cmake_minimum_required and sub-projects

Question

I really enjoy having my top-level project, driving compilation of my sub-projects. I currently have a tree such as:

gdcm -- cmake_minimum_required(VERSION 3.8.2 FATAL_ERROR)
├── Utilities
│   ├── gdcmcharls -- cmake_minimum_required(VERSION 2.8.7)

I'd like to keep this as close as possible to each different upstream projects I have a convienent copy of. However when I'd like to test a new feature of CMake, I cannot simply change the top-level CMakeList.txt file and hope that new policies will be transmitted to individual sub-project (by definition of the nested command cmake_minimum_required).

Is there an idiom to say something like:

if(TOPLEVEL PROJECT)
cmake_minimum_required(VERSION 2.8.7)
else()
cmake_minimum_required(VERSION ${INHERITED_VERSION})
endif()

Answer 1

Something like this should work:

if(NOT DEFINED CMAKE_MINIMUM_REQUIRED_VERSION)
  cmake_minimum_required(VERSION 2.8.7)
endif()

I would not call cmake_minimum_required if it's already defined, since this will overwrite the policies that you manually set in your main project.