How do I flush the input buffer correctly after I get the next char without having to press enter twice! C++

Question

Consider the following code:

#include <iostream>

using namespace std;

int main()
{
    char input;
    while (1) {
        cout << "Enter: ";
        cin.get(input);
        cin.ignore(100, '\n'); //requires another `enter`
    }
    return 0;
}

I want to get the next character in the input buffer, and I want that input buffer to be cleared afterwards. I know that we can use cin.ignore() to do the cleaning, however, if I used it then I'll have to press enter twice (in case of inputting enter alone) to enter my input! How can I prevent that from happening?

why do not do if(input == '\n') continue; instead of ignore ? — Gabrielle de Grimouard, Feb 02 '16 at 08:06
You cannot "clear" the input buffer, you have to consume the characters somehow. — wilx, Feb 02 '16 at 08:06
@Pierre: That is wrong. See http://stackoverflow.com/a/2979217/341065. — wilx, Feb 02 '16 at 08:08
@GabrieldeGrimouard Because, even if it works, it is still a bit of unnecessary coding and not very efficient. I want a better way, if possible. — Anich1935, Feb 02 '16 at 08:18
a jmp instruction is way more efficient than use cin.ignore. — Gabrielle de Grimouard, Feb 02 '16 at 08:27
Not reproducible here. It works just as you wish: hitting enter only once. — DarioP, Feb 02 '16 at 08:39
The current code only requires one keypress per prompt (or zero, if you typed more than 100 characters -- to fix this replace `100` by `SIZE_MAX` or some other large number) — M.M, Feb 02 '16 at 09:11
@GabrieldeGrimouard I would love to see that `jmp instruction` if you won't mind. Would you show that using some code? — Anich1935, Feb 02 '16 at 10:10
@Anich1935 continue is just a jmp instruction to the begening of your while... — Gabrielle de Grimouard, Feb 02 '16 at 10:13

score 1 · Answer 1 · answered Feb 02 '16 at 08:08

1

#include <iostream>
using namespace std;
int main()
{
    char input;
    while (1)
    {
        cout << "Enter: ";
        cin.get(input);
        cout << input << endl;
        cin.ignore(255, '\n');
        cin.clear();
    }
    return 0;
}

you need to clear your cin flags: cin.clear();

answered Feb 02 '16 at 08:08

Axios

49
2

What OS are you using by the way? – Axios Feb 02 '16 at 08:24

LogicStuff · Answer 2 · 2016-02-02T09:13:45.213

1

A workaround in this case might be to use std::string and std::getline:

#include <iostream>
#include <string>

int main()
{
    while(true)
    {
        std::cout << "Enter: " << std::flush;
        std::string input;

        if(std::getline(std::cin, input) && !input.empty())
        {
            input.front(); // this is your character
        }
    }

    return 0;
}

edited Feb 02 '16 at 09:13

answered Feb 02 '16 at 09:07

LogicStuff

19,397
6
54
74

1

A slight down-side to this is that it allocates memory unnecessarily. But on the up-side it is simple code. – M.M Feb 02 '16 at 09:12

score 0 · Answer 3 · answered Feb 02 '16 at 08:33

std::cin.ignore() will block until it reads count chars or sees its delimiter. There is no reliable way to check if std::cin will block before calling an input function. The only reliable way to prevent it from blocking when input == '\n' is to check for that condition before calling ignore().

#include <iostream>
#include <limits>

int main()
{
    char input;
    while (true) {
        std::cout << "Enter: ";
        std::cin.get(input);
        if (input != '\n') {
            // Note: using std::numeric_limits<std::streamsize>::max()
            // disables std::istream::ignore's count check entirely
            std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
        }
    }
}

How do I flush the input buffer correctly after I get the next char without having to press enter twice! C++

3 Answers3