Iterating over a QMap with for

Question

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

QMap extensions;
//.. 

for(auto e : extensions)
{
  fou         


        

Answer 1

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
struct RangeWrapper {
    typedef typename Map::iterator MapIterator;
    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
RangeWrapper

There is another wrapper here.