How to understand the difference between two command line options for cmake? -

describe difference between these 2 command lines:

c:\xxxxx> cmake -help  usage    $ cmake [options] <path-to-source>   $ cmake [options] <path-to-existing-build> 

specify source directory (re-)generate build system in current working directory. specify existing build directory re-generate build system.

the last description not give me how use first, or second.

could explain me?

when use in-tree build (cmake .), there no difference.

when out-of-tree build, there difference.

suppose project lives in ~/foo , current directory ~/foo/build

you have run cmake .. first build. subsequent reconfigures, can use cmake . because there build there.