How do you check to see if the user didn't input anything at a cin command and simply pressed enter?
When reading from std::cin, it's preferable not to use the stream extraction operator >>
as this can have all sorts of nasty side effects. For example, if you have this code:
std::string name;
std::cin >> name;
And I enter John Doe
, then the line to read from cin
will just hold the value John
, leaving Doe
behind to be read by some future read operation. Similarly, if I were to write:
int myInteger;
std::cin >> myInteger;
And I then type in John Doe
, then cin
will enter an error state and will refuse to do any future read operations until you explicitly clear its error state and flush the characters that caused the error.
A better way to do user input is to use std::getline to read characters from the keyboard until the user hits enter. For example:
std::string name;
getline(std::cin, name); // getline doesn't need the std:: prefix here because C++ has ADL.
ADL stands for argument-dependent lookup. Now, if I enter John Doe
, the value of name
will be John Doe
and there won't be any data left around in cin
. Moreover, this also lets you test if the user just hit enter:
std::string name;
getline(std::cin, name);
if (name.empty()) {
/* ... nothing entered ... */
}
The drawback of using this approach is that if you want to read in a formatted data line, an int
or a double
you'll have to parse the representation out of the string. I personally think this is worth it because it gives you a more fine-grained control of what to do if the user enters something invalid and "guards" cin
from ever entering a fail state.
I teach a C++ programming course, and have some lecture notes about the streams library that goes into a fair amount of detail about how to read formatted data from cin
in a safe way (mostly at the end of the chapter). I'm not sure how useful you'll find this, but in case it's helpful I thought I'd post the link.
Hope this helps!