Re: std::map initialization - reserving hashtable size

 James Kanze <>
Thu, 02 Aug 2007 11:58:47 -0000
On Aug 2, 1:57 am, wrote:

  I am using the map stl and is there a way I could specify the hash
table size in the map in the beginning - (similar to
vector.reserve(size) ).

std::map doesn't use a hash table, so it's not relevant.
std::map is a node based container, which means that each entry
is a separate allocation. Knowing the size up front couldn't
affect its performance.

Is there a way I could specify the entries at
the beginning.

Yes. Use the two iterator constructor. Something like:

    typedef std::map< int, int > Map ;

    struct MapInit
        int key ;
        int value ;
                        operator Map::value_type() const
            return Map::value_type( key, value ) ;
    } ;

    MapInit const initTable[] =
        { 1, 2 },
        { 3, 4 },
        // ...
    } ;

    Map myMap( begin( initTable ), end( initTable ) ) ;

(You'll need the usual begin and end function templates for

    template< typename T, size_t N >
    begin( T (&array)[ N ] )
        return array ;

    template< typename T, size_t N >
    end( T (&array)[ N ] )
        return array + N ;

   Is there an implementation-independent hint that I could give to
map so that it performs better when I add more entries to it.

If you know where the element is to go, you can tell the map
that when you insert it. This can save a few comparisons.

James Kanze (GABI Software)
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