QVector move semantics

Jimmy Loyola

Regarding Qt 4
I guess first is copied and second is moved

QVector<T> a;
//...
// a is copied or moved in b?
QVector<T> b = a;

Another case

QVector<T> get()
{
  QVector<T> k;
    //...
  return k;
}

// k is copied or moved in x?
QVector<T>  x = get();

jsulm

@Jimmy-Loyola I don't think there is move constructor in QVector in Qt4.
But this is not that important in Qt containers as they use implicit sharing: https://doc.qt.io/qt-5/implicit-sharing.html

Pl45m4

Also:
( = operator)
https://doc.qt.io/qt-5/qvector.html#operator-eq

Move-ctor first in Qt 5.2
https://doc.qt.io/qt-5/qvector.html#QVector-4

Jimmy Loyola

@jsulm I've got it. Thank you.

J.Hilk

@Jimmy-Loyola also, if you explicitly want to move, there's always
https://en.cppreference.com/w/cpp/utility/move

that, as a template, can be applied to QtContainers, if they have a move constructor

Pl45m4

@J-Hilk said in QVector move semantics:

if they have a move constructor

QVector doesnt seem to have one in Qt 4 :)
Docs say, it was introduced in Qt 5.2.

SimonSchroeder

In the first case

QVector<T> b = a;

you are requesting a copy. As others have mentioned implicit sharing will not copy the data at this point, but it is basically a copy on write (if either a or b are changing the data). If you want to have a move you need to specify it explicitely:

QVector<T> b = std::move(a);

This depends on the Qt version if there is an actual move constructor available. This is general C++11 and not specific to Qt. Note that

QVector<T> b = std::move(a);

uses the move constructor if available (even though there is a = on this line), whereas

b = std::move(a);

uses move assignment (i.e. operator=) if available.

The second case is a little trickier to answer. This has to do with return value optimization (allowed for long) and copy elision (allowed in C++11 and required in C++14 or C++17 – I don't remember). With copy elision compilers with the newest standard with neither copy nor move the variable in

QVector<T> x = get();

Instead, get()'s k-variable is directly constructed inside x's memory location (https://en.cppreference.com/w/cpp/language/copy_elision). I hope, I don't mix up 'required' and 'optional' here.

There is a third variant to consider:

QVector<T> get()
{
  QVector<T> k;
  //...
  return std::move(k);
}

QVector<T> x = get();

This is a mistake people needed to be reminded of when C++11 was new. In the case presented by you return value optimization is possible. As soon as you add std::move in the return statement, a move is required. Instead of helping the compiler you are taking away the possibility for optimization.

Jimmy Loyola

@SimonSchroeder Your answer is very interesting. I will go into this.
Having all these things together can cause some confusion.
Thank you!