Iterating over a QMap with for

Question

I've a QMap object and I am trying to write its content to a file.

QMap<QString, QString> extensions;
//.. 

for(auto e : extensions)
{
  fout << e.first << "," << e.second << '\n';
}  

Why do I get: error: 'class QString' has no member named 'first' nor 'second'

Is e not of type QPair?

Answer 1

If you want the STL style with first and second, do this:

for(auto e : extensions.toStdMap())
{
  fout << e.first << "," << e.second << '\n';
}

If you want to use what Qt offers, do this:

for(auto e : extensions.keys())
{
  fout << e << "," << extensions.value(e) << '\n';
}

Answer 2

C++11 range-based-for uses the type of the dereferenced iterator as the automatically deduced "cursor" type. Here, it is the type of the expression *map.begin().
And since QMap::iterator::operator*() returns a reference to the value (of type QString &), the key isn't accessible using that method.

You should use one of the iterator methods described in the documentation but you should avoid using

• keys() because it involves creating a list of keys and then searching the value for each key, or,
• toStdMap() because it copies all the map elements to another one,

and that wouldn't be very optimal.

---

You could also use a wrapper to get QMap::iterator as the auto type:

template<class Map>
struct RangeWrapper {
    typedef typename Map::iterator MapIterator;
    Map &map;

    RangeWrapper(Map & map_) : map(map_) {}

    struct iterator {
        MapIterator mapIterator;
        iterator(const MapIterator &mapIterator_): mapIterator(mapIterator_) {}
        MapIterator operator*() {
            return mapIterator;
        }
        iterator & operator++() {
            ++mapIterator;
            return *this;
        }
        bool operator!=(const iterator & other) {
            return this->mapIterator != other.mapIterator;
        }
    };
    iterator begin() {
        return map.begin();
    }
    iterator end() {
        return map.end();
    }
};

// Function to be able to use automatic template type deduction
template<class Map>
RangeWrapper<Map> toRange(Map & map)
{
    return RangeWrapper<Map>(map);
}

// Usage code
QMap<QString, QString> extensions;
...
for(auto e : toRange(extensions)) {
    fout << e.key() << "," << e.value() << '\n';
}

There is another wrapper here.

Answer 3

For people interested in optimizations, I have tried several approaches, did some micro benchmarks, and I can conclude that STL style approach is significantly faster.

I have tried adding integers with these methods :

• QMap::values()
• Java style iterator (as advised in the documentation)
• STL style iterator (as advised in the documentation too)

And I have compared it with summing integers of a QList/QVector

Results :

Reference vector :   244  ms
Reference list :     1239  ms

QMap::values() :     6504  ms
Java style iterator :    6199  ms
STL style iterator :     2343  ms

Code for those interested :

#include <QDateTime>
#include <QMap>
#include <QVector>
#include <QList>
#include <QDebug>

void testQMap(){
    QMap<int, int> map;
    QVector<int> vec;
    QList<int> list;

    int nbIterations = 100;
    int size = 1000000;
    volatile int sum = 0;

    for(int i = 0; i<size; ++i){
        int randomInt = qrand()%128;
        map[i] = randomInt;
        vec.append(randomInt);
        list.append(randomInt);
    }


    // Rererence vector/list
    qint64 start = QDateTime::currentMSecsSinceEpoch();
    for(int i = 0; i<nbIterations; ++i){
        sum = 0;
        for(int j : vec){
            sum += j;
        }
    }
    qint64 end = QDateTime::currentMSecsSinceEpoch();
    qDebug() << "Reference vector : \t" << (end-start) << " ms";

    qint64 startList = QDateTime::currentMSecsSinceEpoch();
    for(int i = 0; i<nbIterations; ++i){
        sum = 0;
        for(int j : list){
            sum += j;
        }
    }
    qint64 endList = QDateTime::currentMSecsSinceEpoch();
    qDebug() << "Reference list : \t" << (endList-startList) << " ms";

    // QMap::values()
    qint64 start0 = QDateTime::currentMSecsSinceEpoch();
    for(int i = 0; i<nbIterations; ++i){
        sum = 0;
        QList<int> values = map.values();
        for(int k : values){
            sum += k;
        }
    }
    qint64 end0 = QDateTime::currentMSecsSinceEpoch();
    qDebug() << "QMap::values() : \t" << (end0-start0) << " ms";


    // Java style iterator
    qint64 start1 = QDateTime::currentMSecsSinceEpoch();
    for(int i = 0; i<nbIterations; ++i){
        sum = 0;
        QMapIterator<int, int> it(map);
        while (it.hasNext()) {
            it.next();
            sum += it.value();
        }
    }
    qint64 end1 = QDateTime::currentMSecsSinceEpoch();
    qDebug() << "Java style iterator : \t" << (end1-start1) << " ms";


    // STL style iterator
    qint64 start2 = QDateTime::currentMSecsSinceEpoch();
    for(int i = 0; i<nbIterations; ++i){
        sum = 0;
        QMap<int, int>::const_iterator it = map.constBegin();
        auto end = map.constEnd();
        while (it != end) {
            sum += it.value();
            ++it;
        }
    }
    qint64 end2 = QDateTime::currentMSecsSinceEpoch();
    qDebug() << "STL style iterator : \t" << (end2-start2) << " ms";


    qint64 start3 = QDateTime::currentMSecsSinceEpoch();
    for(int i = 0; i<nbIterations; ++i){
        sum = 0;
        auto end = map.cend();
        for (auto it = map.cbegin(); it != end; ++it)
        {
            sum += it.value();
        }
    }
    qint64 end3 = QDateTime::currentMSecsSinceEpoch();

    qDebug() << "STL style iterator v2 : \t" << (end3-start3) << " ms";
}

Edit July 2017 : I ran this code again on my new laptop (Qt 5.9, i7-7560U) and got some interesting changes

Reference vector :   155  ms 
Reference list :     157  ms
QMap::values():      1874  ms 
Java style iterator: 1156  ms 
STL style iterator:  1143  ms

STL style and Java style have very similar performances in this benchmark

Answer 4

QMap::iterator uses key() and value() - which can be found easily in the documentation for Qt 4.8 or the documentation for Qt-5.

Edit:

A range-based for loop generates codes similar to this (see CPP reference):

{
    for (auto __begin = extensions.begin(), __end = extensions.end();
            __begin != __end; ++__begin) {
        auto e = *__begin; // <--- this is QMap::iterator::operator*()
        fout << e.first << "," << e.second << '\n';
    }
} 

QMap::iterator::iterator*() is equivalent to QMap::iterator::value(), and does not give a pair.

The best way to write this is without range-based for loop:

auto end = extensions.cend();
for (auto it = extensions.cbegin(); it != end; ++it)
{
    std::cout << qPrintable(it.key()) << "," << qPrintable(it.value());
}

Answer 5

In "old" C++, using Qt, you would do it like this:

QMap< QString, whatever > extensions;
//...
foreach( QString key, extensions.keys() )
{
    fout << key << "," << extensions.value( key ) << '\n';
}

I don't have a C++11 compiler here but maybe the following will work:

for( auto key: extensions.keys() )
{
    fout << key << "," << extensions.value( key ) << '\n';
}

You can also use iterators instead, check out hmuelners link if you prefer using them

Answer 6

Since Qt 5.10 you can use a simple wrapper class to use a range based for loop, but still be able to access both the key and value of the map entries.

Put the following code somewhere at the top of your source file or in a header that you include:

template<class K,class V>
struct QMapWrapper {
    const QMap<K,V> map;
    QMapWrapper(const QMap<K,V>& map) : map(map) {}
    auto begin() { return map.keyValueBegin(); }
    auto end()   { return map.keyValueEnd();   }
};

To iterate over all entries you can simply write:

QMap<QString, QString> extensions;
//.. 

for(auto e : QMapWrapper(extensions))
{
  fout << e.first << "," << e.second << '\n';
}

The type of e will be std::pair<const QString&, const QString&> as is partially specified in the QKeyValueIterator documentation.

The member variable map is an implicitly shared copy of the map, to avoid a segmentation fault in case this is used with temporary values. Hence as long as you do not modify the map within the loop, this only has a small constant overhead.

---

The above example uses class template argument deduction, which was introduced in C++17. If you're using an older standard, the template parameters for QMapWrapper must be specified when calling the constructor. In this case a factory method might be useful:

template<class K,class V>
QMapWrapper<K,V> wrapQMap(const QMap<K,V>& map) {
    return QMapWrapper<K,V>(map);
}
    

This is used as:

for(auto e : wrapQMap(extensions))
{
  fout << e.first << "," << e.second << '\n';
}

Answer 7

Ivan Čukić of KDAB has a blog post that explains how to iterate over a QMap with C++17 structured bindings without copying the container:

template <typename T>
class asKeyValueRange
{
public:
    asKeyValueRange(T& data) : m_data{data} {}

    auto begin() { return m_data.keyValueBegin(); }

    auto end() { return m_data.keyValueEnd(); }

private:
    T& m_data;
};

...

QMap<QString, QString> extensions;

for (auto&& [key, value]: asKeyValueRange(extensions))
{
    fout << key << ',' << value << '\n';
} 

Answer 8

Another convenient method, from the QMap Docs. It allows explicit access to key and value (Java-Style iterator):

QMap<QString, QString> extensions;
// ... fill extensions
QMapIterator<QString, QString> i(extensions);
while (i.hasNext()) {
    i.next();
    qDebug() << i.key() << ": " << i.value();
}

In case you want to be able to overwrite, use QMutableMapIterator instead.

There's another convenient Qt method, if you're only interested in reading the values, without the keys (using Qts foreach and c++11):

QMap<QString, QString> extensions;
// ... fill extensions
foreach (const auto& value, extensions)
{
    // to stuff with value
}

Answer 9

I used something like this, to achieve my own result. Just in case someone needed the keys and values separately.

{
   QMap<int,string> map; 

   map.insert(1,"One");
   map.insert(2,"Two");
   map.insert(3,"Three");
   map.insert(4,"Four");   

   fout<<"Values in QMap 'map' are:"<<endl;
   foreach(string str,map)
   {
     cout<<str<<endl;
   };


   fout<<"Keys in QMap 'map' are:"<<endl;
   foreach(int key,map.keys())
   {
     cout<<key<<endl;
   }; 
}  

Answer 10

Since Qt 6.4 you can use the method asKeyValueRange() as follows:

for(auto pair : extensions.asKeyValueRange()) {
    pair.first;    // key access
    pair.second;   // value access
}

This even works for the fancy structured bindings.

for(auto& [key, value] : extensions.asKeyValueRange()) {
    fout << key << "," << value << '\n';
}