char randChar()
{
    return 'a' + rand() % 26;
}

string randString(int maxLen = 20)
{
    size_t len = rand() % maxLen;

    string result(len, ' ');
    std::generate_n(result.begin(), len, randChar);

    return result;
}
// это компилится

//ниже не компилится ( с ошибкой:
/* /usr/include/c++/4.7/bits/stl_algo.h: In instantiation of ‘_OIter std::generate_n(_OIter, _Size, _Generator) [with  _OIter = __gnu_cxx::__normal_iterator<std::basic_string<char>*, std::vector<std::basic_string<char> > >; _Size = long unsigned int; _Generator = std::basic_string<char> (*)(int)]’:
main.cpp:63:50:   required from here
/usr/include/c++/4.7/bits/stl_algo.h:5113:2: error: too few arguments to function */

vector<string> genRandomStrings(int maxSize)
{
    size_t n = rand() % maxSize;

    vector<string> result(n, "");
    std::generate_n(result.begin(), n, randString);
    //result[0] = randString();

    return result;
}