Stack Overflow Asked by Vencat on January 18, 2021
I have to transform a QMap<QString, int>. I tried to use std::transform as it works for QList, but it’s failed to compile.
#include <QCoreApplication>
#include <algorithm>
#include <QMap>
int main(int argc, char *argv[]) {
QCoreApplication a(argc, argv);
QMap<QString, int> map;
map["one"] = 1;
map["three"] = 3;
map["seven"] = 7;
QMap<QString, int> map2;
std::transform(map.begin(), map.end(), std::back_inserter(map2), [](const QPair<QString, int>& item) {
return qMakePair(item.first+"testing", item.second);
});
return a.exec();
}
I’m getting the below error while compiling the above code. I would like to achieve this without general for loop as my project using cppcheck and it throws warnings.
/usr/include/c++/7/bits/stl_iterator.h:490: error: no type named ‘value_type’ in ‘class QMap<QString, int>’
In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
from /usr/include/c++/7/algorithm:61,
from ../../../../Qt5.12.4/5.12.4/gcc_64/include/QtCore/qglobal.h:142,
from ../../../../Qt5.12.4/5.12.4/gcc_64/include/QtCore/qcoreapplication.h:43,
from ../../../../Qt5.12.4/5.12.4/gcc_64/include/QtCore/QCoreApplication:1,
from ../test/main.cpp:1:
/usr/include/c++/7/bits/stl_iterator.h: In instantiation of ‘class std::back_insert_iterator<QMap<QString, int> >’:
../test/main.cpp:15:65: required from here
/usr/include/c++/7/bits/stl_iterator.h:490:7: error: no type named ‘value_type’ in ‘class QMap<QString, int>’
operator=(const typename _Container::value_type& __value)
^~~~~~~~
/usr/include/c++/7/bits/stl_algo.h:4306: error: no match for call to ‘(main(int, char**)::<lambda(const QPair<QString, int>&)>) (int&)’
In file included from /usr/include/c++/7/algorithm:62:0,
from ../../../../Qt5.12.4/5.12.4/gcc_64/include/QtCore/qglobal.h:142,
from ../../../../Qt5.12.4/5.12.4/gcc_64/include/QtCore/qcoreapplication.h:43,
from ../../../../Qt5.12.4/5.12.4/gcc_64/include/QtCore/QCoreApplication:1,
from ../test/main.cpp:1:
/usr/include/c++/7/bits/stl_algo.h: In instantiation of ‘_OIter std::transform(_IIter, _IIter, _OIter, _UnaryOperation) [with _IIter = QMap<QString, int>::iterator; _OIter = std::back_insert_iterator<QMap<QString, int> >; _UnaryOperation = main(int, char**)::<lambda(const QPair<QString, int>&)>]’:
../test/main.cpp:17:4: required from here
/usr/include/c++/7/bits/stl_algo.h:4306:24: error: no match for call to ‘(main(int, char**)::<lambda(const QPair<QString, int>&)>) (int&)’
*__result = __unary_op(*__first);
~~~~~~~~~~^~~~~~~~~~
/usr/include/c++/7/bits/stl_algo.h:4306:24: note: candidate: QPair<QString, int> (*)(const QPair<QString, int>&) <conversion>
/usr/include/c++/7/bits/stl_algo.h:4306:24: note: candidate expects 2 arguments, 2 provided
../test/main.cpp:15:102: note: candidate: main(int, char**)::<lambda(const QPair<QString, int>&)>
std::transform(map.begin(), map.end(), std::back_inserter(map2), [](const QPair<QString, int>& item) {
^
../test/main.cpp:15:102: note: no known conversion for argument 1 from ‘int’ to ‘const QPair<QString, int>&’
A major problem is std::back_inserter(map2) wants to call push_back, which is not a member of QMap (nor std::map).
You also have the issue that the standard inserters want a value_type defined in the container, that QMap doesn't have. In older versions, QMap::value_type was used for a different meaning to std::map::value_type.
You could define an insert iterator yourself
#include <QCoreApplication>
#include <algorithm>
#include <QMap>
template <typename K, typename T>
struct QMapInsertIterator {
QMap<K, T> & map;
QMapInsertIterator& operator=(const QPair<K, T> & pair) { map.insert(pair.first, pair.second); return *this; }
QMapInsertIterator& operator++() { return *this; }
QMapInsertIterator& operator*() { return *this; }
};
template <typename K, typename T>
QMapInsertIterator<K, T> QMapInserter(QMap<K, T> & map) {
return { map };
}
Which directly replaces std::back_inserter
int main(int argc, char *argv[]) {
QCoreApplication a(argc, argv);
QMap<QString, int> map;
map["one"] = 1;
map["three"] = 3;
map["seven"] = 7;
QMap<QString, int> map2;
std::transform(map.begin(), map.end(), QMapInserter(map2), [](const QPair<QString, int>& item) {
return qMakePair(item.first+"testing", item.second);
});
return a.exec();
}
Correct answer by Caleth on January 18, 2021
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