doc_src_containers.cpp

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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
 
#undef QT_NO_FOREACH // this file contains unported legacy Q_FOREACH uses
 
class Employee
{
public:
    Employee() {}
    Employee(const Employee &other);
 
    Employee &operator=(const Employee &other);
 
private:
    QString myName;
    QDate myDateOfBirth;
};
 
QList<QString> list = {"A", "B", "C", "D"};
for (const auto &item : list) {
   ...
}
 
QList<QString> list = {"A", "B", "C", "D"};
for (const auto &item : std::as_const(list)) {
    ...
}
 
QList<QString> list = {"A", "B", "C", "D"};
for (qsizetype i = 0; i < list.size(); ++i) {
    const auto &item = list.at(i);
    ...
}
 
QList<QString> list = {"A", "B", "C", "D"};
 
QListIterator<QString> i(list);
while (i.hasNext())
    QString s = i.next();
 
 
QListIterator<QString> i(list);
i.toBack();
while (i.hasPrevious())
    QString s = i.previous();
 
 
QMutableListIterator<int> i(list);
while (i.hasNext()) {
    if (i.next() % 2 != 0)
        i.remove();
}
 
 
QMutableListIterator<int> i(list);
i.toBack();
while (i.hasPrevious()) {
    if (i.previous() % 2 != 0)
        i.remove();
}
 
 
QMutableListIterator<int> i(list);
while (i.hasNext()) {
    if (i.next() > 128)
        i.setValue(128);
}
 
 
QMutableListIterator<int> i(list);
while (i.hasNext())
    i.next() *= 2;
 
 
QMap<QString, QString> map = {
    {"Paris", "France"},
    {"Guatemala City", "Guatemala"},
    {"Mexico City", "Mexico"},
    {"Moscow", "Russia"}
};
...
 
QMutableMapIterator<QString, QString> i(map);
while (i.hasNext()) {
    if (i.next().key().endsWith("City"))
        i.remove();
}
 
 
QMap<int, QWidget *> map;
QHash<int, QWidget *> hash;
 
QMapIterator<int, QWidget *> i(map);
while (i.hasNext()) {
    i.next();
    hash.insert(i.key(), i.value());
}
 
 
QMutableMapIterator<int, QWidget *> i(map);
while (i.findNext(widget))
    i.remove();
 
 
QList<QString> list = {"A", "B", "C", "D"};
 
for (auto i = list.begin(), end = list.end(); i != end; ++i)
    *i = (*i).toLower();
 
 
QList<QString> list = {"A", "B", "C", "D"};
 
for (auto i = list.rbegin(), rend = list.rend(); i != rend; ++i)
    *i = i->toLower();
 
 
for (auto i = list.cbegin(), end = list.cend(); i != end; ++i)
    qDebug() << *i;
 
 
QMap<int, int> map;
...
for (auto i = map.cbegin(), end = map.cend(); i != end; ++i)
    qDebug() << i.key() << ':' << i.value();
 
 
// RIGHT
const QList<int> sizes = splitter->sizes();
for (auto i = sizes.begin(), end = sizes.end(); i != end; ++i)
    ...
 
// WRONG
for (auto i = splitter->sizes().begin();
        i != splitter->sizes().end(); ++i)
    ...
 
 
QList<QString> values;
...
QString str;
foreach (str, values)
    qDebug() << str;
 
 
QList<QString> values;
...
QListIterator<QString> i(values);
while (i.hasNext()) {
    QString s = i.next();
    qDebug() << s;
}
 
 
QList<QString> values;
...
foreach (const QString &str, values)
    qDebug() << str;
 
 
QList<QString> values;
...
foreach (const QString &str, values) {
    if (str.isEmpty())
        break;
    qDebug() << str;
}
 
 
QMap<QString, int> map;
...
foreach (const QString &str, map.keys())
    qDebug() << str << ':' << map.value(str);
 
 
QMultiMap<QString, int> map;
...
foreach (const QString &str, map.uniqueKeys()) {
    foreach (int i, map.values(str))
        qDebug() << str << ':' << i;
}
 
 
CONFIG += no_keywords
 
 
target_compile_definitions(my_app PRIVATE QT_NO_KEYWORDS)
 
 
 
QString onlyLetters(const QString &in)
{
    QString out;
    for (qsizetype j = 0; j < in.size(); ++j) {
        if (in.at(j).isLetter())
            out += in.at(j);
    }
    return out;
}
 
QList<int> a, b;
a.resize(100000); // make a big list filled with 0.
 
QList<int>::iterator i = a.begin();
// WRONG way of using the iterator i:
b = a;
/*
    Now we should be careful with iterator i since it will point to shared data
    If we do *i = 4 then we would change the shared instance (both vectors)
    The behavior differs from STL containers. Avoid doing such things in Qt.
*/
 
a[0] = 5;
/*
    Container a is now detached from the shared data,
    and even though i was an iterator from the container a, it now works as an iterator in b.
    Here the situation is that (*i) == 0.
*/
 
b.clear(); // Now the iterator i is completely invalid.
 
int j = *i; // Undefined behavior!
/*
    The data from b (which i pointed to) is gone.
    This would be well-defined with STL containers (and (*i) == 5),
    but with QList this is likely to crash.
*/
 
QList<int> list = {1, 2, 3, 4, 4, 5};
QSet<int> set(list.cbegin(), list.cend());
/*
    Will generate a QSet containing 1, 2, 3, 4, 5.
*/
 
QList<int> list = {2, 3, 1};
 
std::sort(list.begin(), list.end());
/*
    Sort the list, now contains { 1, 2, 3 }
*/
 
std::reverse(list.begin(), list.end());
/*
    Reverse the list, now contains { 3, 2, 1 }
*/
 
int even_elements =
        std::count_if(list.begin(), list.end(), [](int element) { return (element % 2 == 0); });
/*
    Count how many elements that are even numbers, 1
*/