1. Conversion between QString, QByteArray, char *
(1) QString -> QByteArray
QString str = "123"; QByteArray byteArray = str.toUtf8(); // 中文 byteArray = str.toLocal8Bit(); // local encoding
(2) QByteArray -> char *
char *pc = byteArray.data();
(3) char * -> QString
char *pc = "abc";
QString qstr = QString("%1").arg(pc);
2. Reading and writing small files
2.1 Read all the data in the file
QString fileName = QString("%1").arg("D:/01.txt");
QFile file(fileName); // path of fileName file
if(file.open(QIODevice::ReadOnly)) // open the file for reading and writing
{
return ;
}
QByteArray data = file.readAll(); // Method 1. Read all the data in the file
int fileSize = file.size(); // Get the length of the file (unit: byte)
data = file.read(fileSize); // Method 2. Read all the data of the file
// Start reading from the specified position
file.seek(100) // This is set to read from position 100
data = file.read(1000) // read 1000 bytes from position 100
file.close(); // Close the file after use
2.2 Read file data line by line
QString fileName = QString("%1").arg("D:/01.txt");
QFile file(fileName); // path of fileName file
if(file.open(QIODevice::ReadOnly)) // open the file for reading
{
return ;
}
while(!file.atEnd()) // Determine whether the file is over
{
// Every time a row of data is read, the cursor seek will automatically jump down, so when the seek reaches the end, atEnd() returns true
QByteArray data = file.readLine();
}
file.close();
2.3 Write data to file
QString fileName = QString("%1").arg("D:/01.txt");
QFile file(fileName); // path of fileName file
if(file.open(QIODevice::WriteOnly)) // open the file for writing
{
return ;
}
QByteArray data;
data.resize(10);
memset(data.data(),0x00,data.size());
file.write(data);
file.close();
2.4 Reading and writing files through memory mapping
uchar *QFileDevice::map(qint64 offset, qint64 size, MemoryMapFlags flags = NoOptions)
① Map the file size bytes into memory from offset. For the mapping to succeed, a file should be open, but the file does not need to remain open after the memory is mapped. Any mappings that have not been unmapped will be automatically unmapped when the QFile is destroyed or a new file is opened using this object.
② The map will have the same open mode (read and/or write) as the file, unless the MapPrivateOption is used, in which case it is always possible to write to the mapped memory.
③ Return a pointer to memory, or return 0 if there is an error.
QString fileName = QString("%1").arg("D:/01.txt");
QFile file(fileName); // path of fileName file
if(file.open(QIODevice::WriteOnly)) // open the file for writing
{
return ;
}
uchar* pfile = file.map(0, file.size()); // returns a pointer to memory, or 0 if there is an error
if(pfile)
{
// do work
// memcpy(pfile,data.data(),size); // write data
// memcpy(data.data(),pfile,size); // read data
file.unmaps(pfile); // To unmap, the QFile object will be automatically unmapped if it is destroyed or a new file is opened
}
file.close();
3. Reading and writing large files
3.1 Split all large files into pieces for reading and writing.
QString fileName = QString("%1").arg("D:/01.txt");
QFile file(fileName); // path of fileName file
if(file.open(QIODevice::WriteOnly)) // open the file for writing
{
return ;
}
qint64 fileSize = file.size(); // length of the file
int readSize = 1024*1024 ; // The length of data read each time can be set freely
// file.pos() gets the position of the file cursor
while(!file.atEnd())
{
QByteArray data = file.read(readSize);
}
file.close();
3.2. Split all large files into read and write line by line.
QString fileName = QString("%1").arg("D:/01.txt");
QFile file(fileName); // path of fileName file
if(file.open(QIODevice::WriteOnly)) // open the file for writing
{
return ;
}
while(!file.atEnd())
{
QByteArray data = file.readLine();
}
file.close();