如何使用 CMake 检测编译器对 C++11 的支持

Question

Is there a way to let CMake detect automatically if a compiler supports C++11 or not?

As it would be nice to inform the users during the CMake run that the code will not compile as the compiler does not support C++11. At the moment I set the C++11 flags. However, if a compiler does not support it the user gets compile errors instead of an error during the CMake run.

Perfect would be something that works like find_package() . However, I have not found any module or function which provides the functionality needed.

Additional it would be nice to have the feature to detect if the compiler needs the flags std=c++0x or std=c++11 .

Is there something available or do I need to develop this on my own?

Below is some code I use so far, however it works only with GNU'c GCC compilers. It would be nice if there would be a more general solution.

if(CMAKE_COMPILER_IS_GNUCXX)
   execute_process(COMMAND ${CMAKE_C_COMPILER} -dumpversion OUTPUT_VARIABLE GCC_VERSION)
   if (GCC_VERSION VERSION_GREATER 4.7 OR GCC_VERSION VERSION_EQUAL 4.7)
        message(STATUS "C++11 activated.")
        add_definitions("-std=gnu++11")
   elseif(GCC_VERSION VERSION_GREATER 4.3 OR GCC_VERSION VERSION_EQUAL 4.3)
        message(WARNING "C++0x activated. If you get any errors update to a compiler which fully supports C++11")
        add_definitions("-std=gnu++0x")
   else ()
        message(FATAL_ERROR "C++11 needed. Therefore a gcc compiler with a version higher than 4.3 is needed.")   
   endif()
else(CMAKE_COMPILER_IS_GNUCXX)
   add_definitions("-std=c++0x") 
endif(CMAKE_COMPILER_IS_GNUCXX)

Answer 1

If you have CMake version 3.1.0 or later you can detect what C++ features your C++ compiler supports

cmake_minimum_required(VERSION 3.1.0 FATAL_ERROR)
project(foobar CXX)
message("Your C++ compiler supports these C++ features:")
foreach(i ${CMAKE_CXX_COMPILE_FEATURES})
  message("${i}")
endforeach()

But normally you don't need to use the CMake variable CMAKE_CXX_COMPILE_FEATURES in your CMake scripts. Instead there are two ways of how to tell CMake under which C++ standard your C++ files should be compiled, either by specifying the C++ standard explicitly or by specifying the required C++ features and let CMake induce the C++ standard. CMake will make sure the C++ compiler is invoked with the correct command line flags (eg -std=c++11).

1. Specifying the C++ standard explicitly

You could specify the C++ standard explicitly, by setting the CMake propertiesCXX_STANDARD and CXX_STANDARD_REQUIRED for your CMake target.

$ cat /tmp/src/CMakeLists.txt
project(foobar CXX)
cmake_minimum_required(VERSION 3.1.0 FATAL_ERROR)
add_executable(prog main.cc)
set_property(TARGET prog PROPERTY CXX_STANDARD 11)
set_property(TARGET prog PROPERTY CXX_STANDARD_REQUIRED ON)
$ cat /tmp/src/main.cc
int main() {
  return 0;
}
$ mkdir /tmp/build
$ cd /tmp/build
$ cmake /tmp/src
-- The CXX compiler identification is GNU 4.8.2
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /tmp/build
$ make VERBOSE=1 | grep main.cc | grep -- "-c"
/usr/bin/c++    -std=gnu++11 -o CMakeFiles/prog.dir/main.cc.o -c /tmp/src/main.cc
$

2. Specifying the required C++ features and let CMake induce the C++ standard

You could use the CMake command target_compile_features to specify the C++ features that are made use of in your CMake target. From this list CMake will induce the C++ standard to be used. The CMake global property CMAKE_CXX_KNOWN_FEATURES lists the C++ features you can choose from.

cmake_minimum_required(VERSION 3.1.0 FATAL_ERROR)
message("Your CMake version supports these C++ features:")
get_property(known_features GLOBAL PROPERTY CMAKE_CXX_KNOWN_FEATURES)
foreach(i ${known_features})
  message("${i}")
endforeach()

For example, this C++ program with the filename main.cc makes use of the C++11 features: cxx_strong_enums , cxx_constexpr , cxx_auto_type

#include <cstdlib>

int main(int argc, char *argv[]) {
  enum class Color { Red, Orange, Yellow, Green, Blue, Violet };
  constexpr float a = 3.1415f;
  auto b = a;
  return EXIT_SUCCESS;
}

This CMakeLists.txt file would build it

cmake_minimum_required(VERSION 3.1.0 FATAL_ERROR)
project(foobar CXX)
add_executable(foobar main.cc)                                                                                                                                                                                                                                                     
set(needed_features
    cxx_strong_enums
    cxx_constexpr
    cxx_auto_type)
target_compile_features(foobar PRIVATE ${needed_features})

Answer 2

At this point, CMake does not have a convenient form to support C++11. Ideally, you would specify a C++11 project like this:

project(foo CXX11)

at the beginning of your CMakeLists.txt . But the CXX11 project type does not exist (yet). Until then, you may use a two-staged technique:

1. Determine the compiler type and version
2. Adjust build flags accordingly.

For example, this is what I use to support C++11 with Clang and GCC:

# Initialize CXXFLAGS.
set(CMAKE_CXX_FLAGS                "-Wall -std=c++11")
set(CMAKE_CXX_FLAGS_DEBUG          "-O0 -g")
set(CMAKE_CXX_FLAGS_MINSIZEREL     "-Os -DNDEBUG")
set(CMAKE_CXX_FLAGS_RELEASE        "-O4 -DNDEBUG")
set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "-O2 -g")

# Compiler-specific C++11 activation.
if ("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU")
    execute_process(
        COMMAND ${CMAKE_CXX_COMPILER} -dumpversion OUTPUT_VARIABLE GCC_VERSION)
    if (NOT (GCC_VERSION VERSION_GREATER 4.7 OR GCC_VERSION VERSION_EQUAL 4.7))
        message(FATAL_ERROR "${PROJECT_NAME} requires g++ 4.7 or greater.")
    endif ()
elseif ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++")
else ()
    message(FATAL_ERROR "Your C++ compiler does not support C++11.")
endif ()

Answer 3

I found this CMake script which claims to do exactly what you need. It can also check for individual C++11 features. I don't think it can decide between std=C++0x and std=C++11 though.

Answer 4

At the time of this writing (pre-GCC 4.8) , it may not be a good idea to detect C++11 flags and add them. This is because changing the standard (at least for GCC) breaks ABI compatibility , which can result in link errors.

Therefore, the use of the C++11 standard should explicitly specified with the compiler setting during the initial CMake configuration of the project , eg

CXX='g++ -std=c++11' cmake /path/to/source

That is, the use of -std=c++11 should be treated like a separate compiler, which should not be mixed or changed in a project.

Answer 5

Use:

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(FATAL_ERROR "Compiler ${CMAKE_CXX_COMPILER} has no C++11 support.")
endif()

This is from Enabling C++11 (C++0x) in CMake with minor changes.

Answer 6

If you are using CMake 3.8 or newer, you can use the feature cxx_std_11 , which requests C++11 or above :

target_compile_features(Hello PUBLIC cxx_std_11)

---

In CMake 3.1* and newer, the proper, simple way to do this is to use the CXX_STANDARD property for a given target. For example, given this simple example using auto (named main.cpp ):

#include <iostream>

int main() {
    auto num = 10;
    std::cout << num << std::endl;
    return 0;
}

The following CMakeLists.txt will enable C++11 support:

cmake_minimum_required(VERSION 3.3)
project(Hello CXX)

set(SOURCE_FILES main.cpp)
add_executable(Hello ${SOURCE_FILES})

set_property(TARGET Hello PROPERTY
    CXX_STANDARD 11
    CXX_STANDARD_REQUIRED ON
)

This will add any flags necessary such as -std=c++11 . Note that the CXX_STANDARD_REQUIRED property will prevent the standard from decaying to an earlier version.

---

Another proper but not as simple way to specify the CMAKE_CXX_KNOWN_FEATURES that you use, such as cxx_auto_type :

cmake_minimum_required(VERSION 3.3)
project(Hello CXX)

set(SOURCE_FILES main.cpp)
add_executable(Hello ${SOURCE_FILES})
target_compile_features(Hello PRIVATE cxx_auto_type)

---

_{* I have not tried this on CMake 3.1, but have verified it works in CMake 3.3. The documentation for 3.1 does document this so it should work.}

Answer 7

We've written a CMake module for detecting and enabling C++11 support which you can find here:
https://github.com/NitroShare/CXX11-CMake-Macros

It's still a work-in-progress, but we are using it for a number of Qt projects that target Windows/Linux/Mac. Currently only MSVC++, GCC, and Clang are supported.

Example:

include(CXX11)

check_for_cxx11_compiler(CXX11_COMPILER)

# If a C++11 compiler is available, then set the appropriate flags
if(CXX11_COMPILER)
    enable_cxx11()
endif()