Qt 6: Move QList contents without copying?

[jsulm]

@JonB Sorry, you're of course right about append!

[Christian Ehrlicher]

@JonB said in Qt 6: Move QList contents without copying?:

Could you explain how it could append a list to an existing list without having to copy the elements onto the existing list while still maintaining contiguous memory like an array/vector? I welcome your input :)

It moves the data from memory a to memory b. It's just not a copy but a move operation. Nothing more.

void addList(QList<T>& list) { fullList.append(std::move(list)); }

How should this be moved at all when list is a const ref? You can move a const object...

[JonB]

@Christian-Ehrlicher said in Qt 6: Move QList contents without copying?:

It moves the data from memory a to memory b. It's just not a copy but a move operation. Nothing more.

For this purpose I am calling that "copy" because (does it not?) have to copy the elements (in the QList, not what they might point to) from memory area a to area b? Certainly if iterates that is O(n), I don't know whether you mean CPUs have an instruction to do that fast instead? Think about if it appends a list of 1,000,000 items to an existing list of one item. Moving and appending a list (like a linked list) could be made an O(1) operation, but not with the vector implementation of Qt6 QList, that is my point. I think of "copy" for O(n), "move" to me implies O(1) (hopefully). Is this not correct?

[Christian Ehrlicher]

@JonB Don't look on the QList but it's contents. The content is moved but not copied - so the move ctor is called and not the copy ctor.

[JonB]

@Christian-Ehrlicher said in Qt 6: Move QList contents without copying?:

so the move ctor is called and not the copy ctor.

I never suggested otherwise. My question/answer at least is based on my assumption that the implementation in move constructor will be O(n) and not O(1) at Qt6 QList which requires contiguous memory. Which is what I would care about (at least as part of the whole) if I were asking the OP's question.

[Christian Ehrlicher]

@JonB said in Qt 6: Move QList contents without copying?:

Which is what I would care about (at least as part of the whole) if I were asking the OP's question.

But even then a move would be faster than (or equal if QList contains a pod) a copy. But the complexity stays, yes. But as I said - it can't work the way it is written.

[JonB]

@Christian-Ehrlicher said in Qt 6: Move QList contents without copying?:

But the complexity stays, yes.

OK.

@Christian-Ehrlicher said in Qt 6: Move QList contents without copying?:

How should this be moved at all when list is a const ref? You can move a const object...

@Christian-Ehrlicher said in Qt 6: Move QList contents without copying?:

But as I said - it can't work the way it is written.

Sorry, don't know about others but I'm not understanding where " list is a const ref" here, where is the const? Please be gentle in your answer ;-)
[Also, did you mean "You can move a const object..." or did you mean "can't"?]

[Christian Ehrlicher]

@JonB said in Qt 6: Move QList contents without copying?:

list is a const ref" here, where is the const? Please be gentle in your answer ;-)

void addList(QList<T>& list) { fullList.append(std::move(list)); }

list is const - so nothing to move here.

[Christian Ehrlicher]

Here I somehow was in the wrong path... sorry for the noise:

class CopyMoveTest
{
public:
  CopyMoveTest() { std::cout << "CopyMoveTest: plain ctor\n"; }
  CopyMoveTest(const CopyMoveTest &) { std::cout << "CopyMoveTest: copy ctor\n"; }
  CopyMoveTest(CopyMoveTest &&) { std::cout << "CopyMoveTest: move ctor\n"; }

  CopyMoveTest operator=(const CopyMoveTest &) { std::cout << "CopyMoveTest: copy operator\n"; return *this; }
  CopyMoveTest operator=(const CopyMoveTest &&) { std::cout << "CopyMoveTest: move operator\n"; return *this; }
};

void refCopy(CopyMoveTest &in)
{
  CopyMoveTest c(in);
}

void rValueRefCopy(CopyMoveTest &&in)
{
  CopyMoveTest c(in);
}

void refMove(CopyMoveTest &in)
{
  CopyMoveTest c(std::move(in));
}

void rValueRefMove(CopyMoveTest &&in)
{
  CopyMoveTest c(std::move(in));
}

int main(int , char **)
{
  CopyMoveTest a;
  std::cout << "\nref source copy\n";
  refCopy(a);
  std::cout << "\nref source move\n";
  refMove(a);
  std::cout << "\nrvalue ref source copy\n";
  rValueRefCopy(std::move(a));
  std::cout << "\nrvalue ref source move\n";
  rValueRefMove(std::move(a));
}

-->
CopyMoveTest: plain ctor

ref source copy
CopyMoveTest: copy ctor

ref source move
CopyMoveTest: move ctor

rvalue ref source copy
CopyMoveTest: copy ctor

rvalue ref source move
CopyMoveTest: move ctor

[SimonSchroeder]

@JonB said in Qt 6: Move QList contents without copying?:

Think about if it appends a list of 1,000,000 items to an existing list of one item. Moving and appending a list (like a linked list) could be made an O(1) operation

Well, you are right about that. However, I would expect that something would be done with the data afterwards. Iterating over the elements of a linked list and the elements of a vector is both O(n). Still, in the general case a vector will be faster because of consecutive memory whereas a list might have many cache misses. If iteration is done more than once the vector might be faster (after copying 1,000,000 items). It is a very tricky problem.