It appears that QDirModel uses rows to store directory entries, which means that columns are not useful at all. So I have a simple hierarchy:
Qt Code: 
  1. ls -R
  2.  
  3. A	C
  4.  
  5. ./A:
  6. B
  7.  
  8. ./A/B:
  9.  
  10. ./C:
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If I run the following code to browse the hierarchy:
Qt Code: 
  1. void
  2. MyClass::Browse(QModelIndex idx, int indent)
  3. {
  4.   QFileInfo info = this->fileInfo(idx);
  5.   for (int i=indent ; i>0 ; i--)
  6.     this->Print(" \r");
  7.   this->Print("%d %d %s", idx.row(), idx.column(), info.fileName().toAscii().constData());
  8.   for (int i=0 ; idx.child(i, 0).isValid() ; i++)
  9.     this->ComputeSizes(idx.child(i, 0), indent+2);
  10. }
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I get the expected output:
Qt Code: 
  1. MyClass: 38 0 test
  2. MyClass:   0 0 A
  3. MyClass:     0 0 B
  4. MyClass:   1 0 C
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But if I change the code so as to browse columns rather that rows as follow:
Qt Code: 
  1. void
  2. MyClass::Browse(QModelIndex idx, int indent)
  3. {
  4.   QFileInfo info = this->fileInfo(idx);
  5.   for (int i=indent ; i>0 ; i--)
  6.     this->Print(" \r");
  7.   this->Print("%d %d %s", idx.row(), idx.column(), info.fileName().toAscii().constData());
  8.   for (int i=0 ; idx.child(0, i).isValid() ; i++)
  9.     this->ComputeSizes(idx.child(0, i), indent+2);
  10. }
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I get an unexpected output:
Qt Code: 
  1. MyClass: 38 0 test
  2. MyClass:   0 0 A
  3. MyClass:     0 0 B
  4. MyClass:     0 1 B
  5. MyClass:     0 2 B
  6. MyClass:     0 3 B
  7. MyClass:   0 1 A
  8. MyClass:   0 2 A
  9. MyClass:   0 3 A
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I would have expected that idx.child(0, i>0) be unvalid!

I have one more question: what is the difference between QDirModel::sibling() and QDirModel::child()?