CMake基礎 第12節 設置C++標准

介紹

自從C++11和C++14發布以來，一個常見的用例是調用編譯器來使用這些標准。隨着CMake的發展，它添加了一些功能來使這一點變得更容易，而CMake的新版本已經改變了實現這一點的方式。下面的示例顯示了設置C++標准的三種不同方法，以及可以使用哪些版本的CMake。

這些例子包括：

• [common-method]. 一個簡單的版本，應該可以與大多數版本的CMake一起使用
• [cxx-standard]. 使用CMake v3.1中引入的CMAKE_CXX_STANDARD變量
• [compile-features]. 使用CMakev3.1中引入的target_compile_features函數

一個簡單的版本

此示例顯示了設置C++標准的通用方法。這可以與大多數版本的CMake一起使用。但是，如果你使用CMake的最新版本，則可以用更方便的方法。

本教程中的文件如下：

A-hello-cmake$ tree
.
├── CMakeLists.txt
├── main.cpp

• [CMakeLists.txt] - 包含要運行的CMake命令。

  # Set the minimum version of CMake that can be used
  # To find the cmake version run
  # $ cmake --version
  cmake_minimum_required(VERSION 2.8)
  
  # Set the project name
  project (hello_cpp11)
  
  # try conditional compilation
  
  #  Check whether the CXX compiler supports a given flag.
  ## CHECK_CXX_COMPILER_FLAG(<flag> <var>)
  #  <flag> - the compiler flag
  #  <var>  - variable to store the result
  #  This internally calls the check_cxx_source_compiles macro and sets CMAKE_REQUIRED_DEFINITIONS to <flag>
  include(CheckCXXCompilerFlag)
  CHECK_CXX_COMPILER_FLAG("-std=c++11" COMPILER_SUPPORTS_CXX11)
  CHECK_CXX_COMPILER_FLAG("-std=c++0x" COMPILER_SUPPORTS_CXX0X)
  
  # check results and add flag
  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()
  
  # Add an executable
  add_executable(hello_cpp11 main.cpp)
  

• [main.cpp] - 一個針對C++11的簡單的“Hello World”CPP文件。

  #include <iostream>
  
  int main(int argc, char *argv[])
  {
      auto message = "Hello C++11";
      std::cout << message << std::endl;
      return 0;
  }
  

概念

檢查編譯標志

CMake支持嘗試使用傳遞給函數CMAKE_CXX_COMPILER_FLAG的任何標志編譯程序。然后將結果存儲在你傳入的變量中。

例如:

include(CheckCXXCompilerFlag)
CHECK_CXX_COMPILER_FLAG("-std=c++11" COMPILER_SUPPORTS_CXX11)

此示例將嘗試使用標志-std=c++11編譯程序，並將結果存儲在變量COMPILER_SUPPORTS_CXX11中。

include(CheckCXXCompilerFlag)這一行告訴CMake包含此函數以使其可用。

添加標志

一旦確定編譯是否支持標志，就可以使用標准的cmake方法將該標志添加到目標。在本例中，我們使用CMAKE_CXX_FLAGS將該標志傳遞到所有目標。

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()

上面的示例只檢查編譯標志的GCC版本，並支持從C++11回退到標准化前的C++0x標志。在實際使用中，你可能希望檢查C14，或者添加對不同編譯設置方法的支持，例如-std=gnu11。

構建示例

下面是構建此示例的示例輸出。

$ mkdir build
$ cd build

$ cmake ..
-- The C compiler identification is GNU 4.8.4
-- The CXX compiler identification is GNU 4.8.4
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- 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
-- Performing Test COMPILER_SUPPORTS_CXX11
-- Performing Test COMPILER_SUPPORTS_CXX11 - Success
-- Performing Test COMPILER_SUPPORTS_CXX0X
-- Performing Test COMPILER_SUPPORTS_CXX0X - Success
-- Configuring done
-- Generating done
-- Build files have been written to: /data/code/01-basic/L-cpp-standard/i-common-method/build

$ make VERBOSE=1
/usr/bin/cmake -H/data/code/01-basic/L-cpp-standard/i-common-method -B/data/code/01-basic/L-cpp-standard/i-common-method/build --check-build-system CMakeFiles/Makefile.cmake 0
/usr/bin/cmake -E cmake_progress_start /data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles /data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles/progress.marks
make -f CMakeFiles/Makefile2 all
make[1]: Entering directory `/data/code/01-basic/L-cpp-standard/i-common-method/build'
make -f CMakeFiles/hello_cpp11.dir/build.make CMakeFiles/hello_cpp11.dir/depend
make[2]: Entering directory `/data/code/01-basic/L-cpp-standard/i-common-method/build'
cd /data/code/01-basic/L-cpp-standard/i-common-method/build && /usr/bin/cmake -E cmake_depends "Unix Makefiles" /data/code/01-basic/L-cpp-standard/i-common-method /data/code/01-basic/L-cpp-standard/i-common-method /data/code/01-basic/L-cpp-standard/i-common-method/build /data/code/01-basic/L-cpp-standard/i-common-method/build /data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles/hello_cpp11.dir/DependInfo.cmake --color=
Dependee "/data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles/hello_cpp11.dir/DependInfo.cmake" is newer than depender "/data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles/hello_cpp11.dir/depend.internal".
Dependee "/data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles/CMakeDirectoryInformation.cmake" is newer than depender "/data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles/hello_cpp11.dir/depend.internal".
Scanning dependencies of target hello_cpp11
make[2]: Leaving directory `/data/code/01-basic/L-cpp-standard/i-common-method/build'
make -f CMakeFiles/hello_cpp11.dir/build.make CMakeFiles/hello_cpp11.dir/build
make[2]: Entering directory `/data/code/01-basic/L-cpp-standard/i-common-method/build'
/usr/bin/cmake -E cmake_progress_report /data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles 1
[100%] Building CXX object CMakeFiles/hello_cpp11.dir/main.cpp.o
/usr/bin/c++    -std=c++11   -o CMakeFiles/hello_cpp11.dir/main.cpp.o -c /data/code/01-basic/L-cpp-standard/i-common-method/main.cpp
Linking CXX executable hello_cpp11
/usr/bin/cmake -E cmake_link_script CMakeFiles/hello_cpp11.dir/link.txt --verbose=1
/usr/bin/c++    -std=c++11    CMakeFiles/hello_cpp11.dir/main.cpp.o  -o hello_cpp11 -rdynamic
make[2]: Leaving directory `/data/code/01-basic/L-cpp-standard/i-common-method/build'
/usr/bin/cmake -E cmake_progress_report /data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles  1
[100%] Built target hello_cpp11
make[1]: Leaving directory `/data/code/01-basic/L-cpp-standard/i-common-method/build'
/usr/bin/cmake -E cmake_progress_start /data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles 0

使用CMAKE_CXX_STANDARD變量

介紹

此示例說明如何使用CMAKE_CXX_STANDARD變量設置C++標准。這是從CMake v3.1開始提供的。

本教程中的文件如下：

A-hello-cmake$ tree
.
├── CMakeLists.txt
├── main.cpp

• [CMakeLists.txt] - 包含要運行的CMake命令

  # Set the minimum version of CMake that can be used
  # To find the cmake version run
  # $ cmake --version
  cmake_minimum_required(VERSION 3.1)
  
  # Set the project name
  project (hello_cpp11)
  
  # set the C++ standard to C++ 11
  set(CMAKE_CXX_STANDARD 11)
  
  # Add an executable
  add_executable(hello_cpp11 main.cpp)
  

• [main.cpp] - 一個針對C++11的簡單的“Hello World”CPP文件

  #include <iostream>
  
  int main(int argc, char *argv[])
  {
      auto message = "Hello C++11";
      std::cout << message << std::endl;
      return 0;
  }
  

概念

使用CXX_STANDARD屬性

設置CMAKE_CXX_STANDARD變量會導致所有目標上的CXX_STANDARD屬性改變。這會影響CMake在編譯時設置適當的標志。

Note	CMAKE_CXX_STANDARD變量會回退到最接近的不會失敗的適當標准。例如，如果請求-std=gnu11，最后可能變成-std=gnu0x。這可能會在編譯時導致意外故障。

構建示例

下面是構建此示例的示例輸出：

$ mkdir build
$ cd build


$ cmake ..
-- The C compiler identification is GNU 5.4.0
-- The CXX compiler identification is GNU 5.4.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- 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: /data/code/01-basic/L-cpp-standard/ii-cxx-standard/build

$ make VERBOSE=1
/usr/bin/cmake -H/data/code/01-basic/L-cpp-standard/ii-cxx-standard -B/data/code/01-basic/L-cpp-standard/ii-cxx-standard/build --check-build-system CMakeFiles/Makefile.cmake 0
/usr/bin/cmake -E cmake_progress_start /data/code/01-basic/L-cpp-standard/ii-cxx-standard/build/CMakeFiles /data/code/01-basic/L-cpp-standard/ii-cxx-standard/build/CMakeFiles/progress.marks
make -f CMakeFiles/Makefile2 all
make[1]: Entering directory '/data/code/01-basic/L-cpp-standard/ii-cxx-standard/build'
make -f CMakeFiles/hello_cpp11.dir/build.make CMakeFiles/hello_cpp11.dir/depend
make[2]: Entering directory '/data/code/01-basic/L-cpp-standard/ii-cxx-standard/build'
cd /data/code/01-basic/L-cpp-standard/ii-cxx-standard/build && /usr/bin/cmake -E cmake_depends "Unix Makefiles" /data/code/01-basic/L-cpp-standard/ii-cxx-standard /data/code/01-basic/L-cpp-standard/ii-cxx-standard /data/code/01-basic/L-cpp-standard/ii-cxx-standard/build /data/code/01-basic/L-cpp-standard/ii-cxx-standard/build /data/code/01-basic/L-cpp-standard/ii-cxx-standard/build/CMakeFiles/hello_cpp11.dir/DependInfo.cmake --color=
Dependee "/data/code/01-basic/L-cpp-standard/ii-cxx-standard/build/CMakeFiles/hello_cpp11.dir/DependInfo.cmake" is newer than depender "/data/code/01-basic/L-cpp-standard/ii-cxx-standard/build/CMakeFiles/hello_cpp11.dir/depend.internal".
Dependee "/data/code/01-basic/L-cpp-standard/ii-cxx-standard/build/CMakeFiles/CMakeDirectoryInformation.cmake" is newer than depender "/data/code/01-basic/L-cpp-standard/ii-cxx-standard/build/CMakeFiles/hello_cpp11.dir/depend.internal".
Scanning dependencies of target hello_cpp11
make[2]: Leaving directory '/data/code/01-basic/L-cpp-standard/ii-cxx-standard/build'
make -f CMakeFiles/hello_cpp11.dir/build.make CMakeFiles/hello_cpp11.dir/build
make[2]: Entering directory '/data/code/01-basic/L-cpp-standard/ii-cxx-standard/build'
[ 50%] Building CXX object CMakeFiles/hello_cpp11.dir/main.cpp.o
/usr/bin/c++     -std=gnu++11 -o CMakeFiles/hello_cpp11.dir/main.cpp.o -c /data/code/01-basic/L-cpp-standard/ii-cxx-standard/main.cpp
[100%] Linking CXX executable hello_cpp11
/usr/bin/cmake -E cmake_link_script CMakeFiles/hello_cpp11.dir/link.txt --verbose=1
/usr/bin/c++      CMakeFiles/hello_cpp11.dir/main.cpp.o  -o hello_cpp11 -rdynamic
make[2]: Leaving directory '/data/code/01-basic/L-cpp-standard/ii-cxx-standard/build'
[100%] Built target hello_cpp11
make[1]: Leaving directory '/data/code/01-basic/L-cpp-standard/ii-cxx-standard/build'

使用target_compile_features函數

介紹

此示例說明如何使用target_compile_features函數設置C++標准。

本教程中的文件如下：

A-hello-cmake$ tree
.
├── CMakeLists.txt
├── main.cpp

• [CMakeLists.txt] - 包含要運行的CMake命令

  # Set the minimum version of CMake that can be used
  # To find the cmake version run
  # $ cmake --version
  cmake_minimum_required(VERSION 3.1)
  
  # Set the project name
  project (hello_cpp11)
  
  # Add an executable
  add_executable(hello_cpp11 main.cpp)
  
  # set the C++ standard to the appropriate standard for using auto
  target_compile_features(hello_cpp11 PUBLIC cxx_auto_type)
  
  # Print the list of known compile features for this version of CMake
  message("List of compile features: ${CMAKE_CXX_COMPILE_FEATURES}")
  

• [main.cpp] - 一個針對C++11的簡單的“Hello World”C++文件

  #include <iostream>
  
  int main(int argc, char *argv[])
  {
      auto message = "Hello C++11";
      std::cout << message << std::endl;
      return 0;
  }
  

概念

使用target_compile_features

在目標上調用target_compile_features函數將檢查傳入的功能，並由CMake確定正確的用於目標的編譯器標志。

target_compile_features(hello_cpp11 PUBLIC cxx_auto_type)

與其他target_*函數一樣，你可以指定所選目標的功能范圍。這將填充目標的INTERFACE_COMPILE_FEATURES屬性。可用功能列表可從CMAKE_CXX_COMPILE_FEATURES變量中找到。你可以使用以下代碼獲取可用功能的列表：

message("List of compile features: ${CMAKE_CXX_COMPILE_FEATURES}")

構建示例

下面是構建此示例的輸出。

$ mkdir build
$ cd build

$ cmake ..
-- The C compiler identification is GNU 5.4.0
-- The CXX compiler identification is GNU 5.4.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- 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
List of compile features: cxx_template_template_parameters;cxx_alias_templates;cxx_alignas;cxx_alignof;cxx_attributes;cxx_auto_type;cxx_constexpr;cxx_decltype;cxx_decltype_incomplete_return_types;cxx_default_function_template_args;cxx_defaulted_functions;cxx_defaulted_move_initializers;cxx_delegating_constructors;cxx_deleted_functions;cxx_enum_forward_declarations;cxx_explicit_conversions;cxx_extended_friend_declarations;cxx_extern_templates;cxx_final;cxx_func_identifier;cxx_generalized_initializers;cxx_inheriting_constructors;cxx_inline_namespaces;cxx_lambdas;cxx_local_type_template_args;cxx_long_long_type;cxx_noexcept;cxx_nonstatic_member_init;cxx_nullptr;cxx_override;cxx_range_for;cxx_raw_string_literals;cxx_reference_qualified_functions;cxx_right_angle_brackets;cxx_rvalue_references;cxx_sizeof_member;cxx_static_assert;cxx_strong_enums;cxx_thread_local;cxx_trailing_return_types;cxx_unicode_literals;cxx_uniform_initialization;cxx_unrestricted_unions;cxx_user_literals;cxx_variadic_macros;cxx_variadic_templates;cxx_aggregate_default_initializers;cxx_attribute_deprecated;cxx_binary_literals;cxx_contextual_conversions;cxx_decltype_auto;cxx_digit_separators;cxx_generic_lambdas;cxx_lambda_init_captures;cxx_relaxed_constexpr;cxx_return_type_deduction;cxx_variable_templates
-- Configuring done
-- Generating done
-- Build files have been written to: /data/code/01-basic/L-cpp-standard/iii-compile-features/build


$ make VERBOSE=1
/usr/bin/cmake -H/data/code/01-basic/L-cpp-standard/iii-compile-features -B/data/code/01-basic/L-cpp-standard/iii-compile-features/build --check-build-system CMakeFiles/Makefile.cmake 0
/usr/bin/cmake -E cmake_progress_start /data/code/01-basic/L-cpp-standard/iii-compile-features/build/CMakeFiles /data/code/01-basic/L-cpp-standard/iii-compile-features/build/CMakeFiles/progress.marks
make -f CMakeFiles/Makefile2 all
make[1]: Entering directory '/data/code/01-basic/L-cpp-standard/iii-compile-features/build'
make -f CMakeFiles/hello_cpp11.dir/build.make CMakeFiles/hello_cpp11.dir/depend
make[2]: Entering directory '/data/code/01-basic/L-cpp-standard/iii-compile-features/build'
cd /data/code/01-basic/L-cpp-standard/iii-compile-features/build && /usr/bin/cmake -E cmake_depends "Unix Makefiles" /data/code/01-basic/L-cpp-standard/iii-compile-features /data/code/01-basic/L-cpp-standard/iii-compile-features /data/code/01-basic/L-cpp-standard/iii-compile-features/build /data/code/01-basic/L-cpp-standard/iii-compile-features/build /data/code/01-basic/L-cpp-standard/iii-compile-features/build/CMakeFiles/hello_cpp11.dir/DependInfo.cmake --color=
Dependee "/data/code/01-basic/L-cpp-standard/iii-compile-features/build/CMakeFiles/hello_cpp11.dir/DependInfo.cmake" is newer than depender "/data/code/01-basic/L-cpp-standard/iii-compile-features/build/CMakeFiles/hello_cpp11.dir/depend.internal".
Dependee "/data/code/01-basic/L-cpp-standard/iii-compile-features/build/CMakeFiles/CMakeDirectoryInformation.cmake" is newer than depender "/data/code/01-basic/L-cpp-standard/iii-compile-features/build/CMakeFiles/hello_cpp11.dir/depend.internal".
Scanning dependencies of target hello_cpp11
make[2]: Leaving directory '/data/code/01-basic/L-cpp-standard/iii-compile-features/build'
make -f CMakeFiles/hello_cpp11.dir/build.make CMakeFiles/hello_cpp11.dir/build
make[2]: Entering directory '/data/code/01-basic/L-cpp-standard/iii-compile-features/build'
[ 50%] Building CXX object CMakeFiles/hello_cpp11.dir/main.cpp.o
/usr/bin/c++     -std=gnu++11 -o CMakeFiles/hello_cpp11.dir/main.cpp.o -c /data/code/01-basic/L-cpp-standard/iii-compile-features/main.cpp
[100%] Linking CXX executable hello_cpp11
/usr/bin/cmake -E cmake_link_script CMakeFiles/hello_cpp11.dir/link.txt --verbose=1
/usr/bin/c++      CMakeFiles/hello_cpp11.dir/main.cpp.o  -o hello_cpp11 -rdynamic
make[2]: Leaving directory '/data/code/01-basic/L-cpp-standard/iii-compile-features/build'
[100%] Built target hello_cpp11
make[1]: Leaving directory '/data/code/01-basic/L-cpp-standard/iii-compile-features/build'
/usr/bin/cmake -E cmake_progress_start /data/code/01-basic/L-cpp-standard/