How to apply transform to an STL map in C++

Question

In C++, I\'m using transform to change all the values of a map to uppercase.

  std::map data = getData();

  // make all valu         


        

Answer 1

If you plan on sticking to std::transform, then you need std::inserter():

C++03 MVCE

typedef std::map Map;

struct ToUpper
{
    Map::value_type & operator()(Map::value_type & pair) const
    {
        boost::to_upper(pair.second);
        return pair;
    }
};

int main()
{
    Map m;
    m[0] = "snake_case";
    m[1] = "camelCase";
    m[2] = "PascalCase";
    
    std::transform(m.begin(), m.end(), std::inserter(m, m.end()), ToUpper());
    
    for (Map::const_iterator it = m.begin(); it != m.end(); ++it)
        std::cout << it->first << ", " << it->second << std::endl;
}

---

C++11 (you can do everything in main() really)

int main()
{
    auto m = getData();

    auto toUpper = [] (decltype(m)::value_type & pair)
    {
        boost::to_upper(pair.second);
        return pair;
    };

    std::transform(m.begin(), m.end(), std::inserter(m, m.end()), toUpper);

    for (auto const & pair : m)
        std::cout << pair.first << ", " << pair.second << std::endl;
}

---

C++14 (you can use auto in lambda parameters)

int main()
{
    auto m = getData();

    auto toUpper = [] (auto & pair)
    {
        boost::to_upper(pair.second);
        return pair;
    };
    std::transform(m.begin(), m.end(), std::inserter(m, m.end()), toUpper);

    for (auto const & pair : m)
        std::cout << pair.first << ", " << pair.second << std::endl;
}

---

C++17 (just because i love structured bindings)

int main()
{
    auto m = getData();

    auto toUpper = [] (auto & pair)
    {
        boost::to_upper(pair.second);
        return pair;
    };
    std::transform(m.begin(), m.end(), std::inserter(m, m.end()), toUpper);

    for (auto const & [key, value] : m)
        std::cout << key << ", " << value << std::endl;
}