title: “ QJsonDocument实现Qt下JSON文档读写\t\t”
tags:
- json
- qt
url: 718.html
id: 718
categories: - Qt
date: 2017-12-17 20:43:24
介绍
Qt提供了一系列类以供进行Json 文档的读写，分别为： QJsonDocumentJson文档、QJsonArray数组、QJsonObject对象、QJsonValue值、QJsonParseError错误。
错误分类
Constant
Value
Description
QJsonParseError::NoError
0
No error occurred
QJsonParseError::UnterminatedObject
1
An object is not correctly terminated with a closing curly bracket
QJsonParseError::MissingNameSeparator
2
A comma separating different items is missing
QJsonParseError::UnterminatedArray
3
The array is not correctly terminated with a closing square bracket
QJsonParseError::MissingValueSeparator
4
A colon separating keys from values inside objects is missing
QJsonParseError::IllegalValue
5
The value is illegal
QJsonParseError::TerminationByNumber
6
The input stream ended while parsing a number
QJsonParseError::IllegalNumber
7
The number is not well formed
QJsonParseError::IllegalEscapeSequence
8
An illegal escape sequence occurred in the input
QJsonParseError::IllegalUTF8String
9
An illegal UTF8 sequence occurred in the input
QJsonParseError::UnterminatedString
10
A string wasn’t terminated with a quote
QJsonParseError::MissingObject
11
An object was expected but couldn’t be found
QJsonParseError::DeepNesting
12
The JSON document is too deeply nested for the parser to parse it
QJsonParseError::DocumentTooLarge
13
The JSON document is too large for the parser to parse it
QJsonParseError::GarbageAtEnd
14
The parsed document contains additional garbage characters at the end
Json内容读写
QJsonDocument::toJson可以生成json文档，具有可选参数，可以生成紧凑结构和缩进结构：
Constant
Value
Description
QJsonDocument::Indented
0
Defines human readable output as follows:
{
“Array”: [
true,
999,
“string”
],
“Key”: “Value”,
“null”: null
}
QJsonDocument::Compact
1
Defines a compact output as follows:
{“Array”:[true,999,”string”],”Key”:”Value”,”null”:null}
除此以外还可以用toBinaryData、toVariant用于结果输出 QJsonDocument除了使用构造函数创建以外，还支持静态函数创建，主要用于读取已有文件的内容：
QJsonDocument fromBinaryData(const QByteArray &data, DataValidation validation = Validate)
QJsonDocument fromJson(const QByteArray &json, QJsonParseError error = Q_NULLPTR)
QJsonDocument fromRawData(const char data, int size, DataValidation validation = Validate)
QJsonDocument fromVariant(const QVariant &variant)
QJsonDocument并不会直接操作文件，需要自行利用QFile进行readAll或者Write
fromRawData/fromBinaryData并不会返回QJsonParseError错误而是直接返回DataValidation枚举类型，表明读取的数据是否有效
Constant
Value
Description
QJsonDocument::Validate
0
Validate the data before using it. This is the default.
QJsonDocument::BypassValidation
1
Bypasses data validation. Only use if you received the data from a trusted place and know it’s valid, as using of invalid data can crash the application.
数据类型
QJsonValue用于存储所有值，可以用type判断其类型，含以下类型
Constant
Value
Description
QJsonValue::Null
0x0
A Null value
QJsonValue::Bool
0x1
A boolean value. Use toBool() to convert to a bool.
QJsonValue::Double
0x2
A double. Use toDouble() to convert to a double.
QJsonValue::String
0x3
A string. Use toString() to convert to a QString.
QJsonValue::Array
0x4
An array. Use toArray() to convert to a QJsonArray.
QJsonValue::Object
0x5
An object. Use toObject() to convert to a QJsonObject.
QJsonValue::Undefined
0x80
The value is undefined. This is usually returned as an error condition, when trying to read an out of bounds value in an array or a non existent key in an object.
也可以通过isXXXX用于判断，并通过toXXXX转换为对应类型
读写操作
源码请见GitHub：QtCoreExamples
json范例
{
“Array”: [
true,
999,
“string”
],
“Key”: “Value”,
“null”: null
}
创建
include
include //json文档
include //json数组
include //json对象
include //json值
include //错误处理
include
int main(int argc, char *argv[]) {
QCoreApplication a(argc, argv);
QJsonDocument json;
QJsonArray array;
array.append(true);
array.append(999);
array.append(“string”);
QJsonObject object;
object.insert(“Array”,array);
object.insert(“Key”,”Value”);
//null用空的QJsonValue即可
object.insert(“null”,QJsonValue());
//最外层是大括号所以是object
json.setObject(object);
qDebug()<
}
此时使用QJsonDocument::Compact方式写出，其结果为： “{\”Array\”:[true,999,\”string\”],\”Key\”:\”Value\”,\”null\”:null}”
QDebug会将\n直接输出成\n而不会换行
解析
include
include //json文档
include //json数组
include //json对象
include //json值
include //错误处理
include
int main(int argc, char *argv[]) {
QCoreApplication a(argc, argv);
QJsonDocument json;
QJsonArray array;
array.append(true);
array.append(999);
array.append(“string”);
QJsonObject object;
object.insert(“Array”,array);
object.insert(“Key”,”Value”);
//null用空的QJsonValue即可
object.insert(“null”,QJsonValue());
//最外层是大括号所以是object
json.setObject(object);
auto json_text = json.toJson(QJsonDocument::Compact);
auto json_binary = json.toBinaryData();
QJsonDocument read1 = QJsonDocument::
fromJson(json_text);
QJsonDocument read2 = QJsonDocument::
fromBinaryData(json_binary);
//验证两个是一样的
if(QString(read1.toBinaryData()) ==
QString(read2.toBinaryData()))
qDebug()<<”same”;
//用于判断是否为空，对QJsonXXX对象均支持
if(!read1.isEmpty())
qDebug()<<”empty”;
//下面开始解析代码
if(read1.isObject() ) {
auto read_object = read1.object();
if(!read_object.isEmpty()) {
qDebug()<
qDebug()<
auto read_array = value.toArray();
for(auto one_of_array : read_array)
qDebug()<
}
}
}
return 0;
}
结果
same
empty
“Value”
true
4 QJsonValue(array, QJsonArray([true,999,”string”]))
QJsonValue(bool, true)
QJsonValue(double, 999)
QJsonValue(string, “string”)
根据正常的结构进行判断即可，对于array需要进行遍历，支持C++的for(:)方式遍历
fromJson、fromBinaryData、fromRawData、fromVariant这几个静态函数都不会直接返回成功与否，而是在参数中实现解析结果判断，正式使用时务必进行判断，避免后续代码均出错
其他
- 对于每一步建议明确判断QJsonValut的type如果type错误，会输出为””，比如int类型用toString不会自动转换，而是直接写出””
- 相比于Qt使用core模块的xml读写，json操作很简单，不需要逐行的读取操作使用readNext，获取内容的顺序与文本顺序可以不一致，xml使用请见文章：QXmlStreamReader/QXmlStreamWriter实现Qt下xml文件读写
- 上面介绍的例子最外层为object，也支持最外层为array，使用setArray即可，最外层只能为一种，不能不断的add
- QJsonDocument主要负责的是数据文本格式或者说是表现方式的转换，其余类负责内容
- QJsonArray可以用size获取数组的大小，后续操作感觉类似于QList ，也具有迭代器
- QJsonParseError使用方式自行查看，主要是在解析时出现，但均不会作为返回值直接返回，在QJsonDocument::fromJson处使用，其余几个静态fromXXX函数直接返回枚举类型，此类操作均不可保证绝对正确，故应当做判断避免后续连锁错误。
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