After an in-source build, how to force CMake to build someplace else?

Question

Suppose I have a project named foo, with sources under /foo/src. Also suppose that at some point in the past, someone ran cmake /foo/src, so there are build files there.

Now, I want to perform a build of foo, in /foo/build. But if I cd /foo/build; cmake /foo/src - it treats /foo/src as the build directory, and configures and generates that build files there rather than in the current directory.

Questions:

1. What is the minimal set of files in /foo/src to delete in order for cmake to configure and generate build files in /foo/build?
2. Can I force cmake to use /foo/build without deleting anything from /foo/src?

Answer 1

1. A sufficient set of files to delete

A sufficient, though perhaps not minimal, set of files to delete:

CMakeCache.txt
Makefile
cmake_install.cmake
CMakeFiles/

and if you also have tests enabled, then

CTestTestfile.cmake
Testing/
tests/CTestTestfile.cmake
tests/Makefile
tests/cmake_install.cmake
tests/CMakeFiles/

2. How to force a different build location without deletions:

If you execute

cmake -B /foo/build -S /foo/src

(for non-ancient versions of CMake), CMake will ignore the previous build and build under /foo/build. You don't need it to be your current working directory when issuing this command.

_{The second part of this answer can actually be found within this answer}