Qt reads Json files (including source code + comments)

First, the content of the sample Json file

The following is the content of the Json file used in this article, which contains strings, numbers, booleans, nulls, objects, arrays, combined types, etc. (the reading method is in the third section of this article)

{"str": "strVal","number": 88,"bool": true,"null": null,"object": {"key1": "Val1","key2": "Val2","key3": "Val3"},"array": ["elem1","elem2","elem3","elem4"],"combination": [{"name": "Google","url": "http://www.google.com"},{"name": "Baidu","url": "http://www.baidu.com"},{"name": "SoSo","url": "http://www.SoSo.com"}]}

Second, the classes that will be used in Json reading (introduce the role of the classes used in reading in this article, personal understanding)

1. QJsonParseError: used to report errors during JSON parsing; errors are stored when there is a problem with the JSON string format
2. QJsonDocument: Provides a way to read and write JSON documents;
3. QJsonObject: used to read the object type data and receive the QJsonObject object obtained from the QJsonDocument object
4. QJsonArray: used to read array type data

Third, Json file reading

3.1 Read Json string into QByteArray

 QFile file("./jsonTest.json"); //Create a QFile object and specify the json file path//Open the json file and judge (return 0 if unsuccessful)if(!file.open(QIODevice::ReadOnly))return 0;//read file contents into arrayQByteArray data(file.readAll());file.close(); //Close the file

3.2 Read Json string to QJsonDocument, and use QJsonParseError to judge whether there is an error

 QJsonParseError jError; //Create a QJsonParseError object//Use the fromJson function of QJsonDocument to read the json string, and pass in the QJsonParseError object to get the error valueQJsonDocument jDoc = QJsonDocument::fromJson(data, &jError);//Determine whether the error obtained by the QJsonParseError object contains errors, and returns 0 if it doesif(jError.error != QJsonParseError::NoError)return 0;

3.3 Get QJsonObject and read the values ​​of various types in the Json string

Get the QJsonObject object

 QJsonObject jObj = jDoc.object();

Get value of general type

 //read stringQString strVal = jObj["str"].toString();//Read the value (the corresponding value is converted to the corresponding type)int numVal = jObj["number"].toInt();//read logical valuebool boolVal = jObj["bool"].toBool();//Read the null value (because there is no corresponding to type function, you can use the following type to receive or isNull() function to judge)QVariant nullVal = jObj["null"].toVariant();

Get the object type and its value

 //The object type needs to be stored with a new QJsonObject object, and then use the new QJsonObject to get its valueQJsonObject jObj2 = jObj["object"].toObject();//Get the value of the specified key in jObj2 (the type corresponding to the value is directly converted by the corresponding function)QString key1Val = jObj2["key1"].toString();

Get array type and its values

 //The object type needs to be stored using the QJsonArray objectQJsonArray jArr = jObj["array"].toArray();//Get the value of the specified subscript in jArr (the value can be received in the following way or using the object of QJsonValue)QString arr1Val = jArr.at(0).toString();

Read value of composite type

 //The first layer type of the combined object is an array, which needs to be stored in a QJsonArray objectQJsonArray jArr2 = jObj["combination"].toArray();//The object stored in the array is received using QJsonObjectQJsonObject jObj3 = jArr2.at(0).toObject();//Get the value of the specified key in the objectQString name = jObj3["name"].toString();

Summary

It is recommended to understand the Json file format before learning, and secondly, you need to be careful in the process of writing code, similar object names are easy to confuse

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