How can I std::copy a string to a struct with the std::string fields

I read from a file lots of records into a std::vector like this:

std::fstream in("storage.txt", std::ios::in);
stxxl::vector<Citizen> v;
std::copy(std::istream_iterator<Citizen>(in), std::istream_iterator<Citizen>(), std::back_inserter(v));

The struct Citizen is defined as:

struct Citizen
{
    std::string firstname, lastname;
    int year;
};

That function is created for the isteam_iterator:

std::istream& operator >>(std::istream& i, Citizen& c)
{
    i >> c.firstname >> c.lastname >> c.year;
    return i;
}

During creation of the first struct I get a segmentation fault. Why? I know, the struct's string fields have not enough space. Changing the type to char fieldname[some size] solves the problem, but it is a C style and I don't want to use it.

How can I allocate space for that example?