Factoring out C++ version check to Macros file

include/CLI/App.hpp

@@ -19,6 +19,7 @@
 // CLI Library includes
 #include "CLI/Error.hpp"
 #include "CLI/Ini.hpp"
+#include "CLI/Macros.hpp"
 #include "CLI/Option.hpp"
 #include "CLI/Split.hpp"
 #include "CLI/StringTools.hpp"
@@ -447,7 +448,7 @@ class App {
         return opt;
     }
 
-#if __cplusplus >= 201402L
+#ifdef CLI11_CPP14
     /// Add option for callback (C++14 or better only)
     Option *add_flag(std::string name,
                      std::function<void(size_t)> function, ///< A function to call, void(size_t)

include/CLI/CLI.hpp

@@ -8,6 +8,8 @@
 
 #include "CLI/Version.hpp"
 
+#include "CLI/Macros.hpp"
+
 #include "CLI/StringTools.hpp"
 
 #include "CLI/Error.hpp"

include/CLI/Macros.hpp

@@ -0,0 +1,29 @@
+#pragma once
+
+// Distributed under the 3-Clause BSD License.  See accompanying
+// file LICENSE or https://github.com/CLIUtils/CLI11 for details.
+
+namespace CLI {
+
+// Note that all code in CLI11 must be in a namespace, even if it just a define.
+
+// The following version macro is very similar to the one in PyBind11
+
+#if !defined(_MSC_VER) && !defined(__INTEL_COMPILER)
+#  if __cplusplus >= 201402L
+#    define CLI11_CPP14
+#    if __cplusplus > 201402L /* Temporary: should be updated to >= the final C++17 value once known */
+#      define CLI11_CPP17
+#    endif
+#  endif
+#elif defined(_MSC_VER)
+// MSVC sets _MSVC_LANG rather than __cplusplus (supposedly until the standard is fully implemented)
+#  if _MSVC_LANG >= 201402L
+#    define CLI11_CPP14
+#    if _MSVC_LANG > 201402L && _MSC_VER >= 1910
+#      define CLI11_CPP17
+#    endif
+#  endif
+#endif
+
+} // namespace CLI

include/CLI/Option.hpp

@@ -13,6 +13,7 @@
 #include <vector>
 
 #include "CLI/Error.hpp"
+#include "CLI/Macros.hpp"
 #include "CLI/Split.hpp"
 #include "CLI/StringTools.hpp"
 
@@ -299,7 +300,7 @@ class Option : public OptionBase<Option> {
         return needs(opt1, args...);
     }
 
-#if __cplusplus <= 201703L
+#ifndef CLI11_CPP17
     /// Sets required options \deprecated
     Option *requires(Option *opt) { return needs(opt); }