问题
I want to create headers which use 'optional' from standard C++. However, My headers will be referred from Visual Studio 2015 as well as Visual Studio 2017 projects.
I would like to have something, such that for Visual Studio 2017 ( with C++ 17 lang feature set) , std::optional is used and with Visual Studio 2015, boost::optional gets used.
I am thinking of something like this:
#include <yvals.h>
#if _HAS_CXX17
#include <optional>
template <typename T> using Optional = std::optional<T>;
#else
#include "boost/optional/optional.hpp"
template <typename T> using Optional = boost::optional<T>;
#endif
Is it okay to use '_HAS_CXX17' macro this way? Is there a better way of doing this?
回答1:
The short answer is: No It's not safe to rely on internal preprocessor defines in the implementation of the Visual C++ Runtime, and technically all compiler symbols that begin with a single _ are reserved for use by the implementation.
For example, _NOEXCEPT has been used internally in Visual Studio 2015 and 2017, but as of VS 2017 (15.8 update), this macro no longer exists; the headers just use noexcept directly.
The recommendation to use __has_include is good, but isn't supported prior to VS 2017 (15.3 update).
The other challenge is that __cplusplus doesn't indicate you are using /std:c++17 unless you are using VS 2017 (15.7 update) with the new /Zc:__cplusplus switch which is off by default.
Probably the safest way to do this across a range of VS versions would be:
#if (__cplusplus >= 201703L) || (defined(_MSVC_LANG) && (_MSVC_LANG >= 201703L) && (_MSC_VER >= 1913))
#if __has_include(<optional>)
#include <optional>
template <typename T> using Optional = std::optional<T>;
#else
#include "boost/optional/optional.hpp"
template <typename T> using Optional = boost::optional<T>;
#endif
#else
#include "boost/optional/optional.hpp"
template <typename T> using Optional = boost::optional<T>;
#endif
See Visual C++ Language Conformance
来源：https://stackoverflow.com/questions/52379233/is-has-cxx17-marco-usable-in-custom-project-headers-to-enable-c17-language