c++ - 文字列文から単語を取り出して、C++でこのエラーを取得したい

Question

#include<iostream>
#include<string>
using namespace std;

void extractFirstWord(string& sentence, string& word);
void processBlanks(string& sentence);

int main() {
    string sentence, word;
    cout << "Input a sentence: ";
    getline(cin, sentence);

    while (sentence != "") {
          processBlanks(sentence); // removes all blanks from the front of sentence
          if (sentence.length() > 0) { // removing blanks may have made sentence null - cannot extract from a null string
             extractFirstWord(sentence, word); // gets first word from sentence and puts into word
             cout << word << endl; // output one word at a time
          }
    }

    system("PAUSE");
    return 0;
}

void extractFirstWord(string& sentence, string& word)
    {
        int i=0;
        while(sentence[i]!=' ')
        {
            i++;        
        }
        word=sentence.substr(0,i);
        sentence=sentence.substr(i);
}
// extractFirstWord removes the substring of sentence 
// from the beginning to the first space from sentence 
// and stores the same string into word. sentence is
// shortened accordingly.
// Postcondition: sentence is shortened, and word
//                is appropriately the first word in
//                sentence.

void processBlanks(string& sentence)
    {
        int i=0;
        while(sentence[i]==' '){i++;}
        sentence=sentence.substr(i);
    }

processBlanks は、文の前のスペースをすべて削除します。事後条件: 文の最初の単語の前にスペースがありません。

文字列文から単語を取り出して、C++でこのエラーを取得したい

エラーは -> 文字列添え字が範囲外です