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I want to loop over a vector of objects each containing another vector of objects to be looped over in turn. The subvector/contained vector object members need to be able to be able to be modified and not copy constructed or implicitly shared. They are accessed from different worker methods. See the code below. Does this work? If not what's the best way to do this?

class Worker : public QObject { public: void backup(); void startScans(); void startBackups(); private: QVector<BackupJob> jobs; } class BackupJob { public: QVector<BackupJob> getSubJobs() { return subJobs; } private: QVector<BackupJob> subJobs; } void Worker::backup() { startScans(); startBackups(); } void Worker::startScans() { for (BackupJob& job : jobs) for (BackupJob& subJob : job.getSubJobs()) //do scans on each subjob here } void Worker::startBackups() { for (BackupJob& job : jobs) for (BackupJob& subJob : job.getSubJobs()) //do backups on each subjob here }
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    You can do it like this, but be aware that getSubJobs() is returning a copy of the vector. Perhaps that's okay with Qt's QVector, as it does have implicit sharing to ensure copy construction is fast. You may want to consider providing a backup() method for BackupJob too, and call that recursively, instead of expecting Worker to deal with the details. That allows BackupJob to stay in control of its private members without needing to make them available via accessors. Commented May 28, 2024 at 0:05
  • Sorry I didn't clarify that I need those subvectors modified from different methods in the Worker object. I edited the question appropriately. Commented May 28, 2024 at 0:23
  • It'd help if you added dummy modifying code. Also, are you modifying the vectors, or the objects in the vector? Commented May 28, 2024 at 12:49
  • I'd consider either making Worker friend of BaxkupJob, or change the getter to getSubJobsRef() which returns a reference to the vector. Commented May 28, 2024 at 12:51
  • I'd also consider changing the vectors to maps, if there is an easy unique id to use as a key. That would make things easier as then each job would have a clear identity, making manipulating it more robust. Commented May 28, 2024 at 12:53

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You didn't provide any clue what //do scans on each subjob here is doing so I will make up something.

Lets say Worker should write this tree of jobs as an xml to some stream. Since tree is large SAX must be used.

Fixing your data structure, so copies are not created. QVector uses COW (Copy On Write), so this is not a big problem, but it is better to be safe then sorry:

class BackupJob { public: const QVector<BackupJob>& getSubJobs() const { return subJobs; } private: QVector<BackupJob> subJobs; };

Worker (renamed):

class JobsXmlWriter : public QObject { public: explicit JobsXmlWriter( QVector<BackupJob> jobs, QObject* parent = nullptr) : QObject{parent} , jobs{std::move(jobs)} {} public slots: void writeInto(QIODevice* out) { QXmlStreamWriter xml{out}; writeInto(&xml); } void writeInto(QXmlStreamWriter* xml) { xml->writeStartDocument(); xml->writeStartElement("jobsTree"); for (const auto& job : jobs) { writeJobRecursive(xml, job); } xml.writeEndElement(); xml.writeEndDocument(); } private: void writeJobRecursive(QXmlStreamWriter* xml, const BackupJob& current) { xml->writeStartElement("job"); writeJobDetail(xml, job); writeJobChildren(xml, job); xml->writeEndElement(); } void writeJobDetail(QXmlStreamWriter* xml, const BackupJob& current) { xml->writeAttribute("name", current.getName()); } void writeJobChildren(QXmlStreamWriter* xml, const BackupJob& current) { xml->writeStartElement("children"); for (const auto& subJob : current.getSubJobs()) { writeJobRecursive(xml, subJob); } xml->writeEndElement(); } private: QVector<BackupJob> jobs; };

As you can see recursion is able to iterate over whole tree and write data to xml.

It is possible to write iterative version, but this is more complex. This may be necessary only if tree of your jobs is very deep (for example tree is unbalanced).

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I see where you're going now, sorry it wasn't clear to me at first. The thing there is only one subJobs vector; it's only one level deep so recursion isn't necessary.

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