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.A scalar message field can have one of the following types – the table shows the type specified in the .proto file, and the corresponding type in the automatically generated class:
| .proto Type | Notes | C++ Type | Java Type | Python Type[2] | Go Type |
|---|---|---|---|---|---|
| double | double | double | float | *float64 | |
| float | float | float | float | *float32 | |
| int32 | Uses variable-length encoding. Inefficient for encoding negative numbers – if your field is likely to have negative values, use sint32 instead. | int32 | int | int | *int32 |
| int64 | Uses variable-length encoding. Inefficient for encoding negative numbers – if your field is likely to have negative values, use sint64 instead. | int64 | long | int/long[3] | *int64 |
| uint32 | Uses variable-length encoding. | uint32 | int[1] | int/long[3] | *uint32 |
| uint64 | Uses variable-length encoding. | uint64 | long[1] | int/long[3] | *uint64 |
| sint32 | Uses variable-length encoding. Signed int value. These more efficiently encode negative numbers than regular int32s. | int32 | int | int | *int32 |
| sint64 | Uses variable-length encoding. Signed int value. These more efficiently encode negative numbers than regular int64s. | int64 | long | int/long[3] | *int64 |
| fixed32 | Always four bytes. More efficient than uint32 if values are often greater than 228. | uint32 | int[1] | int | *uint32 |
| fixed64 | Always eight bytes. More efficient than uint64 if values are often greater than 256. | uint64 | long[1] | int/long[3] | *uint64 |
| sfixed32 | Always four bytes. | int32 | int | int | *int32 |
| sfixed64 | Always eight bytes. | int64 | long | int/long[3] | *int64 |
| bool | bool | boolean | bool | *bool | |
| string | A string must always contain UTF-8 encoded or 7-bit ASCII text. | string | String | str/unicode[4] | *string |
| bytes | May contain any arbitrary sequence of bytes. | string | ByteString | str | []byte |
原文：http://www.cnblogs.com/rushoutasia/p/5616440.html