CODEWITHSUNDEEP
c++templatesvector
Oleg
Oleg

Reputation: 1097

Combine all elements from couple of vectors into one using templates

I am trying to combine couple of vectors into one using templates, but I can't make it work because of the compiler error:

error: no matching function for call to   'to_vec_from_vectors(std::vector<std::vector<std::__cxx11::basic_string<char> > >&)'

The code is pretty straightforward. I have two functions, one is iterating via array of vectors, another is adding all the elements to the result vector. Appreciate any help. I have the following code:

template <typename Container>
Container& concat(Container& c1, Container const& c2)
{
    c1.insert(end(c1), begin(c2), end(c2));
    return c1;
}

template <typename Container>
inline auto to_vec_from_vectors(Container& c) -> std::vector<typename decltype(*std::begin(c))::value_type>
{
    std::vector<typename decltype(*std::begin(c))::value_type> v;

    for (auto& e : c)
    {
        concat(v, e);
    }

    return v;
}

int main()
{
    vector<string> malwares1 = {"Malware1", "Malware2"};
    vector<string> malwares2 = {"Malware3", "Malware4"};
    vector<vector<string>> mVector = {malwares1, malwares2};

    vector<string> malwares3 = to_vec_from_vectors(mVector);

    return 0;
}

Upvotes: 2

Views: 71

Answers (2)

rafix07
rafix07

Reputation: 20934

*std::begin(c) returns vector<string>&, you cannot store reference to vector in another vector, use remove_reference_t to discard it:

template <typename Container>
inline 
auto to_vec_from_vectors(Container& c) ->   std::vector<typename std::remove_reference_t<decltype(*std::begin(c))>::value_type>
{

    std::vector<typename std::remove_reference_t<decltype(*std::begin(c))>::value_type> v;

    for (auto& e : c)
    {
        concat(v, e);
    }

    return v;
}

Upvotes: 2

Caleth
Caleth

Reputation: 63039

The type typename decltype(*std::begin(c))::value_type seems to be what is giving you problems. typename Container::value_type::value_type works for me.

#include <vector>
#include <string>

template <typename Container>
Container& concat(Container& c1, Container const& c2)
{
    c1.insert(end(c1), begin(c2), end(c2));
    return c1;
}

template <typename Container>
inline auto to_vec_from_vectors(Container& c) -> std::vector<typename Container::value_type::value_type>
{
    std::vector<typename Container::value_type::value_type> v;

    for (auto& e : c)
    {
        concat(v, e);
    }

    return v;
}

int main()
{
    using std::vector;
    using std::string;

    vector<string> malwares1 = {"Malware1", "Malware2"};
    vector<string> malwares2 = {"Malware3", "Malware4"};
    vector<vector<string>> mVector = {malwares1, malwares2};

    vector<string> malwares3 = to_vec_from_vectors(mVector);

    return 0;
}

Upvotes: 3

Related Questions