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template <typename IteratorT>
static void shiftWeeks(IteratorT begin, IteratorT end)
{
    ....
    static const int DayStep = 7 * (IsReverseIterator<IteratorT> ? -1 : 1);
    ....
}

template <typename, typename = void>
inline constexpr bool IsReverseIterator = false;

template <typename T>
inline constexpr bool IsReverseIterator<T, std::void_t<decltype(std::declval<T>().base())>> = true;

if (!hash.contains(key)) {
    hash.insert(key, 0);
}
hash[key]++;

if (hash.contains(key)) 
    hash[key]++;
else
    hash.insert(key, 1);

hash[key]++;

template <typename IteratorT>
void shift(IteratorT begin, IteratorT end, typename std::iterator_traits<IteratorT>::difference_type offset)

int &value = hash[key];
if (value == 0)
    value = 100;
else
    value++;

void shift(IteratorT begin, IteratorT end, int offset)
{
  for (auto& it = begin; it != end; it++)
  {
    *it = (*it).addDays(offset * 7);
  }
}

void shift(IteratorT begin, IteratorT end, int offset)
{
  while (begin != end)
  {
    *begin++ = (*begin).addDays(offset * 7);
  }
}

class Week
{
public:
	explicit Week(QDate date) : date_(date) {}
	QDate date() const { return date_; }

private:
	QDate date_;
};

	QList<Week*> weeks = {
		new Week(QDate::currentDate().addDays(-7)),
		new Week(QDate::currentDate()),
		new Week(QDate::currentDate().addDays(7))
	};

#include <QtCore/QCoreApplication>
#include <QtCore/QDate>
#include <QtCore/QDebug>
#include <QtCore/QList>
#include <QtCore/QSharedPointer>

class Week
{
public:
	explicit Week(QDate date) : date_(date) { qDebug() << "Week: ctor()"; }
	~Week() { qDebug() << "Week: dtor()"; }

	QDate date() const { return date_; }

private:
	QDate date_;
};

//<<<<<<<<

template <typename, typename = void>
inline constexpr bool IsReverseIterator = false;

template <typename T>
inline constexpr bool IsReverseIterator<T, std::void_t<decltype(std::declval<T>().base())>> = true;

/// Shifts the range of Week*s to left or right on \a offset positions (weeks). The shift direction
/// is determined by the iterators kind - forward ones indicate shifting to left (with insertion on
/// the right side future weeks) and reverse ones - to right (with insertion on the left side past
///  weeks). \a offset may be any value (even greater then the range size). Shifted out Weeks
/// are destroyed, shifted in ones are created with the proper dates.
template <typename IteratorT>
static void shift(IteratorT begin, IteratorT end,
	typename std::iterator_traits<IteratorT>::difference_type offset)
{
	if (Q_UNLIKELY(!offset)) return;

	// std::copy() uses '!=', not '<' for iterator comparison, so values above end are invalid; we
	// decompose offset as offset + offsetExceeding, where the offset < end.
	auto size = std::distance(begin, end);
	auto offsetExceeding = offset <= size ? 0 : offset - size;
	offset -= offsetExceeding;

	// Date step and first shifting into element's date
	static const int DayStep = 7 * (IsReverseIterator<IteratorT> ? -1 : 1);
	QDate date = (*std::prev(end))->date().addDays(DayStep * (offsetExceeding + 1));

	// Shift left/right, shifted out elements are freed
	qDeleteAll(begin, begin + offset);
	auto pos = std::copy(begin + offset, end, begin);

	// Create new elements placing them from left/right side
	for (; pos != end; date = date.addDays(DayStep))
		*pos++ = new Week(date);
}

//>>>>>>>>

// Just syntactic sugar for whole container. Shift direction is determined by offset's sign.
template <typename ContainerT>
static void shift(ContainerT& container, int offset)
{
	if (offset > 0)
		shift(container.begin(), container.end(), offset);
	else if (offset < 0)
		shift(container.rbegin(), container.rend(), -offset);
}

static void print(const QList<Week*>& weeks)
{
	for (auto week : weeks)
		qDebug() << week->date();
}

int main(int argc, char *argv[])
{
	QCoreApplication a(argc, argv);

	QList<Week*> weeks = {
		new Week(QDate::currentDate().addDays(-7)),
		new Week(QDate::currentDate()),
		new Week(QDate::currentDate().addDays(7))
	};
	qDebug() << "\nOriginal:";
	print(weeks);

	qDebug() << "\nShift left on 1:";
	shift(weeks, 1);
	print(weeks);

	qDebug() << "\nShift right on 1, so we get the original range:";
	shift(weeks, -1);
	print(weeks);

	qDebug() << "\nShift left on 4:";
	shift(weeks, 4);
	print(weeks);

	qDebug() << "\nShift right on 4, so we get the original range:";
	shift(weeks, -4);
	print(weeks);

	return a.exec();
}

QT -= gui

CONFIG += c++17 console
CONFIG -= app_bundle

# You can make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

SOURCES += \
        main.cpp

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target