Google Protobuf Primer (2) Language Guide

Google Protobuf Primer (2) Language Guide

• 作者：柳大·Poechant（钟超）
• 邮箱：zhongchao.ustc#gmail.com（# -> @）
• 博客：Blog.youkuaiyun.com/Poechant
• 微博：weibo.com/lauginhom
• 日期：June 9^th, 2012

去年年初初次接触 Google Protobuf，如今已经有不少变化，从这篇开始，续一下 :)

1. Specifying Field Rules

You specify that message fields are one of the following:

• required: a well-formed message must have exactly one of this field.
• optional: a well-formed message can have zero or one of this field (but not more than one).
• repeated: this field can be repeated any number of times (including zero) in a well-formed message. The order of the repeated values will be preserved.

For historical reasons, repeated fields of basic numeric types aren't encoded as efficiently as they could be. New code should use the special option [packed=true] to get a more efficient encoding. For example:

repeated int32 samples = 4 [packed=true];

But when you use it as the following:

message Locations {
    repeated Location location = 1 [packed=true];
}

compile it, then you will got a warning:

[packed = true] can only be specified for repeated primitive fields.

So pay attention to the type of fields you intend to specify.

2. Scalar Value Types

proto Type	Notes	C++ Type	Java Type
double		double	double
float		float	float
int32	variable-len, inefficient for negative num	int32	int
int64	variable-len, inefficient for negative num	int64	long
uint32	variable-len	uint32	int
uint64	variable-len	uint64	long
sint32	variable-len, signed num	int32	int
sint64	variable-len, signed num	int64	long
fixed32	always 4-byte, efficient > 228	int32	int
fixed64	always 8-byte, efficient > 256	int64	long
sfixed32	always 4-byte	int32	int
sfixed64	always 8-btye	int64	long
bool		bool	boolean
string	UTF-8 string, 7-bit ASCII string	string	String
bytes	any arbitrary sequence of bytes	string	ByteString

概括下：

• 可正可负，范围小（2²⁸或2⁵⁶以内）：sint32 或 sint64
• 可正可负，范围大（2²⁸或2⁵⁶以上）：sfixed32 或 sfixed64
• 只会正的，范围小（2²⁸或2⁵⁶以内）：uint32 或 uint64
• 只会正的，范围大（2²⁸或2⁵⁶以上）：fixed32 或 fixed64
• 布尔：bool
• 浮点：float 或 double
• utf8 string 或 7-bit ascii string：string
• ByteString：bytes

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转载请注明来自柳大的优快云博客：Blog.youkuaiyun.com/Poechant，微博：weibo.com/lauginhom

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