std::getline on std::cin

7
votes

Is there any good reason why:

std::string input;
std::getline(std::cin, input);

the getline call won't wait for user input? Is the state of cin messed up somehow?

3
c++getlinecin
post your code, to get an answer which solves your problem rather than speculative answers. - Alok Save
Is there a '\n' sitting in the input buffer from before, perhaps? - hammar
Of course there's a good reason why it's happening - you've messed up somehow :-) Post more code that demonstrates the problem (as others have mentioned). - Praetorian
The cin is allowed to be buffered. Many implementations require a newline in order to flush the input buffer and return the data to the calling program. - Thomas Matthews
Does this answer your question? Why does std::getline() skip input after a formatted extraction? - user202729

3 Answers

6
votes

Most likely you are trying to read a string after reading some other data, say an int.

consider the input:

11
is a prime

if you use the following code:

std::cin>>number;
std::getline(std::cin,input)

the getline will only read the newline after 11 and hence you will get the impression that it's not waiting for user input.

The way to resolve this is to use a dummy getline to consume the new line after the number.

2
votes

I have tested the following code and it worked ok.

#include <iostream>
using namespace std;
int main()
{
    string  input;
    getline(cin, input);
    cout << "You input is: " << input << endl;
    return 0;
}

I guess in your program that you might already have something in you input buffer.

0
votes

This code does not work:

#include <iostream>
#include <string>

int main()
{
int nr;
std::cout << "Number: ";
std::cin >> nr;

std::string  input;
std::cout << "Write something: ";
getline(std::cin, input);
std::cout << "You input is: " << input << std::endl;

return 0;
}

Now it works:

#include <iostream>
#include <string>

int main()
{
int nr;
std::cout << "Number: ";
std::cin >> nr;

std::string x;
std::getline(std::cin,x);

std::string  input;
std::cout << "Write something: ";
getline(std::cin, input);
std::cout << "You input is: " << input << std::endl;

return 0;
}