Knowing initial user input after getline cstring assignment - c++

Question

Here's a C++ function of mine:
void SetUserName(char username[])
{
cout << "\nPlease enter a username.\n"
<< "Must not be longer than 12 characters.\n>> ";
cin.getline(username, MAX) // MAX is globally defined
while(strlen(username) > MAX)
{
cout << "\nUsername too long, try again.\n>> ";
cin.getline(username, MAX);
}
}
Obviously, the while loop never works because the user input is truncated to 12 characters everytime.
How can I effectively determine if the user input was too long, and continue to loop until the conditions are met?
Edit: Using cstring here is a requirement. I already know how easy it is with strings.
Edit #2: This was definitely a fruitful question for me, as it taught me a lot. Final code: http://pastie.org/3537894

Answer 1

C-style terminated strings are rather tricky to work with, and in almost every case I'd recommend the C++ std::string instead. However, since you say you specifically want to read a terminated string into an array, here is one way to do it.
Remember that the array size must be MAX+1, so there's space for MAX characters followed by the terminator.
istream::getline sets the streams failbit flag if the line is too long; you can test this after reading. If only a partial line was extracted, and you want to move on to the next line, then you'll need to clear the error state, and ignore the rest of the line.
while (!std::cin.getline(buffer, MAX+1)) {
std::cout << "Too long\n";
std::cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}

Answer 2

Use proper C++ (in particular, strings and the free getline function):
#include <string>
#include <iostream>
std::string line;
while (std::getline(std::cin, line))
{
if (line.length() > 12)
{
// error, line too long
continue;
}
// process line
}

Answer 3

If you want to find out if std::istream::getline() read an array full of characters as demanded but not an end of line character you need to figure out whether the number of stored characters (minus the terminating null) is identical to the extracted characters. That is, the following determines if there are more then 12 characters on the line (the 13th character is needed for the terminating null):
#include <iostream>
#include <string.h>
int main()
{
char array[13];
if (std::cin.getline(array, 13).gcount() == strlen(array)) {
std::cout << "excess characters on the line\n";
}
}
If you next also want to remove the excess characters from the stream you'd use something like std::cin.ignore(std::numeric_limits<std::streamsize>::max());. Since this is tagged as C, too, I don't know off-hand how to do this with C but I'm pretty sure that there is something similar to gcount().
Actually, looking more closely at the spec std::istream:getline() actually sets std::ios_base::failbit if it doesn't encounter a newline while reading the character (it also sets std::ios_base:failbit when no character is read but it doesn't set std::ios_base::failbit if at least one character is read before end of file is reached). This mean, you also want to clear the stream before ignoring excess characters and you can work off std::ios_base::failbit and std::ios_base::eof():
#include <iostream>
#include <limits>
#include <string.h>
int main()
{
char array[13];
std::cout << "enter password: ";
while (!std::cin.getline(array, 13) && !std::cin.eof())
{
std::cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
std::cout << "password is too long: enter max 12 chars: ";
}
std::cout << "the password is '" << array << "'\n";
}
Since std::ios_base::failbit is set you need to call clear() before you can use the stream for anything.

Answer 4

For the user to enter more characters than you allow, he must go at least one character over the limit. Since it does not matter to you by how many characters the user has "overstepped" your limit, you can pass MAX+1 as your limit, and see if the length is greater than MAX.
Of course you need to make enough space in the buffer to hold the 13-th character and a zero terminator. EDIT You also need to call ignore to skip to the end of the line on each failed attempt.