Re: Template to insert into a map? Is this really necessary?

 James Kanze <>
Thu, 05 Jul 2007 07:30:44 -0700
On Jul 5, 2:49 am, "Jim Langston" <> wrote:

I'm working on a program, and at one time I find it necessary to load
classes into a map. Now, these classes don't have default constructors, =


I can't retrieve them using

So I wound up with a real ugly line:

 DropdownBox& ThisBox = (Dropdowns.insert( Dropdowns.begin(),
std::make_pair<std::string, DropdownBox>( "RoteDots", DropdownBox( Parent,
IDC_RoteDots ) ) ))->second;

Just curious, but why the iterator argument? It's only a hint,
and only helps if the insertion occurs immediately in front of
it---in this case, if the new element will be the first element.

Most of the type, I'll have the map typedef'ed, and use its
value type:

    typedef std::map< std::string, DropdownBox >
                        DDBoxMap ;
    // ...
    DropdownBox& thisBox = dropdowns.insert(
                                    DropdownBox( Parent,
IDC_RoteDots ) )
                                .first->second ;

Of course, most of the time, I'll also want to know if the
insertion succeeded.

My map is fairly simple.

std::map<std::string, DropdownBox> Dropdowns;

Well, I tried to improve it a little, and came up with

 std::map<std::string, DropdownBox>::iterator it = Dropdowns.insert(
Dropdowns.begin(), std::make_pair<std::string, DropdownBox>( "RoteDots",
DropdownBox( Parent, IDC_RoteDots ) ) );
DropdownBox& ThisBox = it->second;

but that's still ugly. So I decided to make a template
function to help. The template itself is kinda messy, but
makes the code easier.

The template:

template<class K, class D>
D& InsertToMap( K& Key, D& Data, std::map<K, D>& Map )
    std::map<K, D>::iterator it = Map.insert( Map.begin(), std::make_pa=


D>( Key, Data ) );
    return it->second;

The real question is: what should the behavior be if the object
is already there: an error, use the already existing object, or
replace it with the new one. For the first two, you can base
your behavior on the return value of insert:

    template< typename Key, typename Value >
        std::map< Key, Value >&
        Key const& key,
        Value const& value )
        typedef std::map< Key, Value >
                            Map ;
        std::pair< Map::iterator, bool >
            = map.insert( Map::value_type( key, value ) ) ;
        if ( ! result.second ) {
            // error handling, or
            // result.first->second = value ;
            // to set the new value.
            // Drop the if to use the existing value.
        return result.first->second ;

James Kanze (GABI Software)
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