C Language Include What You Use (IWYU)


Google's Include What You Use project, or IWYU, ensures source files include all headers used in the code.

Suppose a source file source.c includes a header arbitrary.h which in turn coincidentally includes freeloader.h, but the source file also explicitly and independently uses the facilities from freeloader.h. All is well to start with. Then one day arbitrary.h is changed so its clients no longer need the facilities of freeloader.h. Suddenly, source.c stops compiling — because it didn't meet the IWYU criteria. Because the code in source.c explicitly used the facilities of freeloader.h, it should have included what it uses — there should have been an explicit #include "freeloader.h" in the source too. (Idempotency would have ensured there wasn't a problem.)

The IWYU philosophy maximizes the probability that code continues to compile even with reasonable changes made to interfaces. Clearly, if your code calls a function that is subsequently removed from the published interface, no amount of preparation can prevent changes becoming necessary. This is why changes to APIs are avoided when possible, and why there are deprecation cycles over multiple releases, etc.

This is a particular problem in C++ because standard headers are allowed to include each other. Source file file.cpp could include one header header1.h that on one platform includes another header header2.h. file.cpp might turn out to use the facilities of header2.h as well. This wouldn't be a problem initially - the code would compile because header1.h includes header2.h. On another platform, or an upgrade of the current platform, header1.h could be revised so it no longer includes header2.h, and thenfile.cpp would stop compiling as a result.

IWYU would spot the problem and recommend that header2.h be included directly in file.cpp. This would ensure it continues to compile. Analogous considerations apply to C code too.