So I was to figure out why cin won't block after dumping the input buffer when piping data into the program. As it turned out, I discovered the easiest thing to do was to make piped input an error condition for the program and bail out. Yes, this was a homework assignment.

Anyway, in trying to wrap my brain around the problem, I began by invoking GDB. My compile line was:
g++ -g -D_GLIBCXX_DEBUG src/homework.cpp -o bin/homework
So now I want to look at the status of the input stream in GDB. Unfortunately, when I invoke gdb and set a breakpoint:
(gdb) print cin
$2 = <incomplete type>
(gdb) print cin.rdbuf()->in_avail()
Couldn't find method std::istream::rdbuf
I've installed the debug libraries for stdc++. What am I missing here?