#include <iostream>
#include <algorithm>
#include <vector>
#include <iterator>

template <class T>
void print(const T& v) {
    std::copy(v.begin(), v.end(),
            std::ostream_iterator<typename T::value_type>(std::cout, "n"));
}

std::string lowercase(const std::string& s)
{
    std::string result(s);
    std::transform(result.begin(), result.end(), result.begin(), ::tolower);
    return result;
}

int main() {

    std::vector<std::string> tokens {"Col1", "Col2", "File", "FileIn.dat", "Out", "FileOut.dat"};
    std::transform(tokens.begin(), tokens.end(), tokens.begin(), lowercase);

    // how to replace lowercase() with a lambda that will take the previous
    // element into account while converting an element into lowercase
    print(tokens);

    return 0;
}

{"col1", "col2", "file", "FileIn.dat", "out", "FileOut.dat"};