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$strLenBytes
Returns the number of UTF-8 encoded bytes in the specified string.返回指定字符串中UTF-8编码的字节数。
$strLenBytes has the following operator expression syntax:具有以下运算符表达式语法：
{ $strLenBytes: <string expression> }
The argument can be any valid expression as long as it resolves to a string. 参数可以是任何有效的表达式，只要它解析为字符串。For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。
If the argument resolves to a value of 如果参数解析为null or refers to a missing field, $strLenBytes returns an error.null值或引用缺少的字段，$strLenBytes将返回错误。
The $strLenBytes operator counts the number of UTF-8 encoded bytes in a string where each character may use between one and four bytes.$strLenBytes运算符计算字符串中UTF-8编码的字节数，其中每个字符可以使用一到四个字节。
For example, US-ASCII characters are encoded using one byte. 例如，US-ASCII字符使用一个字节进行编码。Characters with diacritic markings and additional Latin alphabetical characters (i.e. Latin characters outside of the English alphabet) are encoded using two bytes. 带有变音符号标记的字符和其他拉丁字母字符（即英语字母表以外的拉丁字符）使用两个字节进行编码。Chinese, Japanese and Korean characters typically require three bytes, and other planes of unicode (emoji, mathematical symbols, etc.) require four bytes.中文、日文和韩文字符通常需要三个字节，而其他unicode平面（表情符号、数学符号等）需要四个字节。
The $strLenBytes operator differs from $strLenCP operator which counts the code points in the specified string regardless of how many bytes each character uses.$strLenBytes运算符与$strLenCP运算符不同，后者统计指定字符串中的代码点，而不管每个字符使用多少字节。
| Example | Results | Notes |
|---|---|---|
{ $strLenBytes: "abcde" }
| 5 | |
{ $strLenBytes: "Hello World!" }
| 12 | |
{ $strLenBytes: "cafeteria" }
| 9 | |
{ $strLenBytes: "cafétéria" }
| 11 | é |
{ $strLenBytes: "" }
| 0 | |
{ $strLenBytes: "$€λG" }
| 7 | €λ |
{ $strLenBytes: "寿司" }
| 6 |
A collection named 名为food contains the following documents:food的集合包含以下文档：
{ "_id" : 1, "name" : "apple" }
{ "_id" : 2, "name" : "banana" }
{ "_id" : 3, "name" : "éclair" }
{ "_id" : 4, "name" : "hamburger" }
{ "_id" : 5, "name" : "jalapeño" }
{ "_id" : 6, "name" : "pizza" }
{ "_id" : 7, "name" : "tacos" }
{ "_id" : 8, "name" : "寿司" }
The following operation uses the 以下操作使用$strLenBytes operator to calculate the length of each name value:$strLenBytes运算符计算每个name值的length：
db.food.aggregate(
[
{
$project: {
"name": 1,
"length": { $strLenBytes: "$name" }
}
}
]
)
The operation returns the following results:该操作返回以下结果：
{ "_id" : 1, "name" : "apple", "length" : 5 }
{ "_id" : 2, "name" : "banana", "length" : 6 }
{ "_id" : 3, "name" : "éclair", "length" : 7 }
{ "_id" : 4, "name" : "hamburger", "length" : 9 }
{ "_id" : 5, "name" : "jalapeño", "length" : 9 }
{ "_id" : 6, "name" : "pizza", "length" : 5 }
{ "_id" : 7, "name" : "tacos", "length" : 5 }
{ "_id" : 8, "name" : "寿司", "length" : 6 }
The documents with _id: 3 and _id: 5 each contain a diacritic character (é and ñ respectively) that requires two bytes to encode. _id:3和_id:5的文档各自包含一个需要两个字节进行编码的变音字符（分别é和ñ）。The document with _id: 8 contains two Japanese characters that are encoded using three bytes each. _id:8的文档包含两个日语字符，每个字符使用三个字节进行编码。This makes the 这使得具有length greater than the number of characters in name for the documents with _id: 3, _id: 5 and _id: 8._id:3、_id:5和_id:8的文档的长度大于name中的字符数。