Why does reading an invalid input from a stream set an integer to 0 instead of preserving its previous value?

Some C++ streams do not leave the old value untouched.
For example:
Let's say you have this as your input:
32 David

When this line executes:

cin >> firstname >> age;

firstname is a string, so it reads "32".

Then age is an int, so the stream tries to read "David" as an integer. That fails.

When formatted extraction into an integer fails, the stream sets fail-bit, and the target integer is set to 0. So your age becomes 0, not -1.

You can try adding these lines, so after reading input, this checks whether the read succeeded

cin >> first_name >> age;
if (!cin) {
    cout  age) {
    cout