build artifacts in source directory

[mwoehlke-kitware]

It appears that the built libraries get written to the source directory. Please don't do that; build artifacts belong in the build directory.

[ahornung]

Arguably, the compilation results are placed in bin and lib alongside the source directories in the main project root, whereas the build directory contains temporary artifacts only. This is common in quite a few projects and provides a re-usable project layout similar to an install space.

Do you have a specific case in mind where this is problematic?

[mwoehlke-kitware]

Do you have a specific case in mind where this is problematic?

It prevents thoroughly nuking a build by simply removing the build directory; CMake users tend to expect this to work. Multiple build trees from a single source will be horribly broken. Also, it grossly violates Standard Practice. (In general, it should be possible to build a CMake project with the source in a read-only mount point. Note also that it is "source directory", singular, i.e. the root of the git clone, which usually matches CMAKE_SOURCE_DIR.)

This is common in quite a few projects and provides a re-usable project layout similar to an install space.

Can you show me another project that does this? I've seen quite a few projects using CMake, and I can't think of a single one besides octomap that forces its build artifacts to be in the source tree in this manner. (I'm ignoring ones using ExternalProject in bad ways, but even those are few.)

To be clear: the problem isn't putting all the libraries in lib; that is moderately common and perfectly acceptable. The problem is putting lib in the source tree. It belongs in the build tree.

[ahornung]

The projects that I had in mind were not necessarily CMake projects, and the layout originates from a time when building completely in-source was common so maybe that's an explanation.

All basic instructions I've come across actually tell you to create build in the main source tree (aka mkdir build; cd build; cmake ..; make) and then call this an out-of-source build. But is the build directory then not also contained in the source?

Your scenarios all make sense to support, so the change is best planned for the next major release. Could you provide a PR with a suggestion?

[mwoehlke-kitware]

The projects that I had in mind were not necessarily CMake projects, and the layout originates from a time when building completely in-source was common so maybe that's an explanation.

Sounds plausible. Most of the problems I think can be fixed by using CMAKE_[CURRENT_]BINARY_DIR instead of CMAKE_[CURRENT_]SOURCE_DIR when specifying locations of build artifacts.

All basic instructions I've come across actually tell you to create build in the main source tree (aka mkdir build; cd build; cmake ..; make) and then call this an out-of-source build. But is the build directory then not also contained in the source?

In some technical sense, yes... but all build artifacts should still be contained within the build directory. (This matters when you instead create e.g. build-debug, build-release, etc.) Note that the use cases I mentioned previously all work with this pattern.

Also, those are typical "lazy" instructions; not that there is anything wrong with them, but there is no requirement that the build directory be a subdirectory of the source directory. Personally, I keep sources and builds about six levels deep in the FS, with only the first two in common. (When possible, they're on different physical drives, for performance reasons.)

IOW, this pattern is just as valid:

$ git clone some/project src
$ mkdir build && cd build
$ cmake ../src

[ahornung]

Do you have a concrete proposal for a change as PR? I'm preparing a new release and now would be a good time to change things.

[ahornung]

PR is here: #270