Changing CMAKE_CXX_FLAGS in project

The correct way to set the C++ standard in CMake 3.1 and later is:

set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED on)

It is possible to specify the standard for one individual target also:

set_property(TARGET mylib PROPERTY CXX_STANDARD 11)

Since CMake 3.8 there is a new option to the target_compile_features command that allows to set the required standard for a target:

target_compile_features(mylib PUBLIC cxx_std_11)

The advantage would be that it propagates the requirement to dependent targets. If you compile a library with the cxx_std_11 required feature, any binary that links to it will automatically have this requirement set.

The most straightforward solution should be using add_compile_options() if you are using version 2.8.12 or newer. For older versions you can "abuse" add_definitions(). While it is only meant for add -D flags, it also works with any other compiler flag. However, I think it is not meant to be used that way and could break in a future version.

add_compile_options(-std=c++0x) # CMake 2.8.12 or newer

or

add_definitions(-std=c++0x) # CMake 2.8.11 or older

Starting with CMake 3.3 you can also make this flag only apply to a specific language (e.g. only C or C++) using the strange generator expressions syntax:

 add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-std=c++14> $<$<COMPILE_LANGUAGE:C>:-std=c99>)

However this will not work with the Visual studio generator, so make sure to only use this for Make/Ninja generators or use target_compile_options() to set it on a per-target scope.

Does it help to use the FORCE flag?

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

Perhaps this would work better:

set_source_files_properties(${SOURCES}
       PROPERTIES
       COMPILE_FLAGS  "-std=c++0x")