有关如何读取，写入和打印QJson代码的最小示例(使用QJsonDocument，QJsonArray，QJsonObject和QFile) [英] Minimal Example on how to read, write and print QJson code (with QJsonDocument, QJsonArray, QJsonObject, QFile)

问题描述

我正在寻找一个最小的完整的可执行qt或c ++代码示例来解析和编写此Json代码:

I am searching for a minimal full executable qt or c++ code example to parse and write this Json code:

{
    "FirstName": "John",
    "LastName": "Doe",
    "MiddleName": null,
    "Age": 43,

    "Address": {
        "Street": "Downing Street 10",
        "City": "London",
        "Country": "Great Britain"
    },
    "Phone numbers": [
        "[phone]",
        "[phone]"
    ]
    "Valid":true,
}

• 上面的示例由一个具有5个键/值对的对象组成.其中两个值是字符串，一个是数字，
• 一个是另一个对象，最后一个是数组.
• 有效的JSON文档是数组或对象，因此文档始终以方括号或大括号开头.
• Json有还有2个键/值对-值'null'和'布尔'
• Json has 2 more key/value pairs - value 'null' and 'bool'

是的，我看到了保存游戏示例" 并试图找出答案.

And yes, I have seen a "Save Game Example" and tried to figure it out.

但是将近一周之后，我放弃了将没有枚举，QVectors和3个不同的头文件的最小Example转移到我的项目中，以处理代码段.无关紧要的是小部件还是核心代码.

But after nearly a week I gave up to transfer a minimal Example without enums, QVectors and 3 different header Files over to my project to handle the code snippet. Doesn't matter if its for a widget or core code.

我已经成功地完成了xml读写程序，但是似乎我错过了一些重要的要点，并且得到了与解析可能或不一定有关的json错误.如果没有最少的完全可以运行的代码示例，我将无法排除它.

I already did a xml read and write program successfully but it seems I miss some Important point and get errors with json that may or may not have to do with the parsing. I am not able to rule it out without a minimal fully working code example.

所以我的问题是:能否请您提供一个编写，读取和打印Json文件的最小示例?谢谢你.

推荐答案

我想它也可能对其他人有帮助，我想通了， 感谢@Botje的榜样，这确实有所帮助. 通常，我很容易记得将数据表示为QByteArray 在内存中.

I guess it might help someone else too, I figured it all out, thanks to @Botje for his example, really helped. In general it is easy for me to remember to represent data as QByteArray in the memory.

欢迎访问并使用存储在json文件中的每个单个值的最小示例:

A minimal example to access and also use every single value stored in a json file, you are welcome:

(是的，我本可以将其编写得更加紧凑，但是我想这会使它更容易理解，因此为了可读性，我省去了函数/类/结构.)

#include <QCoreApplication>
#include <QString>
#include <QVariant>
#include <QFile>
#include <QByteArray>
#include <QTextStream>
#include <QDebug>

//json specific
#include <QJsonParseError>
#include <QJsonDocument>
#include <QJsonArray>
#include <QJsonObject>
#include <QJsonParseError>
#include <QJsonValue>


    void writeJsonFile();
    void readJsonFile();


int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    writeJsonFile();
    readJsonFile();

    return a.exec();
}

void writeJsonFile()
{
    qDebug() << "Write Json File: ";

    //1. Create a QJsonObject and add values to it
    QJsonObject jsonObj;
    jsonObj["FirstName"] = "John";
    //no value will be written as null in the json file
    jsonObj["MiddleName"];
    jsonObj["LastName"] = "Doe";
    jsonObj["Age"] = 43;

    //2. Create Item of Json Object content (object of object)
    QJsonObject jsonItemObj;
    jsonItemObj["Street"] = "Downing Street 10";
    jsonItemObj["City"] = "London";
    jsonItemObj["Country"] = "Great Britain";
    //3. Add jsonItemObj to jsonObj and give it an object Name
    jsonObj["Address"] = jsonItemObj;

    //4. Create jsonArray and fill it with values - similar to filling a vector
    QJsonArray jsonArray;
    jsonArray.append("[phone]");
    jsonArray.append("[phone]");

    //Add a bool to the Object
    jsonObj["Valid"] = true;

    //5. Add jsonArray to jsonObj and give it an object Name
    jsonObj["Phone numbers"] = jsonArray;
    //(It can also be added to the jsonItemObj to be inline with the Address section)
    //with jsonItemObj["Phone numbers"] = jsonArray or as much objects of objects
    //you need


    /* As I understood it, most Qt classes use a QByteArray to handle data internally
     * because it is really fast/efficient,
     * also QFile QIODevice, it is a good idea to hold the read/write
     * QIODevice data as QByteArray in the Memory
     */

    //6. Create a QByteArray and fill it with QJsonDocument (json formatted)
    QByteArray byteArray;
    byteArray = QJsonDocument(jsonObj).toJson();

    //7. Open a QFile and write the byteArray filled with json formatted data
    //thanks to the QJsonDocument() Class to the file
    QFile file;
    file.setFileName("file.json");
    if(!file.open(QIODevice::WriteOnly)){
        qDebug() << "No write access for json file";
        return;
    }

    //8. finally write the file and close it
    file.write(byteArray);
    file.close();

    //9. Print out the byteArray to the terminal
    QTextStream textStream(stdout);
    textStream << "Rendered json byteArray text: " << endl;
    textStream << byteArray << endl;
}

void readJsonFile()
{
    qDebug() << "Read Json File:";

    //1. Open the QFile and write it to a byteArray and close the file
    QFile file;
    file.setFileName("file.json");
    if(!file.open(QIODevice::ReadOnly)){
        qDebug() << "Json filef couldn't be opened/found";
        return;
    }

    QByteArray byteArray;
    byteArray = file.readAll();
    file.close();

    //2. Format the content of the byteArray as QJsonDocument
    //and check on parse Errors
    QJsonParseError parseError;
    QJsonDocument jsonDoc;
    jsonDoc = QJsonDocument::fromJson(byteArray, &parseError);
    if(parseError.error != QJsonParseError::NoError){
        qWarning() << "Parse error at " << parseError.offset << ":" << parseError.errorString();
        return;
    }

    //3. Create a jsonObject and fill it with the byteArray content, formatted
    //and holding by the jsonDocument Class
    QJsonObject jsonObj;
    jsonObj = jsonDoc.object();

    //4. Now picking the jsonValues and printing them out or do what ever you need
    QJsonValue jsonVal;
    QTextStream textStream(stdout);

    jsonVal = jsonObj.value("FirstName");
    textStream << jsonVal.toString() << endl;

    jsonVal = jsonObj.value("MiddleName");
    //null gets back to an empty fild - added the sting "null/empty" to make it visible
    textStream << jsonVal.toVariant().toString() << "null/empty" << endl;

    jsonVal = jsonObj.value("LastName");
    textStream << jsonVal.toString() << endl;

    //The number has to be converted to an int and than to a string to print it
    jsonVal = jsonObj.value("Age");
    textStream << QString::number(jsonVal.toInt()) << endl;

    //5. Now we need to fill the object of the object. To do that, we need
    //the Item Object and a jsonSubVal object for json without a loop
    QJsonObject jsonItemObj;
    QJsonValue jsonSubVal;

    jsonVal = jsonObj.value(QString("Address"));
    jsonItemObj = jsonVal.toObject();

       jsonSubVal = jsonItemObj["Street"];
       textStream << jsonSubVal.toString() << endl;

       jsonSubVal = jsonItemObj["City"];
       textStream << jsonSubVal.toString() << endl;

       jsonSubVal = jsonItemObj["Country"];
       textStream << jsonSubVal.toString() << endl;

    //6. now the Phone Numbers array with a loop
    QJsonArray jsonArray;
    jsonArray = jsonObj["Phone numbers"].toArray();

    for(int i = 0; i < jsonArray.size(); i++)
        textStream << jsonArray[i].toString() << endl;

    textStream << "or with foreach" << endl;

    foreach(QJsonValue v, jsonArray)
        textStream << v.toString() << endl;

    //7. And finally the bool value:
    jsonVal = jsonObj.value(QString("Valid"));
    textStream << jsonVal.toVariant().toString() << endl;

    textStream << "or as number, not a string: ";

    textStream << (QString::number(jsonVal.toInt())) << endl;

    textStream << "\nThe whole file as printed in the file \n" <<
                  jsonDoc.toJson(QJsonDocument::Indented);

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