Question

Considering the following:

char c;
cin >> c;
cin.unget();

Assuming the char input was successful, is unget guaranteed to be able to back up at least that one character? If I ask for, and successfully get a string, can I be guaranteed to be allowed to call unget all the way to the beginning of that string?

Answer 1

You are guaranteed to be able to unget at least 1 character. Any more than one is up to the implementation and circumstances, so you shouldn’t assume that you can unget more than one.

EDIT: Sorry I was thinking of libc’s int unget(int ch, FILE *stream). Which the standard says:

One character of pushback is
guaranteed. If the ungetc function is
called too many times on the same
stream without an intervening read or
file positioning operation on that
stream, the operation may fail.

I will see if I can find exactly what is said about basic_istream<>& unget()

EDIT: OK, so here’s what the c++ standard says about basic_istream<>& unget() (bolding added by me):

Behaves as an unformatted input
function (as described in 27.6.1.3,
paragraph 1). After constructing a
sentry object, if !good() calls
setstate(failbit) which may throw an
exception, and return. If rdbuf() is
not null, calls rdbuf()->sungetc(). If
rdbuf() is null, or if sungetc()
returns traits::eof(), calls
setstate(badbit) (which may throw
ios_base::failure (27.4.4.3)).

So the important bit is that it calls sungetc(), so let’s see what the standard says about that:

If the input sequence putback position
is not available, returns pbackfail().
Otherwise, decrements the next pointer
for the input sequence and returns
traits::to_int_type(*gptr()).

I see nothing that explicitly states a limitation, so it’s worth a try. If I understand this correctly, it will adjust pointers in the backing stream buffer. So as long as there is enough “history” in the buffer, it should continue to succeed.

Unlike C, however, it seems that you are never guaranteed that it will work, but you will likely be able to putback more than one character.

So my advice is to not depend on putting back more than one character, and always check for failure.