Question

Given this code:

#include <cstdio>
#include <iostream>
#include <string>
using std::cin;
using std::cout;
using std::string;

int main() {
        int a;
        string b;

        cin >> a;
        cin >> b;

        return 0;
}

I tried compiling it with g++ and running it.
When assigning a char to a, at the first cin, the following instruction seems to be skipped.

Even if add two getchar() instructions between the last two lines, only the second getchar() seems to be executed.
Can somebody accurately explain what’s happening at low level, which seemingly results in an apparent non execution of those lines?

EDIT:

Using this debug code:

#include <cstdio>
#include <iostream>
#include <string>
using std::cin;
using std::cout;
using std::endl;
using std::string;

int main() {
        int a;
        string b;

        cin >> a;
        cin >> b;
        cout << "a is "<< a << endl;
        cout << "b is "<< b << endl;
        getchar();

        return 0;
}

INPUT
1test

OUTPUT
a is 1
b is test
* No getchar executed *

INPUT
1
test

OUTPUT
a is 1
b is test

INPUT
ttest

OUTPUT
a is 0
b is

INPUT
t

// Skips the second cin

OUTPUT
a is 0
b is

NOTE:
getchar() was not executed even once.

Answer 1

Two things, judging from your output. The first is when you enter
"ttest", the cin >> a; fails. This puts cin in an error state,
where it will remain until the error is cleared. And as long as it is
in an error state, all other operations are no-ops. You really need to
test the results of the input before trying to use the values:

std::cin >> a;
if ( !cin ) {
    std::cerr << "That wasn't an integer" << std::endl;
    std::cin.clear();
}
std::cin >> b;
if ( !cin ) {
    std::cerr << "Where was the string" << std::endl;
    std::cin.clear();
}

(And don’t use a non-initialized variable, like a, until it has been
successfully input.)

The second is that the >> operator only extracts the characters
necessary for its target: >> to an int will stop at the first
non-numeric character, and >> to a std::string at the first white
space (in both cases, after having skipped leading white space). This
means that after something like "1test\n", there will still be a
'\n' in the buffer. And while it’s generally a bad idea to mix
FILE* (like getchar()) and iostream, if they’re correctly
synchronized, getchar() will read this '\n' immediately and return.

If you’re reading line oriented input, the best solution is to use
getline(), and then put the line into a std::istringstream to parse
it. So your code might end up looking like:

std::string line:
std::getline(std::cin, line);
if ( ! std::cin ) {
    //  Something unexpected went wrong...
    std::cin.clear();
} else {
    std::istringstream l( line );
    l >> a >> b;
    if ( !l ) {
        //  Format error in input...
    } else {
        //  use your data here...
    }
}
std::cin.get();  //  Wait for one more character...