CODEWITHSUNDEEP
c++c++11initializationstdmap
dmoody256
dmoody256

Reputation: 583

initialize part of a const std::map from another const std::map

I have a const std::map< int, std::string> initialized like so:

const std::map< int, std::string > firstMap = {
    { 1, "First" },
    { 2, "Second"}
};

Then I want to make another const std::map which uses the first map as part of its initial values and also has extends the original data. So it would be something similar to this I guess:

const std::map< int, std::string > secondMap = {
    { <firstMap>},
    { 3, "Third"}
};

so that the secondMap has the three pairs. Is this possible?

EDIT: Also the maps are declared extern.

Upvotes: 5

Views: 444

Answers (2)

R Sahu
R Sahu

Reputation: 206577

While @Rakete111's answer is perfectly valid for what you need, I suggest elevating that to a function. Maybe you can use it multiple places.

std::map<int, std:::string> combine(std::map<int, std::string> const& map1,
                                    std::map<int, std::string> const& map2)
{
   std::map<int, std:::string> res(map1);
   res.insert(map2.begin(), map2.end());
   return res;
}

and then use

const auto secondMap = combine(firstMap,
                               std::map<int, std::string>{{3, "Third"}});

Upvotes: 5

Rakete1111
Rakete1111

Reputation: 48948

No, std::map doesn't have the right constructor for this. What you can do however is use a lambda to initialize the variable in place.

const auto secondMap = [&firstMap] {
    std::map<int, std::string> map(firstMap);
    map[3] = "Third";
    return map;
}();

Upvotes: 9

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