本文标题:QJsonDocument实现Qt下JSON文档读写 本文地址:http://techieliang.com/2017/12/718/
1. 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。
1.1. 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 |
1.2. 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. |
1.3. 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
2. 读写操作
2.1. json范例
- {
- "Array": [
- true,
- 999,
- "string"
- ],
- "Key": "Value",
- "null": null
- }
3. 创建
- #include <QCoreApplication>
- #include <QJsonDocument>//json文档
- #include <QJsonArray>//json数组
- #include <QJsonObject>//json对象
- #include <QJsonValue>//json值
- #include <QJsonParseError>//错误处理
- #include <QDebug>
- 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()<<json.toJson(QJsonDocument::Compact);
- return 0;
- }
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
4. 解析
- #include <QCoreApplication>
- #include <QJsonDocument>//json文档
- #include <QJsonArray>//json数组
- #include <QJsonObject>//json对象
- #include <QJsonValue>//json值
- #include <QJsonParseError>//错误处理
- #include <QDebug>
- 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()<<read_object.value("Key").toString();
- qDebug()<<read_object.value("null").isNull();
- QJsonValue value = read_object.value("Array");
- qDebug()<<value.type()<<value;
- if(value.isArray()) {
- auto read_array = value.toArray();
- for(auto one_of_array : read_array)
- qDebug()<<one_of_array;
- //此处建议判断好具体类型,因为array里面也可能有object
- }
- }
- }
- 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
5. 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<QVariant> ,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。