Sort actual QMap by key

Question

I have QMap<QString, MyClass*>. I need to sort it by key using natural comparison. I do:

std::map<QString, MyClass*> map = c.toStdMap();
std::sort(map.begin(), map.end(), strnatcmp1<std::pair<QString, MyClass*>>);

However, this does not even compile. And even if it did, as far as I understand, it would sort a copy of my original QMap.

Is there any way to sort my QMap by key with the function for comparison keys provided?

Answer 1

You have several appoaches that you could take:

• Store the keys as integers after converting QString to an integer.

• You can use the compare class for std::map as per the example:

• You can try to use specialize this template function that QMap uses for the comparison.

main.cpp

#include <QMap>
#include <QDebug>
#include <map>


struct str_num_compare {
  bool operator() (const QString& lhs, const QString& rhs) const
  {return lhs.toInt()<rhs.toInt();}
};

int main()
{
    QStringList stringList{"1", "10", "11", "2", "3", "4"};

    QMap<int, QString> map;
    foreach (const QString &string, stringList)
        map.insert(string.toInt(), string);
    qDebug() << "Integer key approach:" << map;

    std::map<QString, QString, str_num_compare> std_map;
    foreach (const QString &string, stringList)
        std_map[string] = string;
    qDebug() << "QString key approach with std::map:";
    for (auto item : std_map)
        qDebug() << item.first;


    return 0;
}

main.pro

TEMPLATE = app
TARGET = main
QT = core
CONFIG += c++11
SOURCES += main.cpp

Build and Run

qmake && make && ./main

Output

Integer key approach: QMap((1, "1")(2, "2")(3, "3")(4, "4")(10, "10")(11, "11"))
QString key approach:
"1"
"2"
"3"
"4"
"10"
"11"