CODEWITHSUNDEEP
c++templatesstl
PapaDiHatti
PapaDiHatti

Reputation: 1921

Copy map/multimap keys to vector or set

I have written below code to convert keys in map or multimap to set:

template<typename STLContainer>
inline auto CopyContanerKeyToSet(const STLContainer& cont)
{
    std::set<decltype(cont.begin()->first)> lset;
    std::transform(cont.begin(),cont.end(),std::inserter(lset,lset.end()),[](const  auto it) { return it.first;});
    return lset
}

Now there is requirement that sometimes I need to convert keys into vector as well. So I just want to know how to write template function that can accept vector or set as template argument and after that create that container accordingly.

Upvotes: 1

Views: 309

Answers (1)

NathanOliver
NathanOliver

Reputation: 180415

We can solve this with a Template template parameter. This allows us to specify just the main type without specifying the template type of that type. Doing that gives us

template< template<typename ...> class OutputContainer, typename STLContainer>
inline auto CopyContanerKeyToSet(const STLContainer& cont)
{
    OutputContainer<typename STLContainer::key_type> lset;
    std::transform(cont.begin(),cont.end(),std::inserter(lset,lset.end()),[](const  auto it) { return it.first;});
    return lset;
}

And then we can use that with something like this

int main()
{
    std::map<std::string, int> foo{ {"this", 1}, {"second", 1} };
    auto output = CopyContanerKeyToSet<std::vector>(foo);
    for (const auto& e : output)
        std::cout << e << " ";
}

Which gives us

second this

Live Example

I also changed <decltype(cont.begin()->first)> to <typename STLContainer::key_type> as value_type of the map/multimap has a const key_type for the std::pair which we do not want for the vector/set.

Upvotes: 2

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