why when cin is always 0?

[ssol]

in this code,
int k;
cin>>k
if I type not int like 'a' or '\'
they are all recognized as 0 and code is operated strangely
why this happened??

[JLBorges]

When input fails (failed to read an integer), zero is written into the integer k,
and the stream (cin) is put into a failed state.

We can check for successful input with

if( std::cin >> k )
{
    // success. use k
    // ...
}
else 
{
    // input failed, k is set to zero.
    // ...
}

[ssol]

@JLBorges
Thanks for your kind advise!
But how can I differentiate (fail case) and (success case but type 0)
k=0 in both cases...

[JLBorges]

#include <iostream>

int main()
{
    int k ;
    std::cout << "enter an integer: " ;

    if( std::cin >> k )
    {
        std::cout << "successful input. the value of k is " << k << '\n' ;
        if( k == 0 ) std::cout << "(the user entered zero)\n" ;
    }
    else
    {
        std::cout << "input failure. the user did not enter an integer\n" ;
    }
}