Changing CMAKE_CXX_FLAGS in project

Question

I have the following content in my CMakeLists.txt:

project( Matfile )

SET ( CMAKE_CXX_FLAGS \"-std=c++0x\" )

set ( SOURCES
      \"foo.cpp\"
      \"bar.cp         


        

Answer 1

I had this situation with C++ / clr project, where I needed to modify two variables - CMAKE_CXX_FLAGS and CMAKE_CXX_FLAGS_DEBUG. CMAKE_CXX_FLAGS can be modified using set_source_files_properties / COMPILE_FLAGS, but not CMAKE_CXX_FLAGS_DEBUG. I've concluded to create separate, project specific cmake file, in separate folder. If project is isolated - then variable modifying will work only within current C++/clr project.

I guess if you want to have multiple C++ projects and multiple C++/clr projects, makes sense to separate them at least in two different folder (if not more) - native C++ specific, and managed C++ specific.

In similar manner you can find out on cmake test application: https://github.com/Kitware/CMake/blob/master/Tests/CSharpLinkToCxx/CMakeLists.txt

Here is the same thing, only with more explanation:

# These variables affects also other C++ projects, that's why separate cmake for this c++ dll

# Clr compiling requires these:

# "Enable C++ Exceptions" - "Yes (/EHsc)" => "Yes with SEH Exceptions (/EHa)"
string(REPLACE "/EHsc" "/EHa" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")

# "Basic Run-time checks" - "Both (/RTC1, equiv. to /RTCsu) (/RTC1)" => "Default"
string(REPLACE "/RTC1" "" CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG}")

This is one approach. If you however want to generate native c++ and managed c++ projects in a loop - one approach is to disable global flags:

# Disable global flag, configure on per project basis, using target_compile_options, target_link_options
string(REPLACE "/RTC1" "" CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG}")
string(REPLACE "/INCREMENTAL" "" CMAKE_SHARED_LINKER_FLAGS_DEBUG "${CMAKE_SHARED_LINKER_FLAGS_DEBUG}")
#string(REPLACE "/EHsc" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")

and then configure each project setting individually for each project, for example like this:

if(${is_managed})
    # Managed C++ code
    set_target_properties(${project} PROPERTIES VS_GLOBAL_CLRSupport "true")
    set_property(TARGET ${project} PROPERTY VS_DOTNET_TARGET_FRAMEWORK_VERSION "v4.7.2")

    # Being able to debug dll
    target_link_options(${project} PRIVATE "$<$<CONFIG:Debug>:/ASSEMBLYDEBUG>")

    # Disable incremental linking
    target_link_options(${project} PRIVATE "$<$<CONFIG:Debug>:/INCREMENTAL:NO>")

    # "Enable C++ Exceptions" - "Yes with SEH Exceptions (/EHa)"
    set(compile_flags /EHa)
else()
    # Native C++ code
    # "Basic Run-time checks" - "Both (/RTC1, equiv. to /RTCsu) (/RTC1)" => "Default"
    target_compile_options(${project} PRIVATE "$<$<CONFIG:Debug>:/RTC1>")

    # Enable incremental linking
    target_link_options(${project} PRIVATE "$<$<CONFIG:Debug>:/INCREMENTAL>")

    # "Enable C++ Exceptions" - "Yes (/EHsc)"
    set(compile_flags /EHsc)
endif()
set_target_properties(${project} PROPERTIES COMPILE_FLAGS ${compile_flags})

Answer 2

This is the solution I currently use:

if(CMAKE_COMPILER_IS_GNUCXX)
    add_definitions(-std=c++0x)
    add_definitions(-std=gnu++11)
endif()

Or, if you have an older version of cmake and you want to see it show up in cmake-gui:

set_property(CACHE CMAKE_CXX_FLAGS PROPERTY VALUE "-std=c++0x")

Answer 3

I've come up with a better method that works on older versions of cmake, e.g. Ubuntu 14 has 2.8.12, and target expressions are 3.3 feature.

Here is how you would set some C++ specific flags:

STRING(REGEX REPLACE "<FLAGS>" "<FLAGS> -std=gnu++11 -fpermissive -fexceptions " CMAKE_CXX_COMPILE_OBJECT ${CMAKE_CXX_COMPILE_OBJECT})

Answer 4

Does it help to use the FORCE flag?

SET ( CMAKE_CXX_FLAGS "-std=c++0x" CACHE STRING "compile flags" FORCE)