v811_spc009/external/flatbuffers/docs/source/FlatBuffers.md

FlatBuffers

Overview

[FlatBuffers](@ref flatbuffers_overview) is an efficient cross platform serialization library for C++, C#, C, Go, Java, Kotlin, JavaScript, Lobster, Lua, TypeScript, PHP, Python, Rust and Swift. It was originally created at Google for game development and other performance-critical applications.

It is available as Open Source on GitHub under the Apache license, v2 (see LICENSE.txt).

Why use FlatBuffers?

• Access to serialized data without parsing/unpacking - What sets FlatBuffers apart is that it represents hierarchical data in a flat binary buffer in such a way that it can still be accessed directly without parsing/unpacking, while also still supporting data structure evolution (forwards/backwards compatibility).

• Memory efficiency and speed - The only memory needed to access your data is that of the buffer. It requires 0 additional allocations (in C++, other languages may vary). FlatBuffers is also very suitable for use with mmap (or streaming), requiring only part of the buffer to be in memory. Access is close to the speed of raw struct access with only one extra indirection (a kind of vtable) to allow for format evolution and optional fields. It is aimed at projects where spending time and space (many memory allocations) to be able to access or construct serialized data is undesirable, such as in games or any other performance sensitive applications. See the [benchmarks](@ref flatbuffers_benchmarks) for details.

• Flexible - Optional fields means not only do you get great forwards and backwards compatibility (increasingly important for long-lived games: don't have to update all data with each new version!). It also means you have a lot of choice in what data you write and what data you don't, and how you design data structures.

• Tiny code footprint - Small amounts of generated code, and just a single small header as the minimum dependency, which is very easy to integrate. Again, see the benchmark section for details.

• Strongly typed - Errors happen at compile time rather than manually having to write repetitive and error prone run-time checks. Useful code can be generated for you.

• Convenient to use - Generated C++ code allows for terse access & construction code. Then there's optional functionality for parsing schemas and JSON-like text representations at runtime efficiently if needed (faster and more memory efficient than other JSON parsers).

  Java, Kotlin and Go code supports object-reuse. C# has efficient struct based accessors.

• Cross platform code with no dependencies - C++ code will work with any recent gcc/clang and VS2010. Comes with build files for the tests & samples (Android .mk files, and cmake for all other platforms).

Why not use Protocol Buffers, or .. ?

Protocol Buffers is indeed relatively similar to FlatBuffers, with the primary difference being that FlatBuffers does not need a parsing/ unpacking step to a secondary representation before you can access data, often coupled with per-object memory allocation. The code is an order of magnitude bigger, too. Protocol Buffers has neither optional text import/export nor schema language features like unions.

But all the cool kids use JSON!

JSON is very readable (which is why we use it as our optional text format) and very convenient when used together with dynamically typed languages (such as JavaScript). When serializing data from statically typed languages, however, JSON not only has the obvious drawback of runtime inefficiency, but also forces you to write more code to access data (counterintuitively) due to its dynamic-typing serialization system. In this context, it is only a better choice for systems that have very little to no information ahead of time about what data needs to be stored.

If you do need to store data that doesn't fit a schema, FlatBuffers also offers a schema-less (self-describing) version!

Read more about the "why" of FlatBuffers in the [white paper](@ref flatbuffers_white_paper).

Who uses FlatBuffers?

• Cocos2d-x, the #1 open source mobile game engine, uses it to serialize all their game data.
• Facebook uses it for client-server communication in their Android app. They have a nice article explaining how it speeds up loading their posts.
• Fun Propulsion Labs at Google uses it extensively in all their libraries and games.

Usage in brief

This section is a quick rundown of how to use this system. Subsequent sections provide a more in-depth usage guide.

• Write a schema file that allows you to define the data structures you may want to serialize. Fields can have a scalar type (ints/floats of all sizes), or they can be a: string; array of any type; reference to yet another object; or, a set of possible objects (unions). Fields are optional and have defaults, so they don't need to be present for every object instance.

• Use flatc (the FlatBuffer compiler) to generate a C++ header (or Java/Kotlin/C#/Go/Python.. classes) with helper classes to access and construct serialized data. This header (say mydata_generated.h) only depends on flatbuffers.h, which defines the core functionality.

• Use the FlatBufferBuilder class to construct a flat binary buffer. The generated functions allow you to add objects to this buffer recursively, often as simply as making a single function call.

• Store or send your buffer somewhere!

• When reading it back, you can obtain the pointer to the root object from the binary buffer, and from there traverse it conveniently in-place with object->field().

In-depth documentation

• How to [build the compiler](@ref flatbuffers_guide_building) and samples on various platforms.
• How to [use the compiler](@ref flatbuffers_guide_using_schema_compiler).
• How to [write a schema](@ref flatbuffers_guide_writing_schema).
• How to [use the generated C++ code](@ref flatbuffers_guide_use_cpp) in your own programs.
• How to [use the generated Java code](@ref flatbuffers_guide_use_java) in your own programs.
• How to [use the generated C# code](@ref flatbuffers_guide_use_c-sharp) in your own programs.
• How to [use the generated Kotlin code](@ref flatbuffers_guide_use_kotlin) in your own programs.
• How to [use the generated Go code](@ref flatbuffers_guide_use_go) in your own programs.
• How to [use the generated Lua code](@ref flatbuffers_guide_use_lua) in your own programs.
• How to [use the generated JavaScript code](@ref flatbuffers_guide_use_javascript) in your own programs.
• How to [use the generated TypeScript code](@ref flatbuffers_guide_use_typescript) in your own programs.
• How to [use FlatBuffers in C with flatcc](@ref flatbuffers_guide_use_c) in your own programs.
• How to [use the generated Lobster code](@ref flatbuffers_guide_use_lobster) in your own programs.
• How to [use the generated Rust code](@ref flatbuffers_guide_use_rust) in your own programs.
• How to [use the generated Swift code](@ref flatbuffers_guide_use_swift) in your own programs.
• [Support matrix](@ref flatbuffers_support) for platforms/languages/features.
• Some [benchmarks](@ref flatbuffers_benchmarks) showing the advantage of using FlatBuffers.
• A [white paper](@ref flatbuffers_white_paper) explaining the "why" of FlatBuffers.
• How to use the [schema-less](@ref flexbuffers) version of FlatBuffers.
• A description of the [internals](@ref flatbuffers_internals) of FlatBuffers.
• A formal [grammar](@ref flatbuffers_grammar) of the schema language.

Online resources

• GitHub repository
• Landing page
• FlatBuffers Google Group
• Discord and Gitter chat.
• FlatBuffers Issues Tracker
• Independent implementations & tools:
  • FlatCC Alternative FlatBuffers parser, code generator and runtime all in C.
• Videos:
  • Colt's DevByte.
  • GDC 2015 Lightning Talk.
  • FlatBuffers for Go.
  • Evolution of FlatBuffers visualization.
• Useful documentation created by others:
  • FlatBuffers in Go
  • FlatBuffers in Android
  • Parsing JSON to FlatBuffers in Java
  • FlatBuffers in Unity
  • FlexBuffers C# and article on its use.