Databases
StackOverflow2013
Postsdbo
POHow to construct a custom const_iterator with const data?
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POHow to construct a custom const_iterator with const data?
    primarykey
    data
    text
    <p>I'm writing the iterator classes for a custom container (some hash map class), let's call this container <code>Map</code>.</p> <p>For a couple of reasons it seems handy to derive <code>iterator</code> from <code>const_iterator</code> and I already did this in some other custom container (an array class), which works great, so I want to stick to this pattern.</p> <p>But now that leads to the following problem (which is quite general/abstract, I'm sorry):</p> <p>Whatever <code>const_iterator</code>'s critical data member is (it might be a <code>Map</code> reference, or a pointer to <code>Map::Elements</code>, many ways are possible), <strong>it needs to be non-const</strong> so both <code>const_iterator</code> and the derived <code>iterator</code> can utilize it meaningfully in their methods.</p> <p>But how can this non-const data member, in a proper way, be initialized when dealing with a const <code>Map</code> object (e.g. calling the method <code>const_iterator Map::Begin( void ) const</code>)?</p> <hr> <p>To illustrate my question bit, I wrote some example code that solves the problem by using const_cast which, I suppose, is bad style:</p> <pre><code>#include &lt;vector&gt; template&lt; typename K, typename T &gt; class Map { public: class const_iterator { public: const_iterator( Map const &amp; a_Map, int a_Index ) : m_Index( a_Index ), m_Map( const_cast&lt; Map &amp; &gt;( a_Map ) ) // Note the use of const_cast { } private: Map &amp; m_Map; int m_Index; }; class iterator : public const_iterator { public: T &amp; operator * ( void ) { return m_Map.mElements[ m_Index ]; } }; public: const_iterator Begin( void ) const { return const_iterator( *this, 0 ); } iterator Begin( void ) { return iterator( *this, 0 ); } private: std::vector&lt; T &gt; m_Elements; }; </code></pre>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. This table or related slice is empty.
    1. USBaltram
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POHow to construct a custom const_iterator with const data?
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POHow to construct a custom const_iterator with const data?
      1. USRobert Harvey
      1. VTFavorite
    3. VO
      singulars
      1. POHow to construct a custom const_iterator with const data?
      1. This table or related slice is empty.
      1. VTUpMod
    1. COTypical iterators don't actually contain your so-called "critical data member" at all. There is usually no connection between a real-life (release) iterator and the container to which it refers.
      singulars
      1. POHow to construct a custom const_iterator with const data?
      1. USKerrek SB
    2. COI know this is probably for academic curiosity but there is a `std::map` which will probably perform better than a `std::vector` being wrapped into a custom map.
      singulars
      1. POHow to construct a custom const_iterator with const data?
      1. USAJG85
    3. CO@Kerrek SB You're absolutely right. Anyway this is just an example, the "critical data member" could be anything (for example a pointer to the first Element in `m_Elements` of the Map that provided the iterator). Whatever approach I use the problem is always the same.
      singulars
      1. POHow to construct a custom const_iterator with const data?
      1. USBaltram
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload