Qt5: How to read/write the file in local file system - c++

Question

I'm new to Qt.In my app,I want to press a button and it will come out a QFileDialog to let me select the file in file system .So how to do that?
After that , here is my problem, I don't know which API in Qt works just like "open" in POSIX ? I think if I can open the file in the right way , this API will return me a file descriptor and I can read/write this file like open did in posix.
I read some documents and found some classes such as QFile QDataStream but I don't know if they are exactly what I want.

Answer 1

Those are exactly what you are looking for.
In particular, you can use some of the static methods of QFileDialog to get a reference to the file you want to open, like:
static QString getOpenFileName(QWidget * parent = 0, const QString & caption = QString(), const QString & dir = QString(), const QString & filter = QString(), QString * selectedFilter = 0, Options options = 0)
and then use a QFile and QDataStream or QTextStream to read the contents.
You'd use QDataStream for reading binary data most of the times, like follows:
QFile f(fileName);
if (f.open(QIODevice::ReadOnly)) {
QDataStream stream(&f);
int data;
stream >> data;
}
Otherwise you can read plain text with QTextStream as follows:
QTextStream stream(&f);
QString line;
do {
line = stream.readLine();
/* do something with the line */
} while (!line.isNull());
Qt docs are pretty complete, you just have to take your time and read them. There's also plenty of examples.

Answer 2

Only Reading:
QString fileName = QFileDialog::getOpenFileName(this,
tr("Open file"), "", tr("all Files ()"));
QFile file(fileName);
if(file.open(QIODevice::ReadOnly)){
QByteArray arr = file.readAll();
file.close();
}
Only Writing:
QString fileName = QFileDialog::getOpenFileName(this,
tr("Open file"), "", tr("all Files ()"));
QFile file(fileName);
if(file.open(QIODevice::WriteOnly)){
file.write(QBtyeArray("Heelo World"));
file.close();
}
Read and Write:
QString fileName = QFileDialog::getOpenFileName(this,
tr("Open file"), "", tr("all Files ()"));
QFile file(fileName);
if(file.open(QIODevice::ReadWrite)){
QByteArray arr = file.readAll();
arr += " From Earth";
file.write(arr);
file.close();
}
if you use QDatastream you do not need resolve how many part you have written before,follow Below Code and I always use this method;
QBuffer buffer;
buffer.open(QIODevice::WriteOnly);
QDatastream out(&buffer);
out << QString("Hello World QString");
out << QByteArray("Hello World QByteArray");
out << int(55);
buffer.close();
QFile file(fileName);
if(file.open(QIIDevice::WriteOnly)){
file.write(buffer.data());
file.close();
}
and reading this file
QFile file(fileName);
if(file.open(QIIDevice::WriteOnly)){
QDatastream in(&file);
QString str;
QByteArray arr;
int integer;
in >> str;
in >> arr;
in >> integer;
file.close();
}
str is "Hello World QString";
arr is "Hello World QByteArray";
integer is 55;
QDataStream is adding extra bytes to file for your parts and if you read it with QDataStream, QDataStream solve how many parts and each part bytes instead of you.