I usually just write "TODO have not done X and Y yet" into the source file that where I was last working. It's deliberately not a comment, so that I can find it by running `make` and looking for the syntax error.
haha I do that too. If I need to leave something as it is but I need to fix it, I just write what I should do but without a "//" so it doesn't compile next day when I press build without thinking.
The other thing is that I usually leave a context file (which says "you were adding X to function Y and then debugging Z") in whatever directory I'm working in so that I remember what I was doing if I was in the middle of something.