FlatBuffer入門筆記

FlatBuffer入門筆記

1 flatbuffer資料

flatbuffer下載地址：https://github.com/google/flatbuffers

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

flatbuffer官方測試用例：https://google.github.io/flatbuffers/flatbuffers_guide_use_cpp.html

2 編寫IDL|Schema

2.1 IDL測試文件

　　引用官方測試使用的IDL

 1 // Example IDL file for our monster's schema.
 2 namespace MyGame.Sample;
 3 enum Color:byte { Red = 0, Green, Blue = 2 }
 4 union Equipment { Weapon } // Optionally add more tables.
 5 struct Vec3 {
 6   x:float;
 7   y:float;
 8   z:float;
 9 }
10 table Monster {
11   pos:Vec3; // Struct.
12   mana:short = 150;
13   hp:short = 100;
14   name:string;
15   friendly:bool = false (deprecated);
16   inventory:[ubyte];  // Vector of scalars.
17   color:Color = Blue; // Enum.
18   weapons:[Weapon];   // Vector of tables.
19   equipped:Equipment; // Union.
20   path:[Vec3];        // Vector of structs.
21 }
22 table Weapon {
23   name:string;
24   damage:short;
25 }
26 root_type Monster;

2.2  生成樁文件

　　利用flatc.exe生成IDL對應的樁代碼，命令格式爲

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

 

1 flatc --cpp monster.fbs

　　flatbuffer使用模板編程，僅生成h文件。對應的文件名爲filename_generated.h。這裏生成monster_generated.h文件。

2.3 IDL數據類型

2.3.1 Table

　　Table是FlatBuffer定義的主要數據類型，一個Table包含一個名稱（如2.1的Monster），以及一組字段（如2.1的Monster）。每一個字段由名稱、類型及默認值組成。

　　FlatBuffer每一個字段都是可選的，這由FlatBUffer的線性實現機制決定：空閒字段僅填充默認佔位值，而不佔用分配size。

2.3.2 Struct

　　Struct只包含數值類型與其餘struct，與Table相比，Struct使用更少的存儲空間以及更快的訪問速度。

2.3.3 Type

　　內建的數值類型有：

• 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)

　　內建的非數值類型有：

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

2.3.4 Enums

　　定義了一系列的命名常量，能夠給定默認值。第一個變量的默認值爲0。

 2.3.5 Namespaces

　　能夠定義嵌套的namespace，用.分割。

2.3.6 Root type

　　定義序列化的root table或者struct。

3 序列化與反序列化

　　對2.1例子的序列化和反序列化過程實如今sample_binary.cpp。

3.1 序列化

 1  // Build up a serialized buffer algorithmically:
 2   flatbuffers::FlatBufferBuilder builder;                               //申請一個flatbuffer
 3 
 4   // First, lets serialize some weapons for the Monster: A 'sword' and an 'axe'.
 5   auto weapon_one_name = builder.CreateString("Sword");                 //在buffer中申請string
 6   short weapon_one_damage = 3;
 7 
 8   auto weapon_two_name = builder.CreateString("Axe");                   //在buffer中申請string
 9   short weapon_two_damage = 5;
10 
11   // Use the `CreateWeapon` shortcut to create Weapons with all fields set.
12   auto sword = CreateWeapon(builder, weapon_one_name, weapon_one_damage); //樁代碼中實現
13   auto axe = CreateWeapon(builder, weapon_two_name, weapon_two_damage);   //
14 
15   // Create a FlatBuffer's `vector` from the `std::vector`.
16   std::vector<flatbuffers::Offset<Weapon>> weapons_vector;
17   weapons_vector.push_back(sword);
18   weapons_vector.push_back(axe);
19   auto weapons = builder.CreateVector(weapons_vector);                    //在buffer中序列化vector
20 
21   // Second, serialize the rest of the objects needed by the Monster.
22   auto position = Vec3(1.0f, 2.0f, 3.0f);                               
23 
24   auto name = builder.CreateString("MyMonster");
25 
26   unsigned char inv_data[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
27   auto inventory = builder.CreateVector(inv_data, 10);
28 
29   // Shortcut for creating monster with all fields set:
30   auto orc = CreateMonster(builder, &position, 150, 80, name, inventory,
31                            Color_Red, weapons, Equipment_Weapon, axe.Union());
32 
33   builder.Finish(orc);  // Serialize the root of the object.              //序列化對象root
34 
35   // We now have a FlatBuffer we can store on disk or send over a network.
36 
37   // ** file/network code goes here :) **

 

 

3.2 反序列化

 1 // access builder.GetBufferPointer() for builder.GetSize() bytes
 2 
 3   // Instead, we're going to access it right away (as if we just received it).
 4 
 5   // Get access to the root:
 6   auto monster = GetMonster(builder.GetBufferPointer());                  //獲取根對象指針
 7 
 8   // Get and test some scalar types from the FlatBuffer.
 9   assert(monster->hp() == 80);
10   assert(monster->mana() == 150);  // default
11   assert(monster->name()->str() == "MyMonster");
12 
13   // Get and test a field of the FlatBuffer's `struct`.
14   auto pos = monster->pos();
15   assert(pos);
16   assert(pos->z() == 3.0f);
17   (void)pos;
18 
19   // Get a test an element from the `inventory` FlatBuffer's `vector`.
20   auto inv = monster->inventory();
21   assert(inv);
22   assert(inv->Get(9) == 9);
23   (void)inv;
24 
25   // Get and test the `weapons` FlatBuffers's `vector`.
26   std::string expected_weapon_names[] = { "Sword", "Axe" };
27   short expected_weapon_damages[] = { 3, 5 };
28   auto weps = monster->weapons();
29   for (unsigned int i = 0; i < weps->size(); i++) {
30     assert(weps->Get(i)->name()->str() == expected_weapon_names[i]);
31     assert(weps->Get(i)->damage() == expected_weapon_damages[i]);
32   }
33   (void)expected_weapon_names;
34   (void)expected_weapon_damages;
35 
36   // Get and test the `Equipment` union (`equipped` field).
37   assert(monster->equipped_type() == Equipment_Weapon);
38   auto equipped = static_cast<const Weapon *>(monster->equipped());
39   assert(equipped->name()->str() == "Axe");
40   assert(equipped->damage() == 5);
41   (void)equipped;

 

4 FlatBuffer原理？網上找的一些文檔

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