Read/Write from QTextStream

[tonde]

I've just started with QT and I found something I can't understand.

Why won't this work?

QTextStream out("test",QIODevice::ReadWrite);
    out<<"hello";
    QString string;
    out >> string;

QTextStream out("test",QIODevice::ReadWrite);
    out<<"hello";
    QString string;
    out >> string;

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Only "test" is added to the string. Couldn't find a way to append "hello" to the string.

[jacek]

The docs say:

QTextStream::QTextStream ( const QByteArray & array, QIODevice::OpenMode openMode = QIODevice::ReadOnly )
Constructs a QTextStream that operates on array, using openMode to define the open mode. The array is accessed as read-only, regardless of the values in openMode.

You have to use a different constructor to create an RW stream:

QString buffer( "test" );
QTextStream out( &buffer, QIODevice::ReadWrite );
out << "hello";
QString string;
out >> string; // == "testhello"

QString buffer( "test" );
QTextStream out( &buffer, QIODevice::ReadWrite );
out << "hello";
QString string;
out >> string; // == "testhello"

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If you just want to append a string use QString::operator+() or QString::append().

[tonde]

The docs say:

You have to use a different constructor to create an RW stream:

QString buffer( "test" );
QTextStream out( &buffer, QIODevice::ReadWrite );
out << "hello";
QString string;
out >> string; // == "testhello"

QString buffer( "test" );
QTextStream out( &buffer, QIODevice::ReadWrite );
out << "hello";
QString string;
out >> string; // == "testhello"

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OK, thanks! I tought that const string would use the "QString" constructor. Apparently it uses the "const ByteArray" constructor.

If you just want to append a string use QString:perator+() or QString::append().

My intention was to learn QTextStream...

[tonde]

There is more to come
It seems that if I try to assign QByteArray to the QTextStream QT is once again using the IODevice::ReadOnly constructor. How can I assign QByteArray to the QTextStream in ReadWrite mode? Threre should be one according to the documentation

QString buffer = "test";
QByteArray array;
array.append(buffer);
QTextStream out(&array,QIODevice::ReadWrite);
out << "hello";
QString string;
out >> string; // == "test"

QString buffer = "test";
QByteArray array;
array.append(buffer);
QTextStream out(&array,QIODevice::ReadWrite);
out << "hello";
QString string;
out >> string; // == "test"

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I even get similar problems when using IODevices:

QTemporaryFile file("test");
if (!file.open()) //will always be opened in QIODevice::ReadWrite mode
	return;
QTextStream inOut(&file);
inOut << "hello";
QString s;
inOut >> s; // == ""

QTemporaryFile file("test");
if (!file.open()) //will always be opened in QIODevice::ReadWrite mode
	return;
QTextStream inOut(&file);
inOut << "hello";
QString s;
inOut >> s; // == ""

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What am I missing here?

[jacek]

For the file you need:

QTextStream inOut(&file);
inOut << "hello";
inOut.flush(); // write to file
inOut.device()->seek( 0 ); // rewind the file
QString s;
inOut >> s; // == "hello"

QTextStream inOut(&file);
inOut << "hello";
inOut.flush(); // write to file
inOut.device()->seek( 0 ); // rewind the file
QString s;
inOut >> s; // == "hello"

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Unfortunately I don't know what's wrong with QByteArray case.

[tonde]

Thanks. I missed that seek() function completely. Implementing seek() to the QByteArray case seems to be working.

example:

QString buffer = "funny world";
 
QByteArray array;
array.append(buffer);
QTextStream out(&array,QIODevice::ReadWrite);
 
//out.pos() == 0 
out << "hello"; //"hello" overwrites "funny"
 
out.seek(0); // set position to 0
 
QString string;
QString string2;
 
// stream == "hello world"
// QTextStream reads into strings word by word delimited by space
out >> string; // == "hello" 
out >> string2; // == "world"

QString buffer = "funny world";
	
QByteArray array;
array.append(buffer);
QTextStream out(&array,QIODevice::ReadWrite);
	
//out.pos() == 0 
out << "hello"; //"hello" overwrites "funny"
	
out.seek(0); // set position to 0
	
QString string;
QString string2;

// stream == "hello world"
// QTextStream reads into strings word by word delimited by space
out >> string; // == "hello" 
out >> string2; // == "world"

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[SunnySan]

can we use the seek function with a delimiter?

e.g. |word1|word2|....
with delimiter "|"

[jacek]

can we use the seek function with a delimiter?

No, you have to use QString::indexOf() or similar method.