C++: how do I check if the cin buffer is empty?

How do you check to see if the user didn't input anything at a cin command and simply pressed enter?

回答1:

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!

回答2:

cin will not continue with the program unless the user enters at least 1 character (enter doesn't count). If the user doesn't give ANY input, cin will just keep waiting for the user to give input and then press enter.

回答3:

The Simple way >>

{
char X=0; //  ASCII( 0 ) means a NULL value
cin>>X;
if(X==0 || X==10) // ASCII( 10 ) means ENTER
cout<<"User din't enter ANYTHING !! ";
}

But a simple problem is....

cin just won't allow you to move further without entering a character

by character here i mean a DIGIT or alphabet or special symbol , not space, enter null etc

Hope this solves your problem, if it doesn't, I'll be glad to help just let me know.

回答4:

int main(){ 
string str[100];
std::cout<<"Hello how are you ? \n";
std::cin>>str;
if(str.length() > 0){
// If input is seen
}
else{
// If input is not seen
}
}

Any problem let me know.