FlatBuffer入门笔记

1 flatbuffer资料

flatbuffer下载地址：https://github.com/google/flatbuffers

flatbuffer官方使用文档：https://google.github.io/flatbuffers/index.html#flatbuffers_overview

flatbuffer官方测试用例：https://google.github.io/flatbuffers/flatbuffers_guide_use_cpp.html

2 编写IDL|Schema

2.1 IDL测试文件

　　引用官方测试使用的IDL

 // Example IDL file for our monster's schema.
 namespace MyGame.Sample;
 enum Color:byte { Red = , Green, Blue =  }
 union Equipment { Weapon } // Optionally add more tables.
 struct Vec3 {
   x:float;
   y:float;
   z:float;
 }
 table Monster {
   pos:Vec3; // Struct.
   mana:short = ;
   hp:short = ;
   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;

2.2 生成桩文件

　　利用flatc.exe生成IDL对应的桩代码，命令格式为

 flatc [ GENERATOR OPTIONS ] [ -o PATH ] [ -I PATH ] [ -S ] FILES...  [ -- FILES...]

 flatc --cpp monster.fbs

　　flatbuffer使用模板编程，仅生成h文件。对应的文件名为filename_generated.h。这里生成monster_generated.h文件。

2.3 IDL数据类型

2.3.1 Table

　　Table是FlatBuffer定义的主要数据类型，一个Table包含一个名称（如2.1的Monster），以及一组字段（如2.1的Monster）。每个字段由名称、类型及默认值组成。

　　FlatBuffer每个字段都是可选的，这由FlatBUffer的线性实现机制决定：空闲字段仅填充默认占位值，而不占用分配size。

2.3.2 Struct

　　Struct只包含数值类型与其他struct，与Table相比，Struct使用更少的存储空间以及更快的访问速度。

2.3.3 Type

　　内建的数值类型有：

8 bit: byte (int8), ubyte (uint8), bool
16 bit: short (int16), ushort (uint16)
32 bit: int (int32), uint (uint32), float (float32)
64 bit: long (int64), ulong (uint64), double (float64)

　　内建的非数值类型有：

vector，用[]表示
string
指向其他Table、struct、enum、unions的引用

2.3.4 Enums

　　定义了一系列的命名常量，可以给定默认值。第一个变量的默认值为0。

2.3.5 Namespaces

　　可以定义嵌套的namespace，用.分割。

2.3.6 Root type

　　定义序列化的root table或者struct。

3 序列化与反序列化

　　对2.1例子的序列化和反序列化过程实现在sample_binary.cpp。

3.1 序列化

  // Build up a serialized buffer algorithmically:
   flatbuffers::FlatBufferBuilder builder;                               //申请一个flatbuffer
 
   // First, lets serialize some weapons for the Monster: A 'sword' and an 'axe'.
   auto weapon_one_name = builder.CreateString("Sword");                 //在buffer中申请string
   short weapon_one_damage = ;
 
   auto weapon_two_name = builder.CreateString("Axe");                   //在buffer中申请string
   short weapon_two_damage = ;
 
   // Use the `CreateWeapon` shortcut to create Weapons with all fields set.
   auto sword = CreateWeapon(builder, weapon_one_name, weapon_one_damage); //桩代码中实现
   auto axe = CreateWeapon(builder, weapon_two_name, weapon_two_damage);   //
 
   // Create a FlatBuffer's `vector` from the `std::vector`.
   std::vector<flatbuffers::Offset<Weapon>> weapons_vector;
   weapons_vector.push_back(sword);
   weapons_vector.push_back(axe);
   auto weapons = builder.CreateVector(weapons_vector);                    //在buffer中序列化vector
 
   // Second, serialize the rest of the objects needed by the Monster.
   auto position = Vec3(1.0f, 2.0f, 3.0f);                               
 
   auto name = builder.CreateString("MyMonster");
 
   unsigned char inv_data[] = { , , , , , , , , ,  };
   auto inventory = builder.CreateVector(inv_data, );
 
   // Shortcut for creating monster with all fields set:
   auto orc = CreateMonster(builder, &position, , , name, inventory,
                            Color_Red, weapons, Equipment_Weapon, axe.Union());
 
   builder.Finish(orc);  // Serialize the root of the object.              //序列化对象root
 
   // We now have a FlatBuffer we can store on disk or send over a network.
 
   // ** file/network code goes here :) **

3.2 反序列化

 // access builder.GetBufferPointer() for builder.GetSize() bytes
 
   // Instead, we're going to access it right away (as if we just received it).
 
   // Get access to the root:
   auto monster = GetMonster(builder.GetBufferPointer());                  //获取根对象指针
 
   // Get and test some scalar types from the FlatBuffer.
   assert(monster->hp() == );
   assert(monster->mana() == );  // default
   assert(monster->name()->str() == "MyMonster");
 
   // Get and test a field of the FlatBuffer's `struct`.
   auto pos = monster->pos();
   assert(pos);
   assert(pos->z() == 3.0f);
   (void)pos;
 
   // Get a test an element from the `inventory` FlatBuffer's `vector`.
   auto inv = monster->inventory();
   assert(inv);
   assert(inv->Get() == );
   (void)inv;
 
   // Get and test the `weapons` FlatBuffers's `vector`.
   std::string expected_weapon_names[] = { "Sword", "Axe" };
   short expected_weapon_damages[] = { ,  };
   auto weps = monster->weapons();
   for (unsigned int i = ; i < weps->size(); i++) {
     assert(weps->Get(i)->name()->str() == expected_weapon_names[i]);
     assert(weps->Get(i)->damage() == expected_weapon_damages[i]);
   }
   (void)expected_weapon_names;
   (void)expected_weapon_damages;
 
   // Get and test the `Equipment` union (`equipped` field).
   assert(monster->equipped_type() == Equipment_Weapon);
   auto equipped = static_cast<const Weapon *>(monster->equipped());
   assert(equipped->name()->str() == "Axe");
   assert(equipped->damage() == );
   (void)equipped;

4 FlatBuffer原理？网上找的一些文档

　　https://www.jianshu.com/p/fa999434776a