Why is the letter 'D' missing in the output?

Question

#include <iostream>
using namespace std;

int main() {

    char ch1 = 'A';
    char ch2 = 'B';
    char ch3 = '\n';
    cout << ch1 << '\t' << ch2 << ch3;
    cout << 'C' << '\t' << 'D' << '\b' << ch1 << ch3;
    //return 0;
    system("pause");
}

Output is:

A        B
C        A

Why is the last letter A and not D?

Answer 1

Everything you cout gets outputted. It's just that a terminal will interpret '\b' as "go back one character". Try redirecting the output to a file and examine it with a (hex)editor to see that all the characters (including '\b') are there.

At a first glance, one might think that terminals print output as-is. That's incorrect, though. Terminals change the way they behave whenever they encounter one of special terminal control sequences or characters. The '\b' (=0x08=backspace) character is one of those. More can be found at http://ascii-table.com/ansi-escape-sequences.php . You can try printing some of those to a terminal and see it change colors, rewrite current lines and so on and so forth. In fact, you can use these special sequences and characters to make complete GUI-like apps in the command-line.

Note however, that not with all programs you can rely on the "redirect to a file" trick to see what terminal control sequences they write to stdout. Many programs detect whether they're writing to a terminal or not and adjust their usage (or lack thereof) of terminal control sequences accordingly.