How to call a variable defined in one function in another function

Fred Barclay

(Apologies for non-correct terms. I'm not a C++ programmer) :-)

I'm working on a GUI that will take a file chosen by the user and delete it using the srm command from the secure-delete package (on Linux). This is the code in Taz.cpp for the user to choose a file :

void Taz::on_chooseButton_clicked()
{
    QFileDialog dialog(this);
    dialog.setViewMode(QFileDialog::Detail);
    dialog.setFileMode(QFileDialog::ExistingFiles);
    QStringList fileNames;
    if (dialog.exec())
        fileNames = dialog.selectedFiles();
}

So if I understand this correctly, fileNames is the variabel referencing whatever file the user has chosen (or is this incorrect)?
(NOTE: code for this taken from https://doc.qt.io/qt-5.7/qfiledialog.html)
My goal for this is for the user to click the "Choose file" button that references the "onn_chooseButton_clicked() slot, which will open a file dialog from which he/she can select the file(s) to delete. If I understand the code correctly the list of files chosen is stored in the fileNames variable.

Next, I want the user to press the "Shred" button, which will call the srm <chosen_files> command. This is where I'm having trouble. Obviously the files being deleted will be different every time, so I need a variable to substitute. Based on my understanding, I should be able to do something like srm fileNames to delete the file(s). However, I'm having a lot of trouble getting this to work. If I do

void Taz::on_shredButton_clicked()
{
    system(qPrintable("srm -r /home/fred/delete_me"))
}

the code will work perfectly and delete the file delete_me when I press the Shred button on the Gui. However, if I try to assign the fileNames variabel in place of the
absolute path, I get nothing. I've tried various combinations such as
system("srm " << fileNames.c_str());
system(qPrintable("srm "<<fileNames));
system(qPrintable("srm %s", fileName);
or

   QProcess process;
    process.start("srm fileNames" );

None of them will work.
If I were in python I would say this is partly due because fileName is not a globally defined variable, but I can't see if that is the case here in C++/Qt.(That is, void signals a new function in C++, right?)

Thanks for any help! I know my abysmal understanding of C++ is to blame, and I can't wait to get this behind me and start back work on my project. :-)

mrjj

Hi and welcome

From dialog you get a QStringList

http://doc.qt.io/qt-5/qstringlist.html

So you should loop it and call pr file

for (int i = 0; i < fileNames.size(); ++i)
cout << fileNames.at(i);

It would be much better to use QProcess if possible.

Fred Barclay

@mrjj Would this be correct?

void Taz::on_chooseButton_clicked()
{
    QFileDialog dialog(this);
    dialog.setViewMode(QFileDialog::Detail);
    dialog.setFileMode(QFileDialog::ExistingFiles);
    QStringList fileNames;
    if (dialog.exec())
        fileNames = dialog.selectedFiles();
    for (int i = 0; i < fileNames.size(); ++i)
        cout << fileNames.at(i);
}

...

void Taz::on_shredButton_clicked()
{
     QProcess process;
    process.start("srm" <<cout );
}

Currently when I try to make this I get an error:

g++ -c -pipe -O2 -std=gnu++0x -Wall -W -D_REENTRANT -fPIC -DQT_NO_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I. -I/opt/Qt/5.6/gcc_64/include -I/opt/Qt/5.6/gcc_64/include/QtWidgets -I/opt/Qt/5.6/gcc_64/include/QtGui -I/opt/Qt/5.6/gcc_64/include/QtCore -I. -I. -I/opt/Qt/5.6/gcc_64/mkspecs/linux-g++ -o taz.o taz.cpp
taz.cpp: In member function ‘void Taz::on_chooseButton_clicked()’:
taz.cpp:40:9: error: ‘cout’ was not declared in this scope
         cout << fileNames.at(i);
         ^
Makefile:945: recipe for target 'taz.o' failed
make: *** [taz.o] Error 1

How should I declare cout?

Thanks!

jeremy_k

@Fred-Barclay said in How to call a variable defined in one function in another function:

taz.cpp: In member function ‘void Taz::on_chooseButton_clicked()’:
taz.cpp:40:9: error: ‘cout’ was not declared in this scope
cout << fileNames.at(i);
^
Makefile:945: recipe for target 'taz.o' failed
make: *** [taz.o] Error 1
How should I declare cout?

#include <iostream> and change the reference to std::cout to fix the error reported by the compiler, but that's not going to produce useful results. cout is an output file stream. cout << fileName.at(i) will write one file name at a time to standard output, without a newline or space to separate them. The commented-out process.start("srm" <<cout ) definitely isn't right.

You may want to find a basic C++ tutorial such as http://www.cplusplus.com/doc/tutorial/classes/

Fred Barclay

@jeremy_k Thanks - I'll read this more thoroughly in a moment, but the commented-out process.start("srm" <<cout ) is a typo. Fixing now. :)

mrjj

hi
Just a note:
The arguments for QProcess should be put in a list

//Example
QString program = "./path/to/srm";
QStringList arguments;
arguments << "file1" << "file 2";
QProcess myProcess(parent);
myProcess.start(program, arguments);

since fileNames is such list, you can use it directly.
as in
myProcess.start(program, fileNames );

Also sorry for cout. what just to write out the names to check
for (int i = 0; i < fileNames.size(); ++i)
cout << fileNames.at(i); // <<< this prints the entry [i] from the list

Fred Barclay

@mrjj Would this be correct? I'm just trying to get the contents of fileName echoed into a text file - before I try calling srm -r fileNames with another button.

void Taz::on_chooseButton_clicked()
{
// New behavior
    QFileDialog dialog(this);
    dialog.setViewMode(QFileDialog::Detail);
    dialog.setFileMode(QFileDialog::ExistingFiles);
    QStringList fileNames;
    if (dialog.exec())
        fileNames = dialog.selectedFiles();

    QObject *parent;
    QString program = "/bin/echo";
    QStringList arguments;
    arguments << "fileNames" << ">> /home/fred/fn.txt";
    QProcess *myProcess = new QProcess(parent);
    myProcess->start(program, arguments);

}

When compiling with make I get a warning:

taz.cpp: In member function ‘void Taz::on_chooseButton_clicked()’:
taz.cpp:41:46: warning: ‘parent’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     QProcess *myProcess = new QProcess(parent);

The GUI does finish compiling, though. However, nothing is echoed into /home/fred/fn.txt. :(
Thanks!

mrjj

Hi
well, this is not ok.
QObject *parent;
..
QProcess *myProcess = new QProcess(parent);
as parent is a dangling pointer so its invalid to do.
( its just points to random location)
( that is what it means with warning: ‘parent’ may be used uninitialized)
so just use
QProcess *myProcess = new QProcess(this); // this being Taz ( mainwinow i guess)

also, you are not really processing the list
but you should get "fileNames" in the fn.txt?

maybe you can do
fileNames << ">> /home/fred/fn.txt";
and
myProcess->start(program, fileNames );

But in any case
"fileNames" do not expand to the real list as it would in say bash.
it just is text. like "hello".

Fred Barclay

@mrjj Thanks - unallocated error fixed. You're right, I'm definitely not processing the list fileNames. I was just hoping I could get something (even the word "fileNames") to be echoed into a file.

This yields nothing; is it what you are talking about?

void Taz::on_chooseButton_clicked()
{
// New behavior
    QFileDialog dialog(this);
    dialog.setViewMode(QFileDialog::Detail);
    dialog.setFileMode(QFileDialog::ExistingFiles);
    QStringList fileNames;
    if (dialog.exec())
        fileNames = dialog.selectedFiles();

    QString program = "/bin/echo";
    QStringList arguments;
    fileNames << ">> /home/fred/fn.txt";
    QProcess *myProcess = new QProcess(this);
    myProcess->start(program, fileNames );
}

BTW: here is my entire taz.cpp file: https://gist.github.com/Fred-Barclay/f0454d31681fe3744659cd18dfd62403

and here is a screenshot (just in case...)

mrjj

Hi
it looks ok now.

you should use qDebug() to check stuff out
(#include <QDebug>)

and do
qDebug() << " numfiles:" << fileNames.size();
It will show in Creator in output window.
Should give you a non zero value.

also
QProcess has a error() function u should also call to see if any errors.

for test u can also try
QProcess sh;
sh.start("sh", QStringList() << "-c" << "ifconfig" << ">> /home/fred/fn.txt");

SGaist

Hi,

IIRC, you should split >> and /home/fred/fn.txt and add both separately to your argument list.

Fred Barclay

@SGaist Hmm... that would make sense but even then nothing seems to happen when or after I press the button ("Choose File" in the screenshot above):

void Taz::on_chooseButton_clicked()
{
// New behavior
    QFileDialog dialog(this);
    dialog.setViewMode(QFileDialog::Detail);
    dialog.setFileMode(QFileDialog::ExistingFiles);
    QStringList fileNames;
    if (dialog.exec())
        fileNames = dialog.selectedFiles();

    QString program = "/bin/echo";
    QStringList arguments;
    arguments << "fileName" << ">>" << "/home/fred/fn.txt";
    QProcess *myProcess = new QProcess(this);
    myProcess->start(program, arguments);
}

SGaist

You should add some error checking to your code. That would help pinpoint the problem.

jeremy_k

@Fred-Barclay said in How to call a variable defined in one function in another function:

@SGaist Hmm... that would make sense but even then nothing seems to happen when or after I press the button ("Choose File" in the screenshot above):

void Taz::on_chooseButton_clicked()
{
// New behavior
    QFileDialog dialog(this);
    dialog.setViewMode(QFileDialog::Detail);
    dialog.setFileMode(QFileDialog::ExistingFiles);
    QStringList fileNames;
    if (dialog.exec())
        fileNames = dialog.selectedFiles();

    QString program = "/bin/echo";
    QStringList arguments;
    arguments << "fileName" << ">>" << "/home/fred/fn.txt";
    QProcess *myProcess = new QProcess(this);
    myProcess->start(program, arguments);
}

Is the expectation that "fileName" will be appended to the file /home/fred/fn.txt?

Neither echo nor QProcess have special handling for ">>" as far as I am aware. Redirection to a file using this syntax is usually the domain of a shell such as bash. QProcess either redirects output to the enclosing program's output, or makes it available via QProcess::readChannel().

What you'll get instead is the string "fileName >> /home/fred/fn.txt" written to standard output.

#include <QCoreApplication>
#include <iostream>
#include <QProcess>
#include <QObject>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    QProcess p;
    p.setProcessChannelMode(QProcess::ForwardedChannels);
    QStringList arguments;
    arguments << "fileName" << ">>" << "/home/fred/fn.txt";
    QObject::connect<void(QProcess::*)(int)>(&p, &QProcess::finished, [=](){ std::cout << "process done" << std::endl; });
    p.start("/bin/echo", arguments);

    return a.exec();
}

mjsurette

Hi. For a non c++ programmer, you seem to be catching on. Your posting of...

void Taz::on_chooseButton_clicked()
{
// New behavior
    QFileDialog dialog(this);
    dialog.setViewMode(QFileDialog::Detail);
    dialog.setFileMode(QFileDialog::ExistingFiles);
    QStringList fileNames;
    if (dialog.exec())
        fileNames = dialog.selectedFiles();

    QString program = "/bin/echo";
    QStringList arguments;
    arguments << "fileName" << ">>" << "/home/fred/fn.txt";
    QProcess *myProcess = new QProcess(this);
    myProcess->start(program, arguments);
}

isn't too bad. Just a little overly complex, so this

void Taz::on_chooseButton_clicked()
{
// New behavior
    QFileDialog dialog(this);
    dialog.setViewMode(QFileDialog::Detail);
    dialog.setFileMode(QFileDialog::ExistingFiles);
    QStringList fileNames;
    if (dialog.exec())
    {
        fileNames = dialog.selectedFiles();
        QProcess::execute("echo", fileNames);
    }
}

should echo the filenames to your Application Output screen. This is actually a lot closer to what you're end game should look like.

HTH

Mike