CMake基础第12节设置C++标准

CMake基础第12节设置C++标准
介绍
⾃从C++11和C++14发布以来，⼀个常见的⽤例是调⽤编译器来使⽤这些标准。

随着CMake的发展，它添加了⼀些功能来使这⼀点变得更容易，⽽CMake的新版本已经改变了实现这⼀点的⽅式。

下⾯的
⽰例显⽰了设置C++标准的三种不同⽅法，以及可以使⽤哪些版本的CMake。

这些例⼦包括：
[common-method]. ⼀个简单的版本，应该可以与⼤多数版本的CMake⼀起使⽤
[cxx-standard]. 使⽤CMake v3.1中引⼊的CMAKE_CXX_STANDARD变量
[compile-features]. 使⽤CMakev3.1中引⼊的target_compile_features函数
⼀个简单的版本
此⽰例显⽰了设置C++标准的通⽤⽅法。

这可以与⼤多数版本的CMake⼀起使⽤。

但是，如果你使⽤CMake的最新版本，则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] - 包含要运⾏的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] - ⼀个针对C++11的简单的“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
检查编译标志
CMake⽀持尝试使⽤传递给函数CMAKE_CXX_COMPILER_FLAG的任何标志编译程序。

然后将结果存储在你传⼊的变量中。

例如:
include(CheckCXXCompilerFlag)
CHECK_CXX_COMPILER_FLAG("-std=c++11" COMPILER_SUPPORTS_CXX11)
此⽰例将尝试使⽤标志-std=c++11编译程序，并将结果存储在变量COMPILER_SUPPORTS_CXX11中。

include(CheckCXXCompilerFlag)这⼀⾏告诉CMake包含此函数以使其可⽤。

添加标志
⼀旦确定编译是否⽀持标志，就可以使⽤标准的cmake⽅法将该标志添加到⽬标。

在本例中，我们使⽤CMAKE_CXX_FLAGS将该标志传递到所有⽬标。

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()
上⾯的⽰例只检查编译标志的GCC版本，并⽀持从C++11回退到标准化前的C++0x标志。

在实际使⽤中，你可能希望检查C14，或者添加对不同编译设置⽅法的⽀持，例如-std=gnu11。

构建⽰例
下⾯是构建此⽰例的⽰例输出。

$ 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-m 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 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 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变量
介绍
此⽰例说明如何使⽤CMAKE_CXX_STANDARD变量设置C++标准。

这是从CMake v3.1开始提供的。

本教程中的⽂件如下：
A-hello-cmake$ tree
.
├── CMakeLists.txt
├── main.cpp
[CMakeLists.txt] - 包含要运⾏的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] - ⼀个针对C++11的简单的“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属性
设置CMAKE_CXX_STANDARD变量会导致所有⽬标上的CXX_STANDARD属性改变。

这会影响CMake在编译时设置适当的标志。

Note CMAKE_CXX_STANDARD变量会回退到最接近的不会失败的适当标准。

例如，如果请求-std=gnu11，最后可能变成-std=gnu0x。

这可能会在编译时导致意外故障。

构建⽰例
下⾯是构建此⽰例的⽰例输出：
$ 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 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 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 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函数
介绍
此⽰例说明如何使⽤target_compile_features函数设置C++标准。

本教程中的⽂件如下：
A-hello-cmake$ tree
.
├── CMakeLists.txt
├── main.cpp
[CMakeLists.txt] - 包含要运⾏的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] - ⼀个针对C++11的简单的“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
在⽬标上调⽤target_compile_features函数将检查传⼊的功能，并由CMake确定正确的⽤于⽬标的编译器标志。

target_compile_features(hello_cpp11 PUBLIC cxx_auto_type)
与其他target_*函数⼀样，你可以指定所选⽬标的功能范围。

这将填充⽬标的INTERFACE_COMPILE_FEATURES属性。

可⽤功能列表可从CMAKE_CXX_COMPILE_FEATURES变量中找到。

你可以使
⽤以下代码获取可⽤功能的列表：
message("List of compile features: ${CMAKE_CXX_COMPILE_FEATURES}")
构建⽰例
下⾯是构建此⽰例的输出。

$ 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_functio -- 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-f 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/he 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/hel 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/。