Disable -Werror for one of CMakeLists.txt

Question

I have the following CMake file:

project(MyLib)
cmake_minimum_required(VERSION 2.8)

if(NOT CMAKE_BUILD_TYPE)
  set(CMAKE_BUILD_TYPE "release")
endif()

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall -Werror")
set(ROOT_DIR ${CMAKE_SOURCE_DIR})
set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${ROOT_DIR}/bin/${CMAKE_BUILD_TYPE})
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${ROOT_DIR}/bin/${CMAKE_BUILD_TYPE})
set(MAIN_LIBRARY_NAME "mylib")

add_subdirectory(src)
add_subdirectory(test_app)
add_subdirectory(test_app1) <--- I want to disable -Werror flag for CMakeLists.txt in this folder.
add_subdirectory(test_app2)

How to disable -Werror flag for one of subdirectories? In the each of sub directories I have CMakeLists.txt too.

Answer 1

Turning my comment into an answer

All variables and directory properties are copied to the subdirectory's context at the moment you call add_subdirectory(). Either modify CMAKE_CXX_FLAGS before this call with something like

string(REPLACE " -Werror" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
add_subdirectory(test_app1) 
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror")

Or use the "old way" were compiler flags could be given with add_definitions() and removed with remove_definitions()

add_definitions(-std=c++11 -Wall -Werror)
...
remove_definitions(-Werror)
add_subdirectory(test_app1)
add_definitions(-Werror)

But you can - in contradiction to my first comment - change flags added add_compile_options() only on target level COMPILE_OPTIONS property and not for complete directories. When the target is created the property is copied as-is from the directory to the target and changing the directory property later won't automatically update the target's property.

If you have a newer CMake version that supports SOURCE_DIR target property you can alternatively go by some crazy generator expression:

add_compile_options(
    -std=c++11 
    -Wall 
    $<$<NOT:$<STREQUAL:$<TARGET_PROPERTY:SOURCE_DIR>,${CMAKE_SOURCE_DIR}/test_app1>>:-Werror>
)

Reference

• Is Cmake set variable recursive?