Abandoning Header Files?

garethw asks: "I'm working on a project where the lead developer, following a suggestion by our tool vendor, wants to get rid of the header files and directly #include source code. The language is a somewhat specialized language, but for all intents and purposes, you can assume it's Java or C. The conventional argument I recall for using header files, and incremental compilation, is that it's faster to use a makefile and conditionally build only those files that have changed. However, it turns out that the brute force of invoking the compiler once on the top-level does actually compile much faster. I feel that there is something about #include'ing source files directly, compiling only the top-level file, just doesn't 'feel' right and I'm at a loss to really give a solid argument as to why. Has anyone actually used this approach? Does anyone have any thoughts on any advantages or drawbacks?"

Comments

Not useful for C

[david.given]

...or, to a lesser extent C++, because of the way C scoping works:

static global variables have scope within the module they're defined in. Which means that two static globals in different source files don't collide, because they're in different modules.

Including everything into one big source file will mean that they're both in the same module, and so will collide. Not good.

Can't say about other languages, though.

Re:Several advantages and disadvantages

[stonecypher]

1. Faster compile of the full product.

Well, back in the real world, in a properly decoupled project incremental linking is a massive speed win, even when building from the top, as there's far less cross-lexing and as the build tables may be handled a small piece at a time, which is important because their parsing in the compiler itself is generally of O(n^2 log n) time or better. Once you've worked on a large project which fails to make proper decouplings, you will become painfully aware of this trend.

Whereas in this particular project the complete build is apparently faster, that is almost certainly the result of a very naive code tree and/or build scheme; the importance of incremental linking towards speed of compile cannot be overestimated, even in the case of compiling from clean.

2. Much better optimization. Compilers can only optimize within a compilation unit.

This simply isn't true. Whereas only some compilers make cross-TU optimizations, that is not the same as cross-TU optimizations being only able to optimize within a translation unit (why do people keep saying compilation unit? There's no such thing!) Besides, you're dramatically underestimating the commonality of link-time cross-tu counterspecialization, which now exists in ICC, BCC, MSCC, ARM ADS, EDG/Comeau, GHOC, and is in experimental development within GCC.

You're not doing it the way everyone expects you to do it. Certain components (the compiler, the linker, and pre-existing code) might have been designed under the assumption that individual files would be compiled separately.

They most certainly have not been. The C and C++ standards do not allow for such ridiculously inappropriate behavior. Where did you get this idea? Compiler writers may not impose arbitrary restrictions on the codebase in any relation to the local filesystem. This is just untrue.

The pre-existing code might have declared static (per-file) variables or functions in a way that could collide with other code (namespaces might help here).

This is a well known gigantic red flag indicating an amateur programmer. File-scoped variables are antiquated even within the pure C community; the only time they're acceptable in most professional programmer's eyes are within a library which is built alone. In fact, you might want to read the things Kernighan himself said about when file-scoped variables are appropriate in K&R 2; the primary author of the language himself says that this is a fundamentally bad technique and should not be done.

Of course, that you're causing problems by misusing the toolchain and allowing bad code to collide when build trees written seperately are blindly merged without the help of a linker is just not surprising.

The compiler and linker might have limits.

Not if they're standards compliant, they mightn't. Did you know that there's a document out there floating around telling compiler authors in concrete detail what they may and may not do? You should read that before commenting on what a compiler may or may not do; you are simply out in left field, here.

As with every issue you'll ever run into, there are two (or three) sides to it.

Not when you know what you're talking about. Whereas many things are issues of pro/con, many simply aren't; you'll be hard pressed to find pros in the distribution of heavy ordinance to delusional sociopaths, you'll be hard pressed to find pros in setting up a "bring a molester to school day," and you'll be hard pressed to find pros in non-decoupled code, once you've actually read the standard and are aware of the real limitations of compiler authors, instead of your guesses about what might maybe happen if someone wasn't paying attention.