std::cin: empty the input buffer without blocking

Question

A followup to this question.

How can I clear the input buffer?

sleep(2);
// clear cin
getchar();

I only want the character that was typed in last, I want to discard any input that was put in while the program was asleep. Is there a way to do that?

Also, I don't want to wait actively to clear cin, thus I can't use ignore().

My approach would have been to get the current size of the buffer and if it's not equal to 0, silently empty the buffer. However, I have not found a way to get the size of the buffer. std::cin.rdbuf()->in_avail() always returns 0 for me. peek() also waits actively.

I don't want to use ncurses.

So the sleep finishes and before the getchar starts the use types a character in. Does that one count? — Ed Heal
– Ed Heal, Commented Jan 20, 2014 at 11:44
That's a case I have not yet considered, but is not important for this question for now. — gartenriese
– gartenriese, Commented Jan 20, 2014 at 11:47
@EdHeal: Personally I'd consider that case and "during the sleep" as practically indistinguishable, particularly since a sleep(2) is for "2 seconds or more". — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Jan 20, 2014 at 13:02

user2249683 · Accepted Answer · 2014-01-20 14:36:59Z

Having a system supporting tcgetattr/tcsetattr:

#include <iostream>
#include <stdexcept>

#include <termios.h>
#include <unistd.h>

class StdInput
{
    // Constants
    // =========

    public:
    enum {
        Blocking = 0x01,
        Echo = 0x02
    };

    // Static
    // ======

    public:
    static void clear() {
        termios attributes = disable_attributes(Blocking);
        while(std::cin)
            std::cin.get();
        std::cin.clear();
        set(attributes);
    }

    // StdInput
    // ========

    public:
    StdInput()
    :   m_restore(get())
    {}

    ~StdInput()
    {
        set(m_restore, false);
    }

    void disable(unsigned flags) { disable_attributes(flags); }
    void disable_blocking() { disable_attributes(Blocking); }
    void restore() { set(m_restore); }

    private:
    static termios get() {
        const int fd = fileno(stdin);
        termios attributes;
        if(tcgetattr(fd, &attributes) < 0) {
            throw std::runtime_error("StdInput");
        }
        return attributes;
    }

    static void set(const termios& attributes, bool exception = true) {
        const int fd = fileno(stdin);
        if(tcsetattr(fd, TCSANOW, &attributes) < 0 && exception) {
            throw std::runtime_error("StdInput");
        }
    }

    static termios disable_attributes(unsigned flags) {
        termios attributes = get();
        termios a = attributes;
        if(flags & Blocking) {
            a.c_lflag &= ~ICANON;
            a.c_cc[VMIN] = 0;
            a.c_cc[VTIME] = 0;
        }
        if(flags & Echo) {
            a.c_lflag &= ~ECHO;
        }
        set(a);
        return attributes;
    }

    termios m_restore;
};


int main()
{
    // Get something to ignore
    std::cout << "Ignore: ";
    std::cin.get();

    // Do something

    StdInput::clear();

    std::cout << " Input: ";
    std::string line;
    std::getline(std::cin, line);
    std::cout << "Output: " << line << std::endl;

    return 0;
}

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