What are the rules of the std::cin object in C++?

I am writing a small program for my personal use to practice learning C++ and for its functionality, an MLA citation generator (I'm writing a large paper with tens of citations).

For lack of a better way to do it (I don't understand classes or using other .cpp files inside your main, so don't bother telling me, I'll work on that when I have more time), I am writing a function for each type of citation. I might break this down into a function for each reused code if I get more time.

My question is: how does the std::cin object work? I am currently reading in with std::cin >> for the strings I expect to be single words, and getline(std::cin, string) for the strings with spaces. I am not getting the right output, though. I just want to know how std::cin works and why I keep unexpectedly skipping over some some inputs (for instance, it skips over webPage instead of giving me a chance to input into it).

void webCit() { std::cout << "Leave any unknowns blank.\n"; std::cout << "Author last name: "; std::string lastName; std::cin >> lastName; if (lastName.size() != 0) { lastName = lastName + ", "; } std::cout << "Author first name: "; std::string firstName; std::cin >> firstName; if (firstName.size() != 0) { firstName = firstName + ". "; } std::cout << "Article title: "; std::string articleTitle; getline(std::cin, articleTitle); if (articleTitle.size() != 0) { articleTitle = "\"" + articleTitle + ".\" "; } std::cout << "Title of web page: "; std::string pageTitle; std::cin >> pageTitle; if (pageTitle.size() != 0) { pageTitle = pageTitle + ". "; } std::cout << "Publication date: "; std::string publicationDate; getline(std::cin, publicationDate); if (publicationDate.size() != 0) { publicationDate = publicationDate + ". "; } std::cout << "Web address: "; std::string webAddress; getline(std::cin, webAddress); webAddress = "<" + webAddress + ">. "; std::cout << "Date accessed: "; std::string dateAccessed; getline(std::cin, dateAccessed); if (dateAccessed.size() != 0) { dateAccessed = dateAccessed + ". "; } std::string citation = lastName + firstName + articleTitle + pageTitle + publicationDate + webAddress + dateAccessed; std::cout << citation; //TEST; remove after } 

EDIT: I/O

Leave any unknowns blank. Author last name: Hooked Author first name: Jerbear Article title: Title of web page: title Publication date: Web address: Date accessed: 4/29/09 Hooked, Jerbear. Title. < 4/29/09. 

As you can see, something is going wrong, because my input is getting skipped over.

2 Answers

What is happening here is that std::cin >> firstName; only reads up to but not including the first whitespace character, which includes the newline (or '\n') when you press enter, so when it gets to getline(std::cin, articleTitle);, '\n' is still the next character in std::cin, and getline() returns immediately.

// cin = "Bloggs\nJoe\nMan of Steel, Woman of Kleenex\n" std::cin >> lastName; std::cin >> firstName; // firstName = "Joe", lastName = "Bloggs", cin = "\nMan of Steel, Woman of Kleenex\n" getline(std::cin, articleTitle); // articleTitle = "", cin = "Man of Steel, Woman of Kleenex\n" 

Adding 'std::cin >> std::ws' (ws meaning whitespace) before your calls to getline() fixes the problem:

std::cin >> firstName >> std::ws; getline(std::cin, articleTitle); 

But it is easier to see where you missed it if you do it in the argument:

std::cin >> firstName; getline(std::cin >> std::ws, articleTitle); 

When you use the >> operator, cin reads up until the next whitespace character, but it doesn't process the whitespace. So when you have

std::cin >> str1; std::getline(std::cin, str2); 

the second call will just process the newline character, and you won't have a chance to type in any input.

Instead, if you're planning to use getline after an operator >>, you can call std::cin.ignore() to eat the newline before you call getline.

Edit: it works as you expected when you do

std::cin >> str1; std::cin >> str2; 

since the second call will ignore all leading whitespace.