Separate a sentence into words

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

// Separate a sentence into words.
int main()
{
    string sentence; // User input.

    cout << "Enter a sentence." << endl;
    getline(cin, sentence); // Get the user's sentence.

    int space_location = 0; // Location of a space.
    int start_position = 0; // Begin a new word.
    int length = 0; // Length of the word.
    string word; // A word.

    cout << "Here is your sentence, one word at a time:" << endl;
    if (!sentence.empty()) // If the user entered a sentence,
    {
        // While there is a space in the sentence,
        while ( (space_location = sentence.find(" ", start_position)) != string::npos)
        {
            // Get the length of the word.
            length = space_location - start_position;

            // Get the word.
            word = sentence.substr(start_position, length);

            // Display the word.
            cout << word << endl;

            // This is the start of the next word.
            start_position = space_location + 1;
        }

        // If there are no more spaces,
        // Get the length of the last word.
        length = sentence.size() - start_position;

        // Get the last word.
        word = sentence.substr(start_position, length);

        // Display the last word.
        cout << word << endl;
    }
    else // If the user did not enter anything,
    {
        cout << "No sentence was entered." << endl;
    }

    // End the program.
    return 0;
}