Cmake policy setting in building Opencv

Question

I am not sure where to set cmake_policy. This link has explained cmake_policy. But not sure where to set cmake policy. I use cmake to build Opencv and I have a lot of warnings like

CMake Warning (dev) at cuda_compile_generated_row_filter.2.cu.o.cmake:137 (if):
  Policy CMP0054 is not set: Only interpret if() arguments as variables or
  keywords when unquoted.  Run "cmake --help-policy CMP0054" for policy
  details.  Use the cmake_policy command to set the policy and suppress this
  warning.

I need to set like cmake_policy(SET CMP0054 NEW). Where to set this policy? I did like

ppp@ppp-Inspiron-7537:~/Softwares/opencv-2.4.9/build$ cmake_policy(SET CMP0054 NEW)
bash: syntax error near unexpected token `SET'

but I got error. How can I set the policy?

Answer 1

Use -Wno-dev option for cmake for suppress warnings intended for developers:

cmake -Wno-dev <other-options> <source-dir>

Alternatively, if you want to set policy for the project, you need to modify its CMakeLists.txt file and add line

cmake_policy(SET CMP0054 OLD)

somewhere at the beginning.

Note, that you need to use OLD value for the property, as the project most likely relies on such variable's dereference. Setting NEW value will probably break project's functionality.

When project's code will be fixed, warnings won't be generated even without explicit policy set.