Individual container based iteration

Question

I have a custom container that I want to use in a loop for a range. The container is somewhat based on a vector, for example:

template<typename T>
class IDMap
{
private:
    struct Item {
        uint16_t mVersion;
        T mItem;

        template <typename... Arguments>
        Item(uint16_t version, Arguments&&... args) : mVersion(version), mItem(args...)
        {
        }
    };


public:
    typedef uint32_t ItemID;

    template <typename... Arguments>
    ItemID AddItem(Arguments&&... args);

    void MarkAsFree(const ItemID id);

    T& GetItem(const ItemID id);

    T* TryGetItem(const ItemID id);

    void Clear();


private:
    std::vector<Item> mItems;
    std::vector<uint16_t> mFreeIndices;
};

I want to repeat the vector mItems, but return only the element mItem, not the whole structure Item. Is there a simple / elegant way to do this?

Answer 1

You must provide a function beginand endthat returns the corresponding iterator, which itself implements the operators ++, !=and *. Functions beginand endcan be free or members.

, , . std::vector::iterator, "" .

, IDMap :

class ItemIterator {
    // based on vector iterator
    std::vector<Item>::iterator i;
public:
    ItemIterator(std::vector<Item>::iterator i) : i(i) {}

    // incrementing
    ItemIterator & operator ++() { ++i; return *this; }
    ItemIterator operator ++(int) { const_iterator old(*this); ++(*this); return old; }

    // comparison
    bool operator!=(const ItemIterator &o) const { return i != o.i; }

    // dereferencing
    const T & operator*() const { return i->mItem; }
};

using iterator = ItemIterator;
using value_type = T;

ItemIterator begin() const { return ItemIterator(mItems.begin()); }
ItemIterator end()   const { return ItemIterator(mItems.end()  ); }

" " , , , "" Item s, . -, .items().

:

class IDMap {
    // (your code)

public:
    struct ItemsAdaptor {
        // (insert above iterator definition + usings)

        ItemsAdaptor(std::vector<Item>::iterator b,
                     std::vector<Item>::iterator e)
            : b{b}, e{e}
        {}

        ItemIterator begin() const { return b; }
        ItemIterator end()   const { return e; }

    private:
        ItemIterator b, e;
    };

    ItemsAdaptor items() const {
        return ItemsAdaptor(mItems.begin(), mItems.end());
    }
};

:

IDMap<int> map = ...;

for (int i : map.items()) {
    ...
}

Answer 2

, , , .

typedef std::vector<Item>::iterator iterator;
typedef std::vector<Item>::const_iterator const_iterator;
iterator begin()
{
   return mItems.begin();
}
const_iterator begin() const;
{
   return mItems.begin();
}
//also add end functions, and viola.

. mItem, .