QList problem

[lvi]

I have a really "funny" (not really) problem involving QList.

I have a class File that, as a member, has a QList<Part>:

class File {
    public:
        QList<Part> parts;
}

class File {
    public:
        QList<Part> parts;
}

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A file object is passed to a worker thread for processing:

class Decoder : public QThread {
    public:
        Decoder(File f) { file = f; }
        void run();
    private:
        File file;
}

class Decoder : public QThread {
    public:
        Decoder(File f) { file = f; }
        void run();
    private:
        File file;
}

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In the run() function, I construct the file from the parts that are associated with that file:

void Decoder::run() {
    QList<Part>::iterator partIter;
    for (partIter = file.parts.begin(); partIter != file.parts.end(); partIter++) {
        // do stuff with *partIter
    }
}

void Decoder::run() {
    QList<Part>::iterator partIter;
    for (partIter = file.parts.begin(); partIter != file.parts.end(); partIter++) {
        // do stuff with *partIter
    }
}

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(simplified representation)

This works perfectly fine if parts holds more than 1 part. If the number of parts is 1, the loop is done twice and of course, I get a segfault in the second iteration.

For completeness I should mention that file has been passed around by signal/slot connections, e.g. someSlot(const File f).

I have no clue why this could be happening. Any ideas?

[tommydent]

Hi,

what exactly is:

// do stuff with *partIter

// do stuff with *partIter

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do you invalidate the iterator somehow?

[lvi]

The only line involving *partIter is this:

QFile partFile((*partIter).filename);
// read data from partFile and use this

QFile partFile((*partIter).filename);
// read data from partFile and use this

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I don't see how this could possibly invalidate partIter. Also, that would not explain why it works with >1 parts in the QList...

[lvi]

Changing

QList<Part>::iterator partIter;

QList<Part>::iterator partIter;

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to

QList<Part>::const_iterator partIter;

QList<Part>::const_iterator partIter;

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seems to solve the problem.
I overlooked this:

emit(fileStatus(file, status));

emit(fileStatus(file, status));

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Even though I thought declaring the signal as fileStatus(const File, int) would prevent trouble, I highly suspect this line was the troublemaker...

I'm still not sure why declaring the const_iterator solved the problem (or did I just hide the problem?).