CODEWITHSUNDEEP
c++iostreamcin
Morten Nissov
Morten Nissov

Reputation: 452

Problems with std::cin.fail()

I am writing some code to read from the terminal with cpp but for some reason it crashed after running out of numbers. From what I have read online I should be able to check if std::cin was successful by using std::cin.fail() but it crashes before.

The code I am running is

#include <iostream>

int main()
{
    int x{};

    while (true)
    {
        std::cin >> x;
        if (!std::cin)
        {
            std::cout << "breaking" << '\n';
            break;
        }
        std::cout << x << '\n';
    }
    return 0;
}

with the input:

test@test:~/learn_cpp/ex05$ ./test
1 2
1
2
^C

I end up having to ctrl+c out of the program. Version info:

gcc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Upvotes: 1

Views: 403

Answers (1)

ChrisMM
ChrisMM

Reputation: 10022

Nothing in your input causes cin to set the fail bit. Hence, the while (true) will just keep going. You can enter a letter, or something which is otherwise not an int, which will set the fail bit, and cause the loop to break.

Note that a new line will be ignored for this purpose.

If you know that all your input will be on a single line, then you can use std::getline to read in the entire line, and then std::stringstream to read the integers from that line.

#include <iostream>
#include <sstream>
#include <string>

int main() {
    int x{};
    std::string buff;
    std::getline( std::cin, buff );
    std::stringstream ss( buff );
    while ( ss >> x ) {
        std::cout << x << '\n';
    }

    return 0;
}

Upvotes: 3

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