Seperating input C++

I am making a text adventure game in C++. Right now I am getting the input like this:

string word1, word2;
cin >> word1 >> word2;
parse(word1, word2);

An example input could be

goto store

Right now, to quit you would have to type quit and any other text to quit.

How can I make it so the input is separated by a space and I can tell if the second string is empty.