Problem with deleting QBuffer

aiphae

I have a FileHandler class to handle simple file operations (mainly .csv). I need it to be able to load files both from path (as QFile) and file data (as QBuffer). Here's the crucial part of the class:

#ifndef FILEHANDLER_H
#define FILEHANDLER_H

#include <QFile>
#include <QBuffer>
#include <QTextStream>
#include <QDir>

class FileHandler
{
public:
    FileHandler(const QString &path);
    FileHandler(const QByteArray &data, const QString &path = "");
    ~FileHandler();

private:
    QFile *file = nullptr;
    QBuffer *buffer = nullptr;
    QTextStream in; // For writing into a file
    QTextStream out; // For reading from a file
    QString path;
    bool useBuffer = false;
};

#endif // FILEHANDLER_H

FileHandler.cpp:

FileHandler::FileHandler(const QString &path)
    : path(path)
    , useBuffer(false)
{
    file = new QFile(path);
    out.setDevice(file);
    in.setDevice(file);
}

FileHandler::FileHandler(const QByteArray &data, const QString &path)
    : path(path)
    , useBuffer(true)
{
    buffer = new QBuffer();
    buffer->setData(data);
    out.setDevice(buffer);
    in.setDevice(buffer);
}

FileHandler::~FileHandler() {
    delete file;
    delete buffer;
}

In my widget I have the following code:

QString filename = QFileDialog::getOpenFileName(this, "Open File", QDir::homePath(), filter);
// Create a temporary handler to check if the file can be opened 
FileHandler *newHandler = nullptr;
if (filename.endsWith(".zip", Qt::CaseInsensitive)) {
    // Reading .csv from an archive
    // ...
    // ...
    newHandler = new FileHandler(file.readAll(), zipFilename);
}
else {
    // If .csv file was selected
    newHandler = new FileHandler(filename);
}

if (csvHandler) {
    csvHandler->close();
    csvHandler->clear();
}
delete csvHandler;

csvHandler = newHandler;

The problem arises only when I'm dealing with a FileHandler constructed with QByteArray, so when FileHandler has QBuffer initialized. The program crashes at the line delete csvHandler; with the "is_block_type_valid(header->_block_use)" OR "_crtisvalidheappointer(block)".

It DOES NOT happen when a FileHandler is constructed just with filepath. So as I understood, there is something wrong with deleteting QBuffer().

The archive processing works well and there is absolutely nothing wrong with it.

Pl45m4

@aiphae said in Problem with deleting QBuffer:

Is there a workaround?

I tried this on a release version and it's working well. But what if I want to debug it?

Simple: Link you release build to Qt release libs and your app's debug build to Qt's debug libs.

How did you install Qt?
You should have them.

Christian Ehrlicher

Where do you initialize file and buffer in the ctors? You currently only initialize them in one of the ctors.
Also I don't see a reason why QFile and QBuffer should be created with new instead using plain objects in the class.

SGaist

@Christian-Ehrlicher they are initialized at declaration time to nullptr.

That said, there's indeed no use for pointers here.

aiphae

@Christian-Ehrlicher said in Problem with deleting QBuffer:

Where do you initialize file and buffer in the ctors? You currently only initialize them in one of the ctors.
Also I don't see a reason why QFile and QBuffer should be created with new instead using plain objects in the class.

I want the class to handle EITHER QFile OR QBuffer. Because of this, I decided to make them pointers to save some memory.

UPD: I remade the code so it doesn't use pointers in the FileHandler class. It didn't help. Still the same error.

Christian Ehrlicher

@SGaist said in Problem with deleting QBuffer:

they are initialized at declaration time to nullptr.

~~Where do you see this?~~
Now there is also the header... hmm

Christian Ehrlicher

@aiphae said in Problem with deleting QBuffer:

. Still the same error.

Please provide some minimal, compilable code and a proper backtrace.

aiphae

@Christian-Ehrlicher said in Problem with deleting QBuffer:

@aiphae said in Problem with deleting QBuffer:

. Still the same error.

Please provide some minimal, compilable code and a proper backtrace.

Maybe this:

FileHandler *csvHandler = nullptr;
QVector<QString> files = {"path_1", "path_2"};

for (const auto &path: files) {
    FileHandler *newHandler = nullptr;
    newHandler = new FileHandler(QFile(path).readAll);
    delete csvHandler;
    csvHandler = newHandler;
}

FileHandler.h and FileHandler.cpp are above.

Christian Ehrlicher

This does not crash for me:

int main(int argc, char** argv)
{
    QApplication a(argc, argv);
    FileHandler* csvHandler = nullptr;
    QVector<QString> files = { "existing_file1.txt", "existing_file2.txt" };
    for (const auto& path : files) {
        QFile f(path);
        f.open(QIODevice::ReadOnly);
        auto newHandler = new FileHandler(f.readAll());
        delete csvHandler;
        csvHandler = newHandler;
    }
    delete csvHandler;
    return 0;
}

aiphae

@Christian-Ehrlicher

That's interesting. In my code I tried to replace newHandler = new FileHandler(file.readAll(), zipFilename); with newHandler = new FileHandler(file.readAll()); (without specifying a filename). No crash now. So the issue is not in QBuffer.

zipFilename is QString zipFilename = zip.getCurrentFileName();. I don't understand what's wrong...

aiphae

@Christian-Ehrlicher

Please try this:

int main(int argc, char** argv)
{
    QApplication a(argc, argv);
    FileHandler* csvHandler = nullptr;
    QVector<QString> files = { "existing_file1.txt", "existing_file2.txt" };
    for (const auto& path : files) {
        QFile f(path);
        f.open(QIODevice::ReadOnly);
        auto newHandler = new FileHandler(f.readAll(), path);
        delete csvHandler;
        csvHandler = newHandler;
    }
    delete csvHandler;
    return 0;
}

In this case the error should appear.

Christian Ehrlicher

No crash here and I also don't see a reason why it should crash.

aiphae

Well it crashes when I'm specifying path and it doesn't when I don't... It's weird.

Christian Ehrlicher

As I said before - please post the stacktrace.

aiphae

@Christian-Ehrlicher

This?

Christian Ehrlicher

You are most likely mixing debug and release libs - I would guess you link your debug app against Qt release libs. This is not supported on windows due to different msvc runtimes.

aiphae

@Christian-Ehrlicher said in Problem with deleting QBuffer:

You are most likely mixing debug and release libs - I would guess you link your debug app against Qt release libs. This is not supported on windows due to different msvc runtimes.

Is there a workaround?

I tried this on a release version and it's working well. But what if I want to debug it?

Pl45m4

@aiphae said in Problem with deleting QBuffer:

Is there a workaround?

I tried this on a release version and it's working well. But what if I want to debug it?

Simple: Link you release build to Qt release libs and your app's debug build to Qt's debug libs.

How did you install Qt?
You should have them.

aiphae

@Pl45m4 said in Problem with deleting QBuffer:

Simple: Link you release build to Qt release libs and your app's debug build to Qt's debug libs.

Yes. Apparently I was linking external release libraries instead of debug ones. The issue is solved. Thank you!