Set installation prefix automatically to custom path if not explicitly specified on the command line

Question

For some internal tests, I would like the install prefix to default to a subdirectory of the build directory, unless explicitly overridden by the user. I know the user can specify a install prefix by:

$ cmake -DCMAKE_INSTALL_PREFIX=/foo/bar ..

But if the user does not specify this, it should default to, e.g. ${PWD}/installed.

The variable CMAKE_INSTALL_PREFIX is already set to /usr/local, so I cannot just check to see if it unset/empty before setting it.

My current solution is to add a custom switch that the user has to invoke to specify that the CMAKE_INSTALL_PREFIX variable gets respected:

option(ENABLE_INSTALL_PREFIX "Install build targets to system (path given by '-DCMAKE_INSTALL_PREFIX' or '${CMAKE_INSTALL_PREFIX}' if not specified)." OFF)
if ( ENABLE_INSTALL_PREFIX )
    set (CMAKE_INSTALL_PREFIX installed CACHE PATH "Installation root")
else()
    set (CMAKE_INSTALL_PREFIX installed CACHE PATH "Installation root" FORCE)
endif()

My questions are:

(a) Are there any issues with the above, beyond the annoyance of the extra flag needing to be passed to CMake to get CMAKE_INSTALL_PREFIX to have an effect?

(b) Is there a better, cleaner, more robust, more idiomatic and/or less annoying way to achieve the above?

Thanks.

Answer 1

CMake sets the boolean variable CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT if CMAKE_INSTALL_PREFIX has not been explicitly specified and is initialized to its default setting. You can override it in the following way:

if (CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
    set (CMAKE_INSTALL_PREFIX "${CMAKE_BINARY_DIR}/installed" CACHE PATH "default install path" FORCE )
endif()