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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?"
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.
It is hard to tell from your statements, but this may stop tools like ccache from working. I use ccache in my projects and it radically cuts down the amount of recompilation required when I do a complete rebuild. Now, an obvious question is why I don't simply rely on makefiles to ensure only changed files ever get rebuilt. This often happens because compilation involves generating new cpp files that are then compiled and I don't want to be grepping through these all the time. I suppose I could move them all to a different directory, but ccache works very well.
The other problem, of course, is that separating your classes into header and implementation means that if you change the implementation, you only need to recompile that one file and relink, rather than recompiling EVERYTHING. This can be a matter of a few seconds vs. several minutes. And implementation does change, a lot... fix a bug, you fix the implementation. The headers change too, but much much less frequently.
Oceania has always been at war with Eastasia.
Speeding up a full build should not be important. The only people who care about it are in your test lab doing daily builds and regression tests, who can start the build overnight and have it ready by morning. Of course, this is the situation in a well-designed application. If you find yourself needing a full rebuild all the time, it means one of two things: 1. you are hacking a core component, or 2. all your components are written with spaghetti code and any change in one forces rebuilds in all the others.
In the first case, try just testing one or two components during development, and then verify all the others when the API is stabilized. This is, incidentally, the advantage you gain from using header files: once the API is stable, you never need to rebuild that component again except to fix bugs (which require rebuilding only that component).
In the second case, you need some serious refactoring. Look at the code and break it up. Encapsulate everything you possibly can. Make stuff private and static. Make everything you don't modify const. Keep it up until each component is accessed only through its API and that API is clean. Trust me, this is possible in any project. The enormous decrease in maintenance costs will more than pay for any time you spend on it.
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.
StoneCypher is Full of BS
First of all, "speed", either compilation-wise or runtime-wise, has nothing to do with why you should use header files.
I too disliked header files, long ago, in my early days of programming C. It seemed pointless, to have two files (or rarely, as many as four), when one would do just as well.
For small projects, I'll still use one large monolithic source file. In that aspect, it makes sense to skip breaking out your data and function definitions.
But when you get to the "real" world... Imagine even a "small" serious project, with perhaps 10k lines of code. Try to find a single function in that file - I hope you feel on good terms with your IDE's search capabilities!
So, break that out into a dozen files - You have your network code in one file, your UI code in another, your file I/O in another, perhaps some database interaction in another, and so on. Okay, that works well... But wait, your network code, your file I/O, and your database code, all make use of the same checksum algorithm! So, you have the same exact code duplicated three times.
That would work, because each file will compile to a module with its own namespace (in most languages). But it wastes space, both in the source and in the compiled code. It also wastes time and can very easily introduce bugs - For example, if you decide you need to switch from MD5 for SHA1 as your checksumming algorithm, you now need to change three places instead of one. If you miss one of those, but use them to compare results between the three different uses, you have a very serious bug that may drive you batty trying to track it down.
So, the obvious solution, break out all your common functions into a toolkit-like source file. Now, you could just #include that in every other file that needs it, but WOW would that cause some serious bloat in the compiled code - In my experience, shared code files frequently end up as the single largest source file in the entire project.
So, use a header file. That way, you don't end up with massive duplication of code, you have the advantage of a logical breakout of your code into similar-purpose files, and you can still make changes to only one file to modify one function.
Incidentally, the above chain of thinking more-or-less describes the evolution of standard libraries... Would your professor actually suggest that you shouldn't "#include<stdio.h>", but instead should manually pull the code for each function you use into your source file? Because, in the degenerative case, he has told you exactly that.