如何让 CMake 基于 GCC 版本通过 std=c++14/c++1y 或 c++17/c++1z？
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How to get CMake to pass either std=c++14/c++1y or c++17/c++1z based on GCC version?
提问by einpoklum
GCC 4.x doesn't accept the --std=c++14switch for C++14 code - it takes --std=c++1yinstead. Later versions take --std=c++1zbut (probably) not --std=c++17which has not been set yet (writing this in 2016). Perhaps there are similar issues with C++11.
GCC 4.x 不接受--std=c++14C++14 代码的开关 ---std=c++1y取而代之。后来的版本采用--std=c++1z但（可能）--std=c++17尚未设置（在 2016 年编写）。也许 C++11 也有类似的问题。
Does CMake have some facility (perhaps as a module) to pass the correct switch according to the GCC version?
CMake 是否有一些工具（也许作为一个模块）来根据 GCC 版本传递正确的开关？
回答by Craig Scott
When wanting to specify a particular C++ version, the recommended way to do this with CMake 3.1 and later is to use the CXX_STANDARD, CXX_STANDARD_REQUIREDand CXX_EXTENSIONStarget properties, or their variable equivalents to specify target defaults. Full details can be found here, but the short version goes something like this:
当想要指定特定的 C++ 版本时，推荐使用 CMake 3.1 及更高版本执行此操作的方法是使用CXX_STANDARD,CXX_STANDARD_REQUIRED和CXX_EXTENSIONStarget 属性，或它们的变量等效项来指定目标默认值。完整的细节可以在这里找到，但简短的版本是这样的：
cmake_minimum_required(VERSION 3.1)
project(Example)
set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)
# ... Define targets, etc. as usual
CMake should then select the appropriate compiler flag for the requested C++ standard based on what the compiler supports, or error out if it doesn't support the requested standard.
然后，CMake 应根据编译器支持的内容为所请求的 C++ 标准选择适当的编译器标志，如果不支持所请求的标准，则出错。
It should also be noted that CMake may upgrade the target to use a later language standard than the one specified by its CXX_STANDARDtarget property. The use of compile feature requirements(as mentioned in @FlorianWolters answer) can raise the language standard requirement. In fact, CMake will always pick the stronger language requirement specified by either the CXX_STANDARDtarget property or the compile feature requirements set on the target. Note also that the CMake documentation as of 3.10.1 does not accurately reflect the way CXX_EXTENSIONSinteracts with compile features, as CXX_EXTENSIONSonly takes effect if CXX_STANDARDis also specified for most common compilers (since they are specified together with the one compiler flag).
还应该注意的是，CMake 可能会升级目标以使用比其CXX_STANDARD目标属性指定的语言标准更高的语言标准。使用编译功能要求（如@FlorianWolters 回答中所述）可以提高语言标准要求。事实上，CMake 将始终选择由CXX_STANDARD目标属性或在目标上设置的编译功能要求指定的更强的语言要求。另请注意，3.10.1 的 CMake 文档并未准确反映CXX_EXTENSIONS与编译功能交互的方式，因为CXX_EXTENSIONS只有在CXX_STANDARD为大多数常见编译器指定时才生效（因为它们与一个编译器标志一起指定）。
回答by Florian Wolters
Modern CMake code should use the target_compile_featurescommand to request a specific C++ standard. This can be specified as build requirement only (PRIVATE), usage requirement only (INTERFACE) or build and usage requirement (PUBLIC).
现代 CMake 代码应该使用该target_compile_features命令来请求特定的 C++ 标准。这可以指定为仅构建要求 ( PRIVATE)、仅使用要求 ( INTERFACE) 或构建和使用要求 ( PUBLIC)。
Example:
例子：
cmake_minimum_required(VERSION 3.9.4)
project(cpp-snippets)
add_executable(cpp-snippets "main.cpp")
target_compile_features(cpp-snippets PRIVATE cxx_std_17)
Refer to the section Requiring Language Standardsin the official CMake documentation for cmake-compile-featuresto learn more.
有关cmake-compile-features 的更多信息，请参阅官方 CMake 文档中的“要求语言标准”部分。
回答by Some programmer dude
Check if the compiler supports the flags? Perhaps something like
检查编译器是否支持这些标志？也许像
include(CheckCXXCompilerFlag)
# Check for standard to use
check_cxx_compiler_flag(-std=c++17 HAVE_FLAG_STD_CXX17)
if(HAVE_FLAG_STD_CXX17)
# Have -std=c++17, use it
else()
check_cxx_compiler_flag(-std=c++1z HAVE_FLAG_STD_CXX1Z)
if(HAVE_FLAG_STD_CXX1Z)
# Have -std=c++1z, use it
else()
# And so on and on...
endif()
endif()
