将 QPixmap 移动到 QLabel 但不复制

Question

Could I move QPixmap into the QLabel? I haven't found any member function which support move semantics or some function could move the QPixmap into the QLabel even in Qt5.The original solution of QLabel is copy the QPixmap you pass into the function.

According to the declaration

setPixmap(QPixmap const &)

Const& wouldn't increase reference count(any mistake?) I don't think they will share the same resource

and the implementation looks more like copy than move(Qt4.8.3)

void QLabel::setPixmap(const QPixmap &pixmap)
{
    Q_D(QLabel);
    if (!d->pixmap || d->pixmap->cacheKey() != pixmap.cacheKey()) {
        d->clearContents();
        d->pixmap = new QPixmap(pixmap);
    }

    if (d->pixmap->depth() == 1 && !d->pixmap->mask())
        d->pixmap->setMask(*((QBitmap *)d->pixmap));

    d->updateLabel();
}

looks like the QLabel will create a brand new copy of the QPixmap if the d->pixmap do not point to anything or they don't have the same cacheKey.

But the behavior I want is something like

QPixmap *temp = origin_pixmap;
origin_pixmap = new_pixmap;
temp->release()

or

std::string A, B;
//.............
A = std::move(B);

Answer 1

Pixmap 实现了Copy On Write概念，所以复制一个像素图并不是一个繁重的操作，只有当你尝试修改原始像素图时才会发生实际的数据复制。