CODEWITHSUNDEEP
c++qt
user2094257
user2094257

Reputation: 1715

QT container class issue

I am trying to create a function that returns a QStringList of companies which were originally stored in a QList<company*> list. When I try to add items to the QStringList, I get the following error:

C:\Qt\Qt5.3.0\Tools\QtCreator\bin\test\companylist.cpp:13: error: passing 'const QStringList' as 'this' argument of 'QList<T>& QList<T>::operator+=(const T&) [with T = QString]' discards qualifiers [-fpermissive]
     m_sortedList += m_companyList.at(i)->toString();
                  ^

Any idea what i'm doing wrong? i've also tried to use m_sortedList.append() with no luck...

My code:

QStringList CompanyList::getCompanyList() const {
    for (int i = 0; i <= m_companyList.length(); ++i) {
        m_sortedList += m_companyList.at(i)->toString();
    }
    m_sortedList.sort();
    return m_sortedList;
}

Upvotes: 0

Views: 58

Answers (1)

Kuba hasn&#39;t forgotten Monica
Kuba hasn&#39;t forgotten Monica

Reputation: 98435

Within getCompanyList() const, all members are const. Thus, the type of m_sortedList is const QStringList and you can't modify it.

There's no reason to have m_sortedList as a member, though, since you always overwrite it. Moreover, you never seem to clean it. If you'd call a (non-const) getCompanyList more than once, you'd get a list with duplicated entries.

To avoid a premature pessimization when growing a list that will have a known size, you should ensure that it has room for enough elements by calling reserve.

What you're looking for is the typical local return value idiom:

// C++11
QStringList CompanyList::getCompanyList() const {
  QStringList result;
  result.reserve(m_companyList.size());
  for (auto c : m_companyList) result << c->toString();
  std::sort(result.begin(), result.end());
  return result;
}

// C++03, this is a more declarative style loved by some OSS projects :)
QStringList CompanyList::getCompanyList() const {
  QStringList result;
  result.reserve(m_companyList.size());
  std::transform(c.begin(), c.end(), std::back_inserter(result),
                 std::mem_fn(&company::toString));
  std::sort(result.begin(), result.end());
  return result;
}

// C++98
QStringList CompanyList::getCompanyList() const {
  QStringList result;
  result.reserve(m_companyList.size());
  foreach (company * c, m_companyList) result << c->toString(); // or Q_FOREACH
  std::sort(result.begin(), result.end());
  return result;
}

Upvotes: 3

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