QT -- qfile file read / write operation

For reading and writing files ,C and C++ Each has its own way . And in the Qt There is also a set in , That is to use QFile Read and write files .

How to open the file

Read the file

First create a new project , The following operations cater to The embedded Linux Development , Decided to ubuntu In the middle of , And use rsync Remote debugging .

First , Build a scene in the form , And fix the form to the size of the development board screen 1024 * 600 .

The construction steps are as follows ： First , Put control LineEdit and PushButton Put in Widget Control , Then click Select Widget Control , Use horizontal alignment , After that, the control TextEdit Drag onto the form （ By stretching , Enlarge the control ）, Last , Select the entire form , Use vertical alignment , It becomes as shown in the figure below .

secondly , Want to achieve such a goal ： When you click Select file Button , A file dialog box pops up , Then put the selected file path into LineEdit Control , The specific contents of the document are displayed in TextEdit in . My directory is /work/resource/ Two files are prepared in , The content of the file is the same , Use different coding , As shown in the figure below .

Switch to code mainwindow.cpp Come on .

in front , We have introduced the knowledge of file dialog , If you are not clear, you can turn to the front and have a look .

The code is as follows ：

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QFileDialog>
#include <QDebug>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    
    ui->setupUi(this);
    // Hit the new button , A file dialog box pops up 
    connect(ui->pushButton,&QPushButton::released,[=](){
    
        // File dialog  --  Return the file path of the selected file 
        QString path = QFileDialog::getOpenFileName(this," Select file ","/home/chantui/work/resourceFile","Images (*.png *.xpm *.jpg);;Text files (*.txt);;XML files (*.xml)");
        // Put the path into  LineEdit  Control 
        // Generally, functions that set text are called setText
        ui->lineEdit->setText(path);

        // Put the read content into  TextEdit  Control 
        QFile file(path);// Parameter is the path to read the file 
        // Set the opening mode （ Read or write or something else ）
        file.open(QIODevice::ReadOnly);
        // adopt readAll Function can read the contents of the file 
        // This function returns one  QByteArray  Value of class type , So you can receive this return value 
        QByteArray bytearray = file.readAll();
        // Put the read data into  textEdit  in , Also use  setText  This function 
        // because  setText  The parameters required for this function are  QString  type , Therefore, format conversion is required 
        // But here, implicit conversion will be performed automatically when compiling , So there is no need for format conversion .
        ui->textEdit->setText(bytearray);
    });
}

MainWindow::~MainWindow()
{
    
    delete ui;
}

The operation results are as follows ：

open utf8 Coded file , give the result as follows ：

open ansi Coded file , give the result as follows ：

QFile The default support is utf8 Format （Unicode A kind of coding ）, and ANSI Format file is GBK code . that QFile Is there no way to read it ？ Not at all , Need to use QT A class of encoding format in QTextCodec .

Use QTextCodec This class needs to add header files #include <QTextCodec>

after , Use a static method in this class codecForName , And pass in the file to be read （ character string ） What is the encoding format of .

//codecForName  Is a static method in this class , Specify here to "gbk" Code format to read 
QTextCodec *codec = QTextCodec::codecForName("gbk");

after , Pass in the path to read the file , Specify how to open , And read the contents of the file .

QFile file(path);// Parameter is the path to read the file 
// Set the opening mode （ Read or write or something else ）
file.open(QIODevice::ReadOnly);
// adopt readAll Function can read the contents of the file 
// This function returns one  QByteArray  Value of class type , So you can receive this return value 
QByteArray bytearray = file.readAll();

Last , It is also a key step , Convert the read file contents into Unicode Coding format （ Need to use toUnicode() This function ）, Show it again .

 ui->textEdit->setText(codec->toUnicode(bytearray));

Run the code to see ：

however , After this conversion , And does not support utf8 The format of the file , Take a look .

therefore , If you want to support at the same time , You need to make a judgment , It happens to be " e-book " In this project , I learned to .^ _ ^

In the previous code , We're going to use readAll() This method reads the text content directly , Now let's introduce another way readAll() — Read by line .

The code is as follows ：

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    
    ui->setupUi(this);
    // Hit the new button , A file dialog box pops up 
    connect(ui->pushButton,&QPushButton::released,[=](){
    
        // File dialog  --  Return the file path of the selected file 
        QString path = QFileDialog::getOpenFileName(this," Select file ","/home/chantui/work/resourceFile","Images (*.png *.xpm *.jpg);;Text files (*.txt);;XML files (*.xml)");
        // Put the path into  LineEdit  Control 
        // Generally, functions that set text are called setText
        ui->lineEdit->setText(path);

        // Put the read content into  TextEdit  Control 
        QFile file(path);// Parameter is the path to read the file 
        // Set the opening mode （ Read or write or something else ）
        file.open(QIODevice::ReadOnly);

        QByteArray array;

        // adopt readline The function reads the contents of the file line by line 
        // adopt atEnd() This method is used to judge whether the end of the file is read 
        while(!file.atEnd())
        {
    
            array += file.readLine();
        }

        ui->textEdit->setText(array);
    });
}

The running result is the same as above （ What I read is utf8 File format ）.

It has been forgotten that a very important thing is not to operate on files , To close the file .

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    
    ui->setupUi(this);
    // Hit the new button , A file dialog box pops up 
    connect(ui->pushButton,&QPushButton::released,[=](){
    
		...
		...
		...
        
        // Close the file object 
        file.close();
    });
}

Write files

If you use write only （ WriteOnly ） To open a file , The contents of the source file will be overwritten . Here we add （Append） To open a file , Write again .

// Set the opening mode （ Open by append , And then write ）
file.open(QIODevice::Append);
file.write(" Work hard to prosper the country ");
file.close();