I often find that the headers section of a file get larger and larger all the time but it never gets smaller. Throughout the life of a source file classes may have moved and been refactored and it's very possible that there are quite a few
#includes that don't need to be there and anymore. Leaving them there only prolong the compile time and adds unnecessary compilation dependencies. Trying to figure out which are still needed can be quite tedious.
Is there some kind of tool that can detect superfluous #include directives and suggest which ones I can safely remove?
Does lint do this maybe?
It's not automatic, but doxygen will produce dependency diagrams for
#included files. You will have to go through them visually, but they can be very useful for getting a picture of what is using what.
There's also a Clang-based tool, include-what-you-use, that can do this. include-what-you-use can even suggest forward declarations (so you don't have to #include so much) and optionally clean up your #includes for you.
Current versions of Eclipse CDT also have this functionality built in: going under the Source menu and clicking Organize Includes will alphabetize your #include's, add any headers that Eclipse thinks you're using without directly including them, and comments out any headers that it doesn't think you need. This feature isn't 100% reliable, however.