Why do I have to press enter Twice?

问题

For some reason in my program when I reach a certain spot, I have to press Enter twice in order to get it to submit. I added the clear to keep it from skipping input and the ignore() to keep it from keeping any extra characters in the buffer. I enter my input and then it drops down to a new line, I hit Enter again and it enter the input and continues the program no problem but I'm wondering why. Here's a code snippet:

    cin.ignore();
    cout << "Enter Student Major (ex. COSC): ";
    cin.getline(student.major, 6);

    for(int i = 0; i < sizeof(student.major); i++)
        student.major[i] = toupper(student.major[i]);

    cin.clear();
    cin.ignore(numeric_limits<streamsize>::max(), '\n');

Any suggestions?

回答1:

It seems to me that you are tossing too many cin.ignore() around, not knowing exactly why they are needed and when to put them there.

There are two common circumstances where cin.ignore() is needed to "make input work right":

1. when mixing formatted and unformatted input;
2. to recover from a formatted input error.

In both cases, you want to get rid of spurious characters from the input buffer; if there isn't any such character (which is probably what happens in your program), cin.ignore() will pause the execution and wait for user input - after all, you asked it to ignore some characters, and dammit, it will obey to its orders.

(although ignore() by default would "eat" just one character, whatever it may be, the execution is paused until a newline is found because by default cin is line buffered - new input is not examined until a newline is recieved)

Case 1:

cin.ignore() calls are often needed if you are performing an unformatted input operation (like getline) after performing a formatted input operation (i.e. using the >> operator).

This happens because the >> operator leaves the newline in the input buffer; that's not a problem if you are performing only formatted input operations (by default they skip all the whitespace before trying to interpret the input), but it's a problem if afterwards you do unformatted input: getline by default reads until it finds a newline, so the "spurious newline" left will make it stop reading immediately.

So, here you will usually call cin.ignore(...) call to get rid of the newline just after the last formatted input operation you do in a row, guaranteeing that the input buffer is empty. Afterwards, you can call getline directly without fear, knowing that you left the buffer empty.

It's a bad idea, instead, to put it before any getline, as you seem to do in your code, since there may be code paths that lead to that getline that have the input buffer clean, so the ignore call will block.

Case 2:

when istream encounters an error in a formatted input operations, it leaves the "bad" characters in the buffer, so if you retry the operation you get stuck endlessly, since the offenders are still there. The usual clear()/ignore() idiom comes to the rescue, removing the whole offending line from the input buffer.

Again, you don't put the clear()/ignore() sequence at random, but only after you get an input error from a formatted input operation (which sets the failbit of the stream).

---

Now, aside from these cases, it's uncommon to use cin.ignore() (unless you actually want to skip characters); don't spread it around randomly "just to be safe", because otherwise you will encounter the problem you described.

回答2:

The answer can be found here.

The extraction ends when n characters have been extracted and discarded or when the character delim is found, whichever comes first. In the latter case, the delim character itself is also extracted.

So in your case, the program will not continue until a '\n' character is received.

回答3:

I think cin.ignore(numeric_limits<streamsize>::max(), '\n'); is expecting a \n in the input and it doesn't find it, so you have to press Enter again for it to find it.

来源：https://stackoverflow.com/questions/13556890/why-do-i-have-to-press-enter-twice