What does cin do when there is an error

Question

#include<iostream>;

int main()
{
    int a = 1;
    int b = 2;
    std::cin >> a >> b;
    std::cout << a << "+" << b << "=" << a+b << std::endl;
    return 0;
}

when I enter 3 4 as input,the output will be 3+4=7,well,it's strange; But when I enter a b,the output is 0+0=0(Why it is 0 and 0?); The most confusing,a 4,it will be 0+0=0(Why not '0+4=4'?????); Then i write another prog.

#include<iostream>;

int main()
{
    int a = 1;
    int b = 2;
    std::cin >> a;
    std::cin.clear();
    std::cin >> b;
    std::cout << a << "+" << b << "=" << a+b << std::endl;
    return 0;
}

When i enter a 4,why is it still 0+0=0?Shouldn't it be 0+4=4?

Thanks to all the warm-hearted!!

I write prog3,to test what will happen when i don't write int a=1;int b=2;

2

#include <iostream>
using namespace std;
int main()
{  
    int a,b;
    cin >> a  ;
    cin >> b;
    cout<< a << "+"<< b <<"="<< a+b << endl;
    return 0;
}

When a bagain,it outputs 0+-1218170892=-1218170892(Why isn't 0+0=0??)

Answer 1

Like all istreams, std::cin has error bits. These bits are set when errors occur. For example, you can find the values of the error bits with functions like good(), bad(), eof(), etc. If you read bad input (fail() returns true), use clear() to clear the flags. You will also likely need an ignore(1); to remove the offending character.

See the State functions section for more information. http://en.cppreference.com/w/cpp/io/basic_ios

Answer 2

The value is set to zero on an error as per C++11: If extraction fails, zero is written to value and failbit is set.

On the 'a 4' example, both values are 0 because the buffer has not been flush/cleared, so the second cin read is still reading the error, and also receives a value of 0.