C++ Frequency of words and phrases

#include <iostream>
#include <fstream>
#include <string>
#include <map>
#include <vector>

using namespace std;

struct Phrase {
  void display() const
  {
    for (int i = 0; i < words.size(); ++i)
      cout << words[i] << " "; 
  }

  vector<string> words;
};

bool operator<(const Phrase& lhs, const Phrase& rhs)
{
  return lhs.words < rhs.words;
}

int main()
{
  ifstream input("input.txt");
  if (!input)
  {
    cout << "Can't open input file: input.txt\n";
    return 1;
  }

  const int LIMIT = 10;
  Phrase cur;
  map<Phrase, int> stat;
  string word;

  while (input >> word)
  {
    for (int i = 0; i < word.size(); ++i)
      word[i] = tolower(word[i]);

    if (cur.words.size() == LIMIT)
    {
      ++stat[cur];
      cur.words.clear();
    }

    cur.words.push_back(word);
  }

  if (!cur.words.empty())
    ++stat[cur];

  cout << "Phrase frequencies:\n";
  map<Phrase, int>::const_iterator iter = stat.begin();
  for (; iter != stat.end(); ++iter)
  {
    iter->first.display();
    cout << ": "
         << iter->second << " times\n";
  }

  return 0;

}