(c++) The benefits / disadvantages of unity builds?

views:

1034

answers:

+4 Q:

(c++) The benefits / disadvantages of unity builds?

Hi, since starting at a new company I've noticed that they use unity cpp files for the majority of our solution, and I was wondering if anyone is able to give me a definitive reason as to why and how these speed up the build process?, as I would've thought that editing one cpp file in the unity files will force recompilation of all of them.

thanks

+10 A:

Very similar question and good answers here: http://stackoverflow.com/questions/543697/include-all-cpp-files-into-a-single-compilation-unit

The summary seems to be that less I/O overhead is the major benefit.

See also The Magic Of Unity Builds as linked in the above question as well.

akent 2009-05-11 13:00:31

excellent answer, thanks a lot for your help, after reading those links im still none the wiser if editing files within the unit cpp force a complete recompilation of the entire source, any ideas about this?

Stowelly 2009-05-11 13:17:06

Yes, it would. Whether that's a bad thing or not depends on how often you have to do a full rebuild already.

Head Geek 2009-05-11 13:26:26

If one file changes, any decent build system would notice a changed dependency and recompile the lot. You may be able to mitigate some of that time with a compiler cache like the excellent ccache -- see http://ccache.samba.org/

akent 2009-05-11 13:30:39

While day to day dev is definintely the big winner, Unity Builds make for rapid release builds compared to the old style builds. For any full rebuild, you can't beat a unity build. If you have a large codebase, you can even break up in the single unity file into a few smaller ones. Couple it with something like Incredibuild and you'll hardly have to wait for a build ever again!

OJ 2009-05-18 22:14:26

+1 A:

An introduction on "Unity Builds" along with benefits, disadvantages and a complete CMake integration can be found at cheind.wordpress.com.

hth, Christoph

Christoph Heindl 2010-01-03 08:16:19

ansaurus

tags:

views:

answers:

(c++) The benefits / disadvantages of unity builds?

related questions