+ is HQ9+ with two differences: there are no "H", "Q", or "9" instructions, and all characters that are not + are explicitly ignored.
There is an interpreter written for + in perl in 60 bytes:
my$r=$ARGV;open my$z,'<',$r or die$!;my$c=()=<$z>=~/\+/g;
There is another interpreter written for + in Lua, Hosted here