will step through a simple example application, which shows you how to:
- Write a FlatBuffer
schemafile. - Use the
flatcFlatBuffer compiler. - Parse JSON files that conform to a schema into FlatBuffer binary files.
- Use the generated files in many of the supported languages (such as C++, Java, and more.)
During this example, imagine that you are creating a game where the main
character, the hero of the story, needs to slay some orcs. We will walk
through each step necessary to create this monster type using FlatBuffers.
Please select your desired language for our quest: \htmlonly
C++ Java C# Go Python JavaScript TypeScript PHP C Dart \endhtmlonly\htmlonly
\endhtmlonly
Where to Find the Example Code
Samples demonstating the concepts in this example are located in the source code
package, under the samples directory. You can browse the samples on GitHub
here.
For your chosen language, please cross-reference with:
Writing the Monsters' FlatBuffer Schema
To start working with FlatBuffers, you first need to create a schema file,
which defines the format for each data structure you wish to serialize. Here is
the schema that defines the template for our monsters:
// Example IDL file for our monster's schema.
namespace MyGame.Sample;
enum Color:byte { Red = 0, Green, Blue = 2 }
union Equipment { Weapon } // Optionally add more tables.
struct Vec3 {
x:float;
y:float;
z:float;
}
table Monster {
pos:Vec3; // Struct.
mana:short = 150;
hp:short = 100;
name:string;
friendly:bool = false (deprecated);
inventory:[ubyte]; // Vector of scalars.
color:Color = Blue; // Enum.
weapons:[Weapon]; // Vector of tables.
equipped:Equipment; // Union.
path:[Vec3]; // Vector of structs.
}
table Weapon {
name:string;
damage:short;
}
root_type Monster;
As you can see, the syntax for the schema
Interface Definition Language (IDL)
is similar to those of the C family of languages, and other IDL languages. Let's
examine each part of this schema to determine what it does.
The schema starts with a namespace declaration. This determines the
corresponding package/namespace for the generated code. In our example, we have
the Sample namespace inside of the MyGame namespace.
Next, we have an enum definition. In this example, we have an enum of type
byte, named Color. We have three values in this enum: Red, Green, and
Blue. We specify Red = 0 and Blue = 2, but we do not specify an explicit
value for Green. Since the behavior of an enum is to increment if
unspecified, Green will receive the implicit value of 1.
Following the enum is a union. The union in this example is not very
useful, as it only contains the one table (named Weapon). If we had created
multiple tables that we would want the union to be able to reference, we
could add more elements to the union Equipment.
After the union comes a struct Vec3, which represents a floating point
vector with 3 dimensions. We use a struct here, over a table, because
structs are ideal for data structures that will not change, since they use
less memory and have faster lookup.
The Monster table is the main object in our FlatBuffer. This will be used as
the template to store our orc monster. We specify some default values for
fields, such as mana:short = 150. All unspecified fields will default to 0
or NULL. Another thing to note is the line
friendly:bool = false (deprecated);. Since you cannot delete fields from a
table (to support backwards compatability), you can set fields as
deprecated, which will prevent the generation of accessors for this field in
the generated code. Be careful when using deprecated, however, as it may break
legacy code that used this accessor.
The Weapon table is a sub-table used within our FlatBuffer. It is
used twice: once within the Monster table and once within the Equipment
enum. For our Monster, it is used to populate a vector of tables via the
weapons field within our Monster. It is also the only table referenced by
the Equipment union.
The last part of the schema is the root_type. The root type declares what
will be the root table for the serialized data. In our case, the root type is
our Monster table.
The scalar types can also use alias type names such as int16 instead
of short and float32 instead of float. Thus we could also write
the Weapon table as:
table Weapon { name:string; damage:int16; }
More Information About Schemas
You can find a complete guide to writing schema files in the
[Writing a schema](@ref flatbuffers_guide_writing_schema) section of the
Programmer's Guide. You can also view the formal
[Grammar of the schema language](@ref flatbuffers_grammar).
Compiling the Monsters' Schema
After you have written the FlatBuffers schema, the next step is to compile it.
If you have not already done so, please follow
[these instructions](@ref flatbuffers_guide_building) to build flatc, the
FlatBuffer compiler.
Once flatc is built successfully, compile the schema for your language:
See [flatcc build instructions](https://github.com/dvidelabs/flatcc#building).
Please be aware of the difference between `flatc` and `flatcc` tools.
For a more complete guide to using the flatc compiler, please read the
[Using the schema compiler](@ref flatbuffers_guide_using_schema_compiler)
section of the Programmer's Guide.
Reading and Writing Monster FlatBuffers
Now that we have compiled the schema for our programming language, we can start creating some monsters and serializing/deserializing them from FlatBuffers.
Creating and Writing Orc FlatBuffers
The first step is to import/include the library, generated files, etc.
using namespace MyGame::Sample; // Specified in the schema.
</div>
<div class="language-java">
~~~{.java}
import MyGame.Sample.*; //The `flatc` generated files. (Monster, Vec3, etc.)
import com.google.flatbuffers.FlatBufferBuilder;
Generated by flatc.
import MyGame.Sample.Color import MyGame.Sample.Equipment import MyGame.Sample.Monster import MyGame.Sample.Vec3 import MyGame.Sample.Weapon
</div>
<div class="language-javascript">
~~~{.js}
// The following code is for JavaScript module loaders (e.g. Node.js). See
// below for a browser-based HTML/JavaScript example of including the library.
var flatbuffers = require('/js/flatbuffers').flatbuffers;
var MyGame = require('./monster_generated').MyGame; // Generated by `flatc`.
//--------------------------------------------------------------------------//
// The following code is for browser-based HTML/JavaScript. Use the above code
// for JavaScript module loaders (e.g. Node.js).
<script src="../js/flatbuffers.js"></script>
<script src="monster_generated.js"></script> // Generated by `flatc`.
import { MyGame } from './monster_generated';
// Contains the `*.php` files for the FlatBuffers library and the `flatc` generated files.
$paths = array(join(DIRECTORY_SEPARATOR, array($root_dir, "php")),
join(DIRECTORY_SEPARATOR, array($root_dir, "samples", "MyGame", "Sample")));
foreach ($paths as $path) {
$file = join(DIRECTORY_SEPARATOR, array($path, $class . ".php"));
if (file_exists($file)) {
require($file);
break;
}
}
}
</div>
<div class="language-c">
~~~{.c}
#include "monster_builder.h" // Generated by `flatcc`.
// Convenient namespace macro to manage long namespace prefix.
#undef ns
#define ns(x) FLATBUFFERS_WRAP_NAMESPACE(MyGame_Sample, x) // Specified in the schema.
// A helper to simplify creating vectors from C-arrays.
#define c_vec_len(V) (sizeof(V)/sizeof((V)[0]))
// Generated by flatc.
import 'monster_my_game.sample_generated.dart' as myGame;
</div>
Now we are ready to start building some buffers. In order to start, we need
to create an instance of the `FlatBufferBuilder`, which will contain the buffer
as it grows. You can pass an initial size of the buffer (here 1024 bytes),
which will grow automatically if needed:
<div class="language-cpp">
~~~{.cpp}
// Create a `FlatBufferBuilder`, which will be used to create our
// monsters' FlatBuffers.
flatbuffers::FlatBufferBuilder builder(1024);
After creating the builder, we can start serializing our data. Before we make
our orc Monster, lets create some Weapons: a Sword and an Axe.
auto weapon_two_name = builder.CreateString("Axe"); short weapon_two_damage = 5;
// Use the CreateWeapon shortcut to create Weapons with all the fields set.
auto sword = CreateWeapon(builder, weapon_one_name, weapon_one_damage);
auto axe = CreateWeapon(builder, weapon_two_name, weapon_two_damage);
</div>
<div class="language-java">
~~~{.java}
int weaponOneName = builder.createString("Sword")
short weaponOneDamage = 3;
int weaponTwoName = builder.createString("Axe");
short weaponTwoDamage = 5;
// Use the `createWeapon()` helper function to create the weapons, since we set every field.
int sword = Weapon.createWeapon(builder, weaponOneName, weaponOneDamage);
int axe = Weapon.createWeapon(builder, weaponTwoName, weaponTwoDamage);
var weaponTwoName = builder.CreateString("Axe"); var weaponTwoDamage = 5;
// Use the CreateWeapon() helper function to create the weapons, since we set every field.
var sword = Weapon.CreateWeapon(builder, weaponOneName, (short)weaponOneDamage);
var axe = Weapon.CreateWeapon(builder, weaponTwoName, (short)weaponTwoDamage);
</div>
<div class="language-go">
~~~{.go}
weaponOne := builder.CreateString("Sword")
weaponTwo := builder.CreateString("Axe")
// Create the first `Weapon` ("Sword").
sample.WeaponStart(builder)
sample.Weapon.AddName(builder, weaponOne)
sample.Weapon.AddDamage(builder, 3)
sword := sample.WeaponEnd(builder)
// Create the second `Weapon` ("Axe").
sample.WeaponStart(builder)
sample.Weapon.AddName(builder, weaponTwo)
sample.Weapon.AddDamage(builder, 5)
axe := sample.WeaponEnd(builder)
Create the first Weapon ('Sword').
MyGame.Sample.Weapon.WeaponStart(builder) MyGame.Sample.Weapon.WeaponAddName(builder, weapon_one) MyGame.Sample.Weapon.WeaponAddDamage(builder, 3) sword = MyGame.Sample.Weapon.WeaponEnd(builder)
Create the second Weapon ('Axe').
MyGame.Sample.Weapon.WeaponStart(builder) MyGame.Sample.Weapon.WeaponAddName(builder, weapon_two) MyGame.Sample.Weapon.WeaponAddDamage(builder, 5) axe = MyGame.Sample.Weapon.WeaponEnd(builder)
</div>
<div class="language-javascript">
~~~{.js}
var weaponOne = builder.createString('Sword');
var weaponTwo = builder.createString('Axe');
// Create the first `Weapon` ('Sword').
MyGame.Sample.Weapon.startWeapon(builder);
MyGame.Sample.Weapon.addName(builder, weaponOne);
MyGame.Sample.Weapon.addDamage(builder, 3);
var sword = MyGame.Sample.Weapon.endWeapon(builder);
// Create the second `Weapon` ('Axe').
MyGame.Sample.Weapon.startWeapon(builder);
MyGame.Sample.Weapon.addName(builder, weaponTwo);
MyGame.Sample.Weapon.addDamage(builder, 5);
var axe = MyGame.Sample.Weapon.endWeapon(builder);
// Create the first Weapon ('Sword').
MyGame.Sample.Weapon.startWeapon(builder);
MyGame.Sample.Weapon.addName(builder, weaponOne);
MyGame.Sample.Weapon.addDamage(builder, 3);
let sword = MyGame.Sample.Weapon.endWeapon(builder);
// Create the second Weapon ('Axe').
MyGame.Sample.Weapon.startWeapon(builder);
MyGame.Sample.Weapon.addName(builder, weaponTwo);
MyGame.Sample.Weapon.addDamage(builder, 5);
let axe = MyGame.Sample.Weapon.endWeapon(builder);
</div>
<div class="language-php">
~~~{.php}
// Create the `Weapon`s using the `createWeapon()` helper function.
$weapon_one_name = $builder->createString("Sword");
$sword = \MyGame\Sample\Weapon::CreateWeapon($builder, $weapon_one_name, 3);
$weapon_two_name = $builder->createString("Axe");
$axe = \MyGame\Sample\Weapon::CreateWeapon($builder, $weapon_two_name, 5);
// Create an array from the two `Weapon`s and pass it to the
// `CreateWeaponsVector()` method to create a FlatBuffer vector.
$weaps = array($sword, $axe);
$weapons = \MyGame\Sample\Monster::CreateWeaponsVector($builder, $weaps);
ns(Weapon_ref_t) weapon_two_name = flatbuffers_string_create_str(B, "Axe"); uint16_t weapon_two_damage = 5;
ns(Weapon_ref_t) sword = ns(Weapon_create(B, weapon_one_name, weapon_one_damage)); ns(Weapon_ref_t) axe = ns(Weapon_create(B, weapon_two_name, weapon_two_damage));
</div>
<div class="language-dart">
~~~{.dart}
// The generated Builder classes work much like in other languages,
final int weaponOneName = builder.writeString("Sword");
final int weaponOneDamage = 3;
final int weaponTwoName = builder.writeString("Axe");
final int weaponTwoDamage = 5;
final swordBuilder = new myGame.WeaponBuilder(builder)
..begin()
..addNameOffset(weaponOneName)
..addDamage(weaponOneDamage);
final int sword = swordBuilder.finish();
final axeBuilder = new myGame.WeaponBuilder(builder)
..begin()
..addNameOffset(weaponTwoName)
..addDamage(weaponTwoDamage);
final int axe = axeBuilder.finish();
// The genearted ObjectBuilder classes offer an easier to use alternative
// at the cost of requiring some additional reference allocations. If memory
// usage is critical, or if you'll be working with especially large messages
// or tables, you should prefer using the generated Builder classes.
// The following code would produce an identical buffer as above.
final String weaponOneName = "Sword";
final int weaponOneDamage = 3;
final String weaponTwoName = "Axe";
final int weaponTwoDamage = 5;
final myGame.WeaponBuilder sword = new myGame.WeaponObjectBuilder(
name: weaponOneName,
damage: weaponOneDamage,
);
final myGame.WeaponBuilder axe = new myGame.WeaponObjectBuilder(
name: weaponTwoName,
damage: weaponTwoDamage,
);
Now let's create our monster, the orc. For this orc, lets make him
red with rage, positioned at (1.0, 2.0, 3.0), and give him
a large pool of hit points with 300. We can give him a vector of weapons
to choose from (our Sword and Axe from earlier). In this case, we will
equip him with the Axe, since it is the most powerful of the two. Lastly,
let's fill his inventory with some potential treasures that can be taken once he
is defeated.
Before we serialize a monster, we need to first serialize any objects that are contained there-in, i.e. we serialize the data tree using depth-first, pre-order traversal. This is generally easy to do on any tree structures.
// Create a vector representing the inventory of the Orc. Each number
// could correspond to an item that can be claimed after he is slain.
unsigned char treasure[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
auto inventory = builder.CreateVector(treasure, 10);
</div>
<div class="language-java">
~~~{.java}
// Serialize a name for our monster, called "Orc".
int name = builder.createString("Orc");
// Create a `vector` representing the inventory of the Orc. Each number
// could correspond to an item that can be claimed after he is slain.
byte[] treasure = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
int inv = Monster.createInventoryVector(builder, treasure);
// Create a vector representing the inventory of the Orc. Each number
// could correspond to an item that can be claimed after he is slain.
// Note: Since we prepend the bytes, this loop iterates in reverse order.
Monster.StartInventoryVector(builder, 10);
for (int i = 9; i >= 0; i--)
{
builder.AddByte((byte)i);
}
var inv = builder.EndVector();
</div>
<div class="language-go">
~~~{.go}
// Serialize a name for our monster, called "Orc".
name := builder.CreateString("Orc")
// Create a `vector` representing the inventory of the Orc. Each number
// could correspond to an item that can be claimed after he is slain.
// Note: Since we prepend the bytes, this loop iterates in reverse.
sample.MonsterStartInventoryVector(builder, 10)
for i := 9; i >= 0; i-- {
builder.PrependByte(byte(i))
}
inv := builder.EndVector(10)
Create a vector representing the inventory of the Orc. Each number
could correspond to an item that can be claimed after he is slain.
Note: Since we prepend the bytes, this loop iterates in reverse.
MyGame.Sample.Monster.MonsterStartInventoryVector(builder, 10) for i in reversed(range(0, 10)): builder.PrependByte(i) inv = builder.EndVector(10)
</div>
<div class="language-javascript">
~~~{.js}
// Serialize a name for our monster, called 'Orc'.
var name = builder.createString('Orc');
// Create a `vector` representing the inventory of the Orc. Each number
// could correspond to an item that can be claimed after he is slain.
var treasure = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
var inv = MyGame.Sample.Monster.createInventoryVector(builder, treasure);
// Create a vector representing the inventory of the Orc. Each number
// could correspond to an item that can be claimed after he is slain.
let treasure = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
let inv = MyGame.Sample.Monster.createInventoryVector(builder, treasure);
</div>
<div class="language-php">
~~~{.php}
// Serialize a name for our monster, called "Orc".
$name = $builder->createString("Orc");
// Create a `vector` representing the inventory of the Orc. Each number
// could correspond to an item that can be claimed after he is slain.
$treasure = array(0, 1, 2, 3, 4, 5, 6, 7, 8, 9);
$inv = \MyGame\Sample\Monster::CreateInventoryVector($builder, $treasure);
// Create a vector representing the inventory of the Orc. Each number
// could correspond to an item that can be claimed after he is slain.
uint8_t treasure[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
flatbuffers_uint8_vec_ref_t inventory;
// c_vec_len is the convenience macro we defined earlier.
inventory = flatbuffers_uint8_vec_create(B, treasure, c_vec_len(treasure));
</div>
<div class="language-dart">
~~~{.dart}
// Serialize a name for our monster, called "Orc".
final int name = builder.writeString('Orc');
// Create a list representing the inventory of the Orc. Each number
// could correspond to an item that can be claimed after he is slain.
final List<int> treasure = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
final inventory = builder.writeListUint8(treasure);
// The following code should be used instead if you intend to use the
// ObjectBuilder classes:
// Serialize a name for our monster, called "Orc".
final String name = 'Orc';
// Create a list representing the inventory of the Orc. Each number
// could correspond to an item that can be claimed after he is slain.
final List<int> treasure = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
We serialized two built-in data types (string and vector) and captured
their return values. These values are offsets into the serialized data,
indicating where they are stored, such that we can refer to them below when
adding fields to our monster.
Note: To create a vector of nested objects (e.g. tables, strings, or
other vectors), collect their offsets into a temporary data structure, and
then create an additional vector containing their offsets.
If instead of creating a vector from an existing array you serialize elements individually one by one, take care to note that this happens in reverse order, as buffers are built back to front.
For example, take a look at the two Weapons that we created earlier (Sword
and Axe). These are both FlatBuffer tables, whose offsets we now store in
memory. Therefore we can create a FlatBuffer vector to contain these
offsets.
// Pass the weaps array into the createWeaponsVector() method to create a FlatBuffer vector.
int weapons = Monster.createWeaponsVector(builder, weaps);
</div>
<div class="language-csharp">
~~~{.cs}
var weaps = new Offset<Weapon>[2];
weaps[0] = sword;
weaps[1] = axe;
// Pass the `weaps` array into the `CreateWeaponsVector()` method to create a FlatBuffer vector.
var weapons = Monster.CreateWeaponsVector(builder, weaps);
// If using the ObjectBuilders, just create an array from the two Weapons
final List<myGame.WeaponBuilder> weaps = [sword, axe];
</div>
<div class="language-cpp">
<br>
Note there's additional convenience overloads of `CreateVector`, allowing you
to work with data that's not in a `std::vector`, or allowing you to generate
elements by calling a lambda. For the common case of `std::vector<std::string>`
there's also `CreateVectorOfStrings`.
</div>
Note that vectors of structs are serialized differently from tables, since
structs are stored in-line in the vector. For example, to create a vector
for the `path` field above:
<div class="language-cpp">
~~~{.cpp}
Vec3 points[] = { Vec3(1.0f, 2.0f, 3.0f), Vec3(4.0f, 5.0f, 6.0f) };
auto path = builder.CreateVectorOfStructs(points, 2);
// Otherwise, using the ObjectBuilder classes:
// The dart implementation provides a simple interface for writing vectors
// of structs, in writeListOfStructs. This method takes
// List<ObjectBuilder> and is used by the generated builder classes.
final List<myGame.Vec3ObjectBuilder> path = [
new myGame.Vec3ObjectBuilder(x: 1.0, y: 2.0, z: 3.0),
new myGame.Vec3ObjectBuilder(x: 4.0, y: 5.0, z: 6.0)
];
</div>
We have now serialized the non-scalar components of the orc, so we
can serialize the monster itself:
<div class="language-cpp">
~~~{.cpp}
// Create the position struct
auto position = Vec3(1.0f, 2.0f, 3.0f);
// Set his hit points to 300 and his mana to 150.
int hp = 300;
int mana = 150;
// Finally, create the monster using the `CreateMonster` helper function
// to set all fields.
auto orc = CreateMonster(builder, &position, mana, hp, name, inventory,
Color_Red, weapons, Equipment_Weapon, axe.Union(),
path);
// Define an equipment union. create calls in C has a single
// argument for unions where C++ has both a type and a data argument.
ns(Equipment_union_ref_t) equipped = ns(Equipment_as_Weapon(axe));
ns(Vec3_t) pos = { 1.0f, 2.0f, 3.0f };
ns(Monster_create_as_root(B, &pos, mana, hp, name, inventory, ns(Color_Red),
weapons, equipped, path));
</div>
<div class="language-dart">
~~~{.dart}
// Using the Builder API:
// Set his hit points to 300 and his mana to 150.
final int hp = 300;
final int mana = 150;
final monster = new myGame.MonsterBuilder(builder)
..begin()
..addNameOffset(name)
..addInventoryOffset(inventory)
..addWeaponsOffset(weapons)
..addEquippedType(myGame.EquipmentTypeId.Weapon)
..addEquippedOffset(axe)
..addHp(hp)
..addMana(mana)
..addPos(vec3Builder.finish(1.0, 2.0, 3.0))
..addPathOffset(path)
..addColor(myGame.Color.Red);
final int orc = monster.finish();
// -Or- using the ObjectBuilder API:
// Set his hit points to 300 and his mana to 150.
final int hp = 300;
final int mana = 150;
// Note that these parameters are optional - it is not necessary to set
// all of them.
// Also note that it is not necessary to `finish` the builder helpers above
// - the generated code will automatically reuse offsets if the same object
// is used in more than one place (e.g. the axe appearing in `weapons` and
// `equipped`).
final myGame.MonsterBuilder orcBuilder = new myGame.MonsterBuilder(
name: name,
inventory: treasure,
weapons: weaps,
equippedType: myGame.EquipmentTypeId.Weapon,
equipped: axe,
path: path,
hp: hp,
mana: mana,
pos: new myGame.Vec3Builder(x: 1.0, y: 2.0, z: 3.0),
color: myGame.Color.Red,
path: [
new myGame.Vec3ObjectBuilder(x: 1.0, y: 2.0, z: 3.0),
new myGame.Vec3ObjectBuilder(x: 4.0, y: 5.0, z: 6.0)
]);
final int orc = orcBuilder.finish(builder);
Note how we create Vec3 struct in-line in the table. Unlike tables, structs
are simple combinations of scalars that are always stored inline, just like
scalars themselves.
Important: Unlike structs, you should not nest tables or other objects,
which is why we created all the strings/vectors/tables that this monster refers
to before start. If you try to create any of them between start and end,
you will get an assert/exception/panic depending on your language.
Note: Since we are passing 150 as the mana field, which happens to be the
default value, the field will not actually be written to the buffer, since the
default value will be returned on query anyway. This is a nice space savings,
especially if default values are common in your data. It also means that you do
not need to be worried of adding a lot of fields that are only used in a small
number of instances, as it will not bloat the buffer if unused.
If you do not wish to set every field in a `table`, it may be more convenient to manually set each field of your monster, instead of calling `CreateMonster()`. The following snippet is functionally equivalent to the above code, but provides a bit more flexibility.
~~~{.cpp} // You can use this code instead of `CreateMonster()`, to create our orc // manually. MonsterBuilder monster_builder(builder); monster_builder.add_pos(&position); monster_builder.add_hp(hp); monster_builder.add_name(name); monster_builder.add_inventory(inventory); monster_builder.add_color(Color_Red); monster_builder.add_weapons(weapons); monster_builder.add_equipped_type(Equipment_Weapon); monster_builder.add_equipped(axe.Union()); auto orc = monster_builder.Finish(); ~~~
~~~{.c} // It is important to pair `start_as_root` with `end_as_root`. ns(Monster_start_as_root(B)); ns(Monster_pos_create(B, 1.0f, 2.0f, 3.0f)); // or alternatively //ns(Monster_pos_add(&pos);
ns(Monster_hp_add(B, hp));
// Notice that Monser_name_add adds a string reference unlike the
// add_str and add_strn variants.
ns(Monster_name_add(B, name));
ns(Monster_inventory_add(B, inventory));
ns(Monster_color_add(B, ns(Color_Red)));
ns(Monster_weapons_add(B, weapons));
ns(Monster_equipped_add(B, equipped));
// Complete the monster object and make it the buffer root object.
ns(Monster_end_as_root(B));
</div>
Before finishing the serialization, let's take a quick look at FlatBuffer
`union Equipped`. There are two parts to each FlatBuffer `union`. The first, is
a hidden field `_type`, that is generated to hold the type of `table` referred
to by the `union`. This allows you to know which type to cast to at runtime.
Second, is the `union`'s data.
In our example, the last two things we added to our `Monster` were the
`Equipped Type` and the `Equipped` union itself.
Here is a repetition these lines, to help highlight them more clearly:
<div class="language-cpp">
~~~{.cpp}
monster_builder.add_equipped_type(Equipment_Weapon); // Union type
monster_builder.add_equipped(axe); // Union data
// in the ObjectBuilder API: equippedTypeId: myGame.EquipmentTypeId.Weapon, // Union type equipped: axe, // Union data
</div>
After you have created your buffer, you will have the offset to the root of the
data in the `orc` variable, so you can finish the buffer by calling the
appropriate `finish` method.
<div class="language-cpp">
~~~{.cpp}
// Call `Finish()` to instruct the builder that this monster is complete.
// Note: Regardless of how you created the `orc`, you still need to call
// `Finish()` on the `FlatBufferBuilder`.
builder.Finish(orc); // You could also call `FinishMonsterBuffer(builder,
// orc);`.
The buffer is now ready to be stored somewhere, sent over the network, be compressed, or whatever you'd like to do with it. You can access the buffer like so:
// Alternatively this copies the above data out of the ByteBuffer for you: byte[] buf = builder.sizedByteArray();
</div>
<div class="language-csharp">
~~~{.cs}
// This must be called after `Finish()`.
var buf = builder.DataBuffer; // Of type `FlatBuffers.ByteBuffer`.
// The data in this ByteBuffer does NOT start at 0, but at buf.Position.
// The end of the data is marked by buf.Length, so the size is
// buf.Length - buf.Position.
// Alternatively this copies the above data out of the ByteBuffer for you:
byte[] buf = builder.SizedByteArray();
// Allocate and extract a readable buffer from internal builder heap.
// The returned buffer must be deallocated using free.
// NOTE: Finalizing the buffer does NOT change the builder, it
// just creates a snapshot of the builder content.
buf = flatcc_builder_finalize_buffer(B, &size);
// use buf
free(buf);
// Optionally reset builder to reuse builder without deallocating // internal stack and heap. flatcc_builder_reset(B); // build next buffer. // ...
// Cleanup. flatcc_builder_clear(B);
</div>
<div class="language-dart">
~~~{.dart}
final Uint8List buf = builder.finish(orc);
Now you can write the bytes to a file, send them over the network.. Make sure your file mode (or tranfer protocol) is set to BINARY, not text. If you transfer a FlatBuffer in text mode, the buffer will be corrupted, which will lead to hard to find problems when you read the buffer.
Reading Orc FlatBuffers
Now that we have successfully created an Orc FlatBuffer, the monster data can
be saved, sent over a network, etc. Let's now adventure into the inverse, and
deserialize a FlatBuffer.
This section requires the same import/include, namespace, etc. requirements as before:
using namespace MyGame::Sample; // Specified in the schema.
</div>
<div class="language-java">
~~~{.java}
import MyGame.Sample.*; //The `flatc` generated files. (Monster, Vec3, etc.)
import com.google.flatbuffers.FlatBufferBuilder;
Generated by flatc.
import MyGame.Sample.Any import MyGame.Sample.Color import MyGame.Sample.Monster import MyGame.Sample.Vec3
</div>
<div class="language-javascript">
~~~{.js}
// The following code is for JavaScript module loaders (e.g. Node.js). See
// below for a browser-based HTML/JavaScript example of including the library.
var flatbuffers = require('/js/flatbuffers').flatbuffers;
var MyGame = require('./monster_generated').MyGame; // Generated by `flatc`.
//--------------------------------------------------------------------------//
// The following code is for browser-based HTML/JavaScript. Use the above code
// for JavaScript module loaders (e.g. Node.js).
<script src="../js/flatbuffers.js"></script>
<script src="monster_generated.js"></script> // Generated by `flatc`.
import { MyGame } from './monster_generated';
</div>
<div class="language-php">
~~~{.php}
// It is recommended that your use PSR autoload when using FlatBuffers in PHP.
// Here is an example from `SampleBinary.php`:
function __autoload($class_name) {
// The last segment of the class name matches the file name.
$class = substr($class_name, strrpos($class_name, "\\") + 1);
$root_dir = join(DIRECTORY_SEPARATOR, array(dirname(dirname(__FILE__)))); // `flatbuffers` root.
// Contains the `*.php` files for the FlatBuffers library and the `flatc` generated files.
$paths = array(join(DIRECTORY_SEPARATOR, array($root_dir, "php")),
join(DIRECTORY_SEPARATOR, array($root_dir, "samples", "MyGame", "Sample")));
foreach ($paths as $path) {
$file = join(DIRECTORY_SEPARATOR, array($path, $class . ".php"));
if (file_exists($file)) {
require($file);
break;
}
}
}
#undef ns #define ns(x) FLATBUFFERS_WRAP_NAMESPACE(MyGame_Sample, x) // Specified in the schema.
</div>
<div class="language-dart">
~~~{.dart}
import 'package:flat_buffers/flat_buffers.dart' as fb;
import './monster_my_game.sample_generated.dart' as myGame;
Then, assuming you have a buffer of bytes received from disk, network, etc., you can create start accessing the buffer like so:
Again, make sure you read the bytes in BINARY mode, otherwise the code below won't work
// Get a pointer to the root object inside the buffer. auto monster = GetMonster(buffer_pointer);
// monster is of type Monster *.
// Note: root object pointers are NOT the same as buffer_pointer.
// GetMonster is a convenience function that calls GetRoot<Monster>,
// the latter is also available for non-root types.
</div>
<div class="language-java">
~~~{.java}
byte[] bytes = /* the data you just read */
java.nio.ByteBuffer buf = java.nio.ByteBuffer.wrap(bytes);
// Get an accessor to the root object inside the buffer.
Monster monster = Monster.getRootAsMonster(buf);
// Get an accessor to the root object inside the buffer. var monster = Monster.GetRootAsMonster(buf);
</div>
<div class="language-go">
~~~{.go}
var buf []byte = /* the data you just read */
// Get an accessor to the root object inside the buffer.
monster := sample.GetRootAsMonster(buf, 0)
// Note: We use `0` for the offset here, which is typical for most buffers
// you would read. If you wanted to read from `builder.Bytes` directly, you
// would need to pass in the offset of `builder.Head()`, as the builder
// constructs the buffer backwards, so may not start at offset 0.
// Get an accessor to the root object inside the buffer. monster = MyGame.Sample.Monster.Monster.GetRootAsMonster(buf, 0)
Note: We use 0 for the offset here, which is typical for most buffers
you would read. If you wanted to read from the builder.Bytes directly,
you would need to pass in the offset of builder.Head(), as the builder
constructs the buffer backwards, so may not start at offset 0.
</div>
<div class="language-javascript">
~~~{.js}
var bytes = /* the data you just read, in an object of type "Uint8Array" */
var buf = new flatbuffers.ByteBuffer(bytes);
// Get an accessor to the root object inside the buffer.
var monster = MyGame.Sample.Monster.getRootAsMonster(buf);
// Get an accessor to the root object inside the buffer. let monster = MyGame.Sample.Monster.getRootAsMonster(buf);
</div>
<div class="language-php">
~~~{.php}
$bytes = /* the data you just read, in a string */
$buf = Google\FlatBuffers\ByteBuffer::wrap($bytes);
// Get an accessor to the root object inside the buffer.
$monster = \MyGame\Sample\Monster::GetRootAsMonster($buf);
// Note: root object pointers are NOT the same as the buffer pointer.
</div>
<div class="language-dart">
~~~{.dart}
List<int> data = ... // the data, e.g. from file or network
// A generated factory constructor that will read the data.
myGame.Monster monster = new myGame.Monster(data);
If you look in the generated files from the schema compiler, you will see it generated
accessors for all non-deprecated fields. For example:
These should hold 300, 150, and "Orc" respectively.
Note: The default value 150 wasn't stored in mana, but we are still able to retrieve it.
To access sub-objects, in the case of our pos, which is a Vec3:
// Note: Whenever you access a new object, like in Pos(), a new temporary
// accessor object gets created. If your code is very performance sensitive,
// you can pass in a pointer to an existing Vec3 instead of nil. This
// allows you to reuse it across many calls to reduce the amount of object
// allocation/garbage collection.
</div>
<div class="language-python">
~~~{.py}
pos = monster.Pos()
x = pos.X()
y = pos.Y()
z = pos.Z()
x, y, and z will contain 1.0, 2.0, and 3.0, respectively.
Note: Had we not set pos during serialization, it would be a NULL-value.
Similarly, we can access elements of the inventory vector by indexing it. You
can also iterate over the length of the array/vector representing the
FlatBuffers vector.
For vectors of tables, you can access the elements like any other vector,
except your need to handle the result as a FlatBuffer table:
Last, we can access our Equipped FlatBuffer union. Just like when we created
the union, we need to get both parts of the union: the type and the data.
We can access the type to dynamically cast the data as needed (since the
union only stores a FlatBuffer table).
if (union_type == Equipment_Weapon) {
auto weapon = static_cast<const Weapon*>(monster->equipped()); // Requires static_cast
// to type const Weapon*.
auto weapon_name = weapon->name()->str(); // "Axe"
auto weapon_damage = weapon->damage(); // 5
}
</div>
<div class="language-java">
~~~{.java}
int unionType = monster.EquippedType();
if (unionType == Equipment.Weapon) {
Weapon weapon = (Weapon)monster.equipped(new Weapon()); // Requires explicit cast
// to `Weapon`.
String weaponName = weapon.name(); // "Axe"
short weaponDamage = weapon.damage(); // 5
}
if (unionType == Equipment.Weapon) { var weapon = monster.Equipped().Value;
var weaponName = weapon.Name; // "Axe"
var weaponDamage = weapon.Damage; // 5
}
</div>
<div class="language-go">
~~~{.go}
// We need a `flatbuffers.Table` to capture the output of the
// `monster.Equipped()` function.
unionTable := new(flatbuffers.Table)
if monster.Equipped(unionTable) {
unionType := monster.EquippedType()
if unionType == sample.EquipmentWeapon {
// Create a `sample.Weapon` object that can be initialized with the contents
// of the `flatbuffers.Table` (`unionTable`), which was populated by
// `monster.Equipped()`.
unionWeapon = new(sample.Weapon)
unionWeapon.Init(unionTable.Bytes, unionTable.Pos)
weaponName = unionWeapon.Name()
weaponDamage = unionWeapon.Damage()
}
}
if union_type == MyGame.Sample.Equipment.Equipment().Weapon:
# monster.Equipped() returns a flatbuffers.Table, which can be used to
# initialize a MyGame.Sample.Weapon.Weapon().
union_weapon = MyGame.Sample.Weapon.Weapon()
union_weapon.Init(monster.Equipped().Bytes, monster.Equipped().Pos)
weapon_name = union_weapon.Name() // 'Axe'
weapon_damage = union_weapon.Damage() // 5
</div>
<div class="language-javascript">
~~~{.js}
var unionType = monster.equippedType();
if (unionType == MyGame.Sample.Equipment.Weapon) {
var weapon_name = monster.equipped(new MyGame.Sample.Weapon()).name(); // 'Axe'
var weapon_damage = monster.equipped(new MyGame.Sample.Weapon()).damage(); // 5
}
if (unionType == MyGame.Sample.Equipment.Weapon) { let weapon_name = monster.equipped(new MyGame.Sample.Weapon()).name(); // 'Axe' let weapon_damage = monster.equipped(new MyGame.Sample.Weapon()).damage(); // 5 }
</div>
<div class="language-php">
~~~{.php}
$union_type = $monster->getEquippedType();
if ($union_type == \MyGame\Sample\Equipment::Weapon) {
$weapon_name = $monster->getEquipped(new \MyGame\Sample\Weapon())->getName(); // "Axe"
$weapon_damage = $monster->getEquipped(new \MyGame\Sample\Weapon())->getDamage(); // 5
}
if (unionType == myGame.EquipmentTypeId.Weapon.value) { myGame.Weapon weapon = mon.equipped as myGame.Weapon;
var weaponName = weapon.name; // "Axe"
var weaponDamage = weapon.damage; // 5
}
</div>
## Mutating FlatBuffers
As you saw above, typically once you have created a FlatBuffer, it is read-only
from that moment on. There are, however, cases where you have just received a
FlatBuffer, and you'd like to modify something about it before sending it on to
another recipient. With the above functionality, you'd have to generate an
entirely new FlatBuffer, while tracking what you modified in your own data
structures. This is inconvenient.
For this reason FlatBuffers can also be mutated in-place. While this is great
for making small fixes to an existing buffer, you generally want to create
buffers from scratch whenever possible, since it is much more efficient and the
API is much more general purpose.
To get non-const accessors, invoke `flatc` with `--gen-mutable`.
Similar to how we read fields using the accessors above, we can now use the
mutators like so:
<div class="language-cpp">
~~~{.cpp}
auto monster = GetMutableMonster(buffer_pointer); // non-const
monster->mutate_hp(10); // Set the table `hp` field.
monster->mutable_pos()->mutate_z(4); // Set struct field.
monster->mutable_inventory()->Mutate(0, 1); // Set vector element.
We use the somewhat verbose term mutate instead of set to indicate that this
is a special use case, not to be confused with the default way of constructing
FlatBuffer data.
After the above mutations, you can send on the FlatBuffer to a new recipient without any further work!
Note that any mutate functions on a table will return a boolean, which is
false if the field we're trying to set is not present in the buffer. Fields
that are not present if they weren't set, or even if they happen to be equal to
the default value. For example, in the creation code above, the mana
field is equal to 150, which is the default value, so it was never stored in
the buffer. Trying to call the corresponding mutate method for mana on such
data will return false, and the value won't actually be modified!
One way to solve this is to call ForceDefaults on a FlatBufferBuilder to
force all fields you set to actually be written. This, of course, increases the
size of the buffer somewhat, but this may be acceptable for a mutable buffer.
If this is not sufficient, other ways of mutating FlatBuffers may be supported
in your language through an object based API (--gen-object-api) or reflection.
See the individual language documents for support.
JSON with FlatBuffers
Using flatc as a Conversion Tool
This is often the preferred method to use JSON with FlatBuffers, as it doesn't require you to add any new code to your program. It is also efficient, since you can ship with the binary data. The drawback is that it requires an extra step for your users/developers to perform (although it may be able to be automated as part of your compilation).
Lets say you have a JSON file that describes your monster. In this example,
we will use the file flatbuffers/samples/monsterdata.json.
Here are the contents of the file:
{
pos: {
x: 1,
y: 2,
z: 3
},
hp: 300,
name: "Orc"
}
You can run this file through the flatc compile with the -b flag and
our monster.fbs schema to produce a FlatBuffer binary file.
./../flatc -b monster.fbs monsterdata.json
The output of this will be a file monsterdata.bin, which will contain the
FlatBuffer binary representation of the contents from our .json file.
Advanced Features for Each Language
Each language has a dedicated Use in XXX page in the Programmer's Guide
to cover the nuances of FlatBuffers in that language.
For your chosen language, see: