CMake how to set the build directory to be different than source directory

It sounds like you want an out of source build. There are a couple of ways you can create an out of source build.

Do what you were doing, run
```
 cd /path/to/my/build/folder
 cmake /path/to/my/source/folder
```
which will cause cmake to generate a build tree in /path/to/my/build/folder for the source tree in /path/to/my/source/folder.

Once you’ve created it, cmake remembers where the source folder is – so you can rerun
cmake on the build tree with
```
 cmake /path/to/my/build/folder
```
or even
```
 cmake .
```
if your current directory is already the build folder.
For CMake 3.13 or later, use these options to set the source and build folders
```
 cmake -B/path/to/my/build/folder -S/path/to/my/source/folder
```
For older CMake, use some undocumented options to set the source and build folders:
```
 cmake -B/path/to/my/build/folder -H/path/to/my/source/folder
```
which will do exactly the same thing as (1), but without the reliance on the current working directory.

CMake puts all of its outputs in the build tree by default, so unless you are liberally using ${CMAKE_SOURCE_DIR} or ${CMAKE_CURRENT_SOURCE_DIR} in your cmake files, it shouldn’t touch your source tree.

The biggest thing that can go wrong is if you have previously generated a build tree in your source tree (i.e. you have an in source build). Once you’ve done this the second part of (1) above kicks in, and cmake doesn’t make any changes to the source or build locations. Thus, you cannot create an out-of-source build for a source directory with an in-source build. You can fix this fairly easily by removing (at a minimum) CMakeCache.txt from the source directory. There are a few other files (mostly in the CMakeFiles directory) that CMake generates that you should remove as well, but these won’t cause cmake to treat the source tree as a build tree.

Since out-of-source builds are often more desirable than in-source builds, you might want to modify your cmake to require out of source builds:

# Ensures that we do an out of source build

MACRO(MACRO_ENSURE_OUT_OF_SOURCE_BUILD MSG)
     STRING(COMPARE EQUAL "${CMAKE_SOURCE_DIR}"
     "${CMAKE_BINARY_DIR}" insource)
     GET_FILENAME_COMPONENT(PARENTDIR ${CMAKE_SOURCE_DIR} PATH)
     STRING(COMPARE EQUAL "${CMAKE_SOURCE_DIR}"
     "${PARENTDIR}" insourcesubdir)
    IF(insource OR insourcesubdir)
        MESSAGE(FATAL_ERROR "${MSG}")
    ENDIF(insource OR insourcesubdir)
ENDMACRO(MACRO_ENSURE_OUT_OF_SOURCE_BUILD)

MACRO_ENSURE_OUT_OF_SOURCE_BUILD(
    "${CMAKE_PROJECT_NAME} requires an out of source build."
)

The above macro comes from a commonly used module called MacroOutOfSourceBuild. There are numerous sources for MacroOutOfSourceBuild.cmake on google but I can’t seem to find the original and it’s short enough to include here in full.

Unfortunately cmake has usually written a few files by the time the macro is invoked, so although it will stop you from actually performing the build you will still need to delete CMakeCache.txt and CMakeFiles.

You may find it useful to set the paths that binaries, shared and static libraries are written to – in which case see how do I make cmake output into a ‘bin’ dir? (disclaimer, I have the top voted answer on that question…but that’s how I know about it).

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