How do I activate C++ 11 in CMake?

Question

When I try to run a 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.

Answer 1

CMake 3.1 introduced the CMAKE_CXX_STANDARD variable that you can use. If you know that you will always have CMake 3.1 or later 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 () 

Answer 2

The CMake command target_compile_features() is used to specify the required C++ feature cxx_range_for. CMake will then induce the C++ standard to be used.

cmake_minimum_required(VERSION 3.1.0 FATAL_ERROR) project(foobar CXX) add_executable(foobar main.cc) target_compile_features(foobar PRIVATE cxx_range_for) 

There is no need to use add_definitions(-std=c++11) or to modify the CMake variable CMAKE_CXX_FLAGS, because CMake will make sure the C++ compiler is invoked with the appropriate command line flags.

Maybe your C++ program uses other C++ features than cxx_range_for. The CMake global property CMAKE_CXX_KNOWN_FEATURES lists the C++ features you can choose from.

Instead of using target_compile_features() you can also specify the C++ standard explicitly by setting the CMake properties CXX_STANDARD and CXX_STANDARD_REQUIRED for your CMake target.

See also my more detailed answer.