C++: how do I remove bad input from cin? - c++

Question

I have done some cin operation and before I use cin I want to clear its state and
flush it. How do I do it? I know cin.clear() clears the error state but to flush the cin
buffer how do I check if it's empty and if not which of the below statements shall I use to empty it so that I can safely use cin later?
cin.ignore(std::numeric_limits<std::streamsize>::max());
or
cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

Answer 1

It seems, you detected a problem and you want to get rid of erronous input, e.g., the entered line. You could try to get rid what std::cin has in its buffer but generally, this doesn't work too well as there is no guarantee that the stream has read, e.g., a complete line. Also, to make it potentially useful you'll need to make sure that stdin and std::cin are not synchronized as otherwise std::cin will never buffer any characters anyway:
std::ios_base::sync_with_stdio(false);
// read data using std::cin
if (!std::cin) {
// clean-up
std::cin.clear();
if (std::cin) {
std::cin.ignore(std::cin.rdbuf()->in_avail()); // ignore what is buffered
}
}
Whether in_avail() will return a non-zero value depends entirely on the used stream buffer and it may ignore more than you actually want to ignore anyway. For example, if std::cin's stream buffer is replaced to use the stream buffer of an std::istringstream it will probably consume all the character from this source.
Probably the most sensible clean-up is to ignore() the next character or the characters on the current line, using either
std::cin.ignore();
or
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
The max() of std::streamsize is used as a magical number to indicate that the count should not be considered a termination criteria. If you don't pass a termination character the stream will ignore characters until the end of the stream which is probably not what you want.

Answer 2

Stream buffers have the in_avail method. But try to simply remove the content regardless.:
template <class charT, class traits>
std::basic_istream<charT, traits>&
empty_buffer(std::basic_istream<charT, traits>& is)
{
std::basic_streambuf<charT, traits>* p = is.rdbuf();
p->pubseekpos(0, std::ios_base::in);
return is.ignore(std::numeric_limits<std::streamsize>::max());
}

Answer 3

Effectively, this can be done with the method is_avail().
Try this:
#include <iostream>
int empty_stream(std::istream & is) {
std::streambuf * pbuf = std::cin.rdbuf();
std::streamsize size = pbuf->in_avail();
if(size)
{
if(!is.good())
return 0;
is.sync();
return 1;
}
else
return -1;
}
int main() {
char str[50];
std::cin >> str;
switch(empty_stream(std::cin))
{
case -1:
std::cout << "Empty buffer" << std::endl;
break;
case 0:
std::cout << "Error flags in stream" << std::endl;
break;
case 1:
std::cout << "Buffer flushed" << std::endl;
break;
}
std::cin.get(); //This shouldn't be bypassed
return 0;
}