With the following piece of code:
QString msg;
msg = "Datalogging Item" + ',' +
QString::number(item) + ',' +
"Slave Index" + ',' +
QString::number(slaveIndex) + ',' +
"Slave Name" + ',' +
slaveName + ',' +
"Experiment Index" + ',' +
QString::number(expIndex) + ',' +
"Experiment Name" + ',' +
expName + ',' +
"Aquisition Frequency " + ',' +
"33 Hz" + "\n";
qDebug() << msg;
I was getting the following debug output
"riment Index0,Slave Index,0,Slave Name,Execute 1,Experiment Index,0,Experiment Name,Read All (Barebone),Aquisition Frequency ,33 Hz\n"
Which is not really what I was expected to get.
But with the code modified with the QString typecast:
QString msg;
msg = QString("Datalogging Item") + QString(',') +
QString::number(item) + QString(',') +
QString("Slave Index") + QString(',') +
QString::number(slaveIndex) + QString(',') +
QString("Slave Name") + QString(',') +
slaveName + QString(',') +
QString("Experiment Index") + QString(',') +
QString::number(expIndex) + QString(',') +
QString("Experiment Name") + QString(',') +
expName + QString(',') +
QString("Aquisition Frequency ")+ QString(',') +
QString("33 Hz") + QString("\n");
qDebug() << msg;
I get what I expected to get:
"Datalogging Item,0,Slave Index,0,Slave Name,Execute 1,Experiment Index,0,Experiment Name,Read All (Barebone),Aquisition Frequency ,33 Hz\n"
Do you have any clue on what is going on? I just don't get what is going on and I'm a newbie to QT.
Thanks.
The expression
"Datalogging Item" + ','
has the type and meaning of
static_cast<const char *>("Datalogging Item" + 44)
You're taking the address of the beginning of the Datalogging string constant and adding 44 to it - advancing it by 44 characters. This is undefined behavior as you're moving a pointer past the object it points to. By coincidence the pointer happens to point to another C string constant.
A short program that reproduces the issue:
#include <iostream>
int main() {
std::cout << "Datalogging Item Foo Bar Baz" + '\n' << std::endl;
}
Output:
g Item Foo Bar Baz
One way of rewriting your expression to make it valid and less verbose would be to use the arg-substitution mechanism of QString:
auto msg = QStringLiteral(
"Datalogging Item,%1,Slave Index,%2,Slave Name,%3,Experiment Index,%4,"
"Experiment Name,%5,Acquisition Frequency,%6\n")
.arg(item).arg(slaveIndex).arg(slaveName).arg(expIndex)
.arg(expName).arg(QStringLiteral("33 Hz"));
If you're using Qt 4, replace QStringLiteral with QString::fromUtf8. The QStringLiteral is a mechanism introduced in Qt 5 that builds up a constant QString instance at compile time, yielding smaller code and better runtime performance than the use of a QString constructor.