-std=c++11 - How to activate C++11 in CMake?





command line (11)


CMake 3.1 introduced the CMAKE_CXX_STANDARD variable that you can use. If you know that you will always have CMake 3.1 available, you can just write this in your top-level CMakeLists.txt file, or put it right before any new target is defined:

set (CMAKE_CXX_STANDARD 11)

If you need to support older versions of CMake, here is a macro I came up with that you can use:

macro(use_cxx11)
  if (CMAKE_VERSION VERSION_LESS "3.1")
    if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
      set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++11")
    endif ()
  else ()
    set (CMAKE_CXX_STANDARD 11)
  endif ()
endmacro(use_cxx11)

The macro only supports GCC right now, but it should be straight-forward to expand it to other compilers.

Then you could write use_cxx11() at the top of any CMakeLists.txt file that defines a target that uses C++11.

CMake issue #15943 for clang users targeting macOS

If you are using CMake and clang to target MacOS there is a bug that can cause the CMAKE_CXX_STANDARD feature to simply not work (not add any compiler flags). Make sure that you do one of the following things:

  • Use cmake_minimum_required to require CMake 3.0 or later, or
  • Set policy CMP0025 to NEW with the following code at the top of your CMakeLists.txt file before the project command:

    # Fix behavior of CMAKE_CXX_STANDARD when targeting macOS.
    if (POLICY CMP0025)
      cmake_policy(SET CMP0025 NEW)
    endif ()
    

When I try to run CMake generated makefile to compile my program, I get the error that

range based for loops are not supported in C++ 98 mode.

I tried adding add_definitions(-std=c++0x) to my CMakeLists.txt, but it did not help. I tried this too:

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

When I do g++ --version, I get:

g++ (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1

I have also tried SET(CMAKE_CXX_FLAGS "-std=c++0x"), which also does not work.

I do not understand how I can activate C++ 11 features using CMake.




I think just these two lines are enough.

set(CMAKE_CXX_STANDARD 11)

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")



The easiest way to set the Cxx standard is:

 set_property(TARGET tgt PROPERTY CXX_STANDARD 11)

See the CMake documentation for more details.




This is another way of enabling C++11 support,

ADD_DEFINITIONS(
    -std=c++11 # Or -std=c++0x
    # Other flags
)

I have encountered instances where only this method works and other methods fail. Maybe it has something to do with the latest version of CMake.




On modern CMake (>= 3.1) best way to set global requirement is:

set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)

It translates to "I want C++11 for all targets, it's not optional, I dont't want to use any GNU or MS extensions." As of c++17, this still is IMHO the best way.

Source: https://crascit.com/2015/03/28/enabling-cxx11-in-cmake/




I am using

include(CheckCXXCompilerFlag)
CHECK_CXX_COMPILER_FLAG("-std=c++11" COMPILER_SUPPORTS_CXX11)
CHECK_CXX_COMPILER_FLAG("-std=c++0x" COMPILER_SUPPORTS_CXX0X)
if(COMPILER_SUPPORTS_CXX11)
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
elseif(COMPILER_SUPPORTS_CXX0X)
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++0x")
else()
        message(STATUS "The compiler ${CMAKE_CXX_COMPILER} has no C++11 support. Please use a different C++ compiler.")
endif()

But if you want to play with C++11, g++ 4.6.1 is pretty old. Try to get a newer g++ version.




For CMake 3.8 and newer you can use

target_compile_features(target PUBLIC cxx_std_11)



As it turns out, SET(CMAKE_CXX_FLAGS "-std=c++0x") does activate many C++11 features. The reason it did not work was that the statement looked like this:

set(CMAKE_CXX_FLAGS "-std=c++0x ${CMAKE_CXX_FLAGS} -g -ftest-coverage -fprofile-arcs")

Following this approach, somehow the -std=c++0x flag was overwritten and it did not work. Setting the flags one by one or using a list method is working.

list( APPEND CMAKE_CXX_FLAGS "-std=c++0x ${CMAKE_CXX_FLAGS} -g -ftest-coverage -fprofile-arcs")



The easiest way:

add_compile_options(-std=c++11)




What works for me is to set the following line in your CMakeLists.txt:

set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")

Setting this command activates the C++11 features for the compiler and after executing the cmake .. command, you should be able to use range based for loops in your code and compile it without any errors.




For languages not specifying a memory model, you are writing code for the language and the memory model specified by the processor architecture. The processor may choose to re-order memory accesses for performance. So, if your program has data races (a data race is when it's possible for multiple cores / hyper-threads to access the same memory concurrently) then your program is not cross platform because of its dependence on the processor memory model. You may refer to the Intel or AMD software manuals to find out how the processors may re-order memory accesses.

Very importantly, locks (and concurrency semantics with locking) are typically implemented in a cross platform way... So if you are using standard locks in a multithreaded program with no data races then you don't have to worry about cross platform memory models.

Interestingly, Microsoft compilers for C++ have acquire / release semantics for volatile which is a C++ extension to deal with the lack of a memory model in C++ http://msdn.microsoft.com/en-us/library/12a04hfd(v=vs.80).aspx. However, given that Windows runs on x86 / x64 only, that's not saying much (Intel and AMD memory models make it easy and efficient to implement acquire / release semantics in a language).





c++11 cmake