CODEWITHSUNDEEP
c++dictionaryinitialization
user3684792
user3684792

Reputation: 2611

c++ initialize a map of maps more nicely

I have just started learning c++ today and am having a hard time finding an elegant way to initialise a map of maps. At the moment, the horrible method is:

typedef std::map < char, map < char, int > >  terribly_initialised_map;

and then

terribly_initialised_map plz_help;
plz_help['m']['e'] = 1;
....
...
..
.

There must be a better way?

Upvotes: 4

Views: 9032

Answers (1)

Qaz
Qaz

Reputation: 61910

In C++11, it's a lot easier:

map_type someMap{
    {'m', {
        {'e', 1},
        {'f', 2}
    }},
    {'a', {
        {'b', 5}
    }}
};

This makes use of list initialization, using std::map's list constructor (the one that takes a std::initializer_list<std::pair<Key, Value>>). The pairs can also be list initialized with the two stored values.

In C++03, you can do decently well with boost::map_list_of, but it might not be much better than what you have for nested maps, especially with the outer call needing to be a specific list_of call to eliminate an ambiguity:

using boost::assign::list_of;
using boost::assign::map_list_of;
map_type someMap = 
    list_of<map_type::value_type>
    ('m', 
        map_list_of
        ('e', 1)
        ('f', 2)
    )
    ('a', 
        map_list_of
        ('b', 5)
    )
;

Upvotes: 11

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