feat: workaround __cplusplus problem on MSVC

tests/test_comments.cpp

@@ -272,7 +272,7 @@ BOOST_AUTO_TEST_CASE(test_construct_value_with_comments)
         BOOST_TEST(v.is_string());
         BOOST_TEST(v.as_string() == "str");
     }
-#if __cplusplus >= 201703L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
     {
         using namespace std::literals::string_view_literals;
         const value_type v("str"sv, {"comment1", "comment2"});

tests/test_find.cpp

@@ -13,7 +13,7 @@
 #include <list>
 #include <deque>
 #include <array>
-#if __cplusplus >= 201703L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
 #include <string_view>
 #endif
 #include <tuple>
@@ -477,7 +477,7 @@ BOOST_AUTO_TEST_CASE_TEMPLATE(test_find_string_type, value_type, test_value_type
         BOOST_TEST("foo" == moved);
     }
 
-#if __cplusplus >= 201703L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
     {
         value_type v{{"key", toml::string("foo", toml::string_t::basic)}};
         BOOST_TEST("foo" == toml::find<std::string_view>(v, "key"));

tests/test_find_or.cpp

@@ -12,7 +12,7 @@
 #include <deque>
 #include <array>
 #include <tuple>
-#if __cplusplus >= 201703L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
 #include <string_view>
 #endif
 

tests/test_find_or_recursive.cpp

@@ -12,7 +12,7 @@
 #include <deque>
 #include <array>
 #include <tuple>
-#if __cplusplus >= 201703L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
 #include <string_view>
 #endif
 

tests/test_get.cpp

@@ -12,7 +12,7 @@
 #include <deque>
 #include <array>
 #include <tuple>
-#if __cplusplus >= 201703L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
 #include <string_view>
 #endif
 
@@ -224,7 +224,7 @@ BOOST_AUTO_TEST_CASE_TEMPLATE(test_get_string_type, value_type, test_value_types
         BOOST_TEST("foobar" == x);
     }
 
-#if __cplusplus >= 201703L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
     {
         value_type v("foo", toml::string_t::basic);
         BOOST_TEST("foo" == toml::get<std::string_view>(v));

tests/test_get_or.cpp

@@ -12,7 +12,7 @@
 #include <deque>
 #include <array>
 #include <tuple>
-#if __cplusplus >= 201703L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
 #include <string_view>
 #endif
 

tests/test_string.cpp

@@ -133,7 +133,7 @@ BOOST_AUTO_TEST_CASE(test_string_add_assign)
         str += str2;
         BOOST_TEST(str.str == "foobar");
     }
-#if __cplusplus >= 201703L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
     // std::string_view
     {
         toml::string str("foo");

tests/test_value.cpp

@@ -9,7 +9,7 @@
 #include <map>
 #include <list>
 
-#if __cplusplus >= 201703L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
 #include <string_view>
 #endif
 
@@ -423,7 +423,7 @@ BOOST_AUTO_TEST_CASE(test_value_string)
     BOOST_TEST(v2.as_boolean() == true);
     BOOST_TEST(v3.as_boolean() == true);
 
-#if __cplusplus >= 201703L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
     std::string_view sv = "foo";
 
     toml::value v7(sv);

toml.hpp

@@ -25,14 +25,6 @@
 #ifndef TOML_FOR_MODERN_CPP
 #define TOML_FOR_MODERN_CPP
 
-#ifndef __cplusplus
-#    error "__cplusplus is not defined"
-#endif
-
-#if __cplusplus < 201103L && _MSC_VER < 1900
-#    error "toml11 requires C++11 or later."
-#endif
-
 #define TOML11_VERSION_MAJOR 3
 #define TOML11_VERSION_MINOR 7
 #define TOML11_VERSION_PATCH 0

toml/string.hpp

@@ -2,12 +2,15 @@
 // Distributed under the MIT License.
 #ifndef TOML11_STRING_HPP
 #define TOML11_STRING_HPP
+
+#include "version.hpp"
+
 #include <cstdint>
 
 #include <algorithm>
 #include <string>
 
-#if __cplusplus >= 201703L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
 #if __has_include(<string_view>)
 #define TOML11_USING_STRING_VIEW 1
 #include <string_view>

toml/traits.hpp

@@ -5,6 +5,7 @@
 
 #include "from.hpp"
 #include "into.hpp"
+#include "version.hpp"
 
 #include <chrono>
 #include <forward_list>
@@ -13,7 +14,7 @@
 #include <type_traits>
 #include <utility>
 
-#if __cplusplus >= 201703L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
 #if __has_include(<string_view>)
 #include <string_view>
 #endif // has_include(<string_view>)
@@ -146,7 +147,7 @@ struct has_specialized_into : decltype(has_specialized_into_impl::check<T>(nullp
 // ---------------------------------------------------------------------------
 // C++17 and/or/not
 
-#if __cplusplus >= 201703L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
 
 using std::conjunction;
 using std::disjunction;
@@ -208,7 +209,7 @@ template<typename T>
 struct is_container : conjunction<
     negation<is_map<T>>,                         // not a map
     negation<std::is_same<T, std::string>>,      // not a std::string
-#if __cplusplus >= 201703L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
 #if __has_include(<string_view>)
     negation<std::is_same<T, std::string_view>>, // not a std::string_view
 #endif // has_include(<string_view>)
@@ -233,7 +234,7 @@ struct is_basic_value<::toml::basic_value<C, M, V>>: std::true_type{};
 // ---------------------------------------------------------------------------
 // C++14 index_sequence
 
-#if __cplusplus >= 201402L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201402L
 
 using std::index_sequence;
 using std::make_index_sequence;
@@ -263,12 +264,12 @@ struct index_sequence_maker<0>
 template<std::size_t N>
 using make_index_sequence = typename index_sequence_maker<N-1>::type;
 
-#endif // __cplusplus >= 2014
+#endif // cplusplus >= 2014
 
 // ---------------------------------------------------------------------------
 // C++14 enable_if_t
 
-#if __cplusplus >= 201402L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201402L
 
 using std::enable_if_t;
 
@@ -277,12 +278,12 @@ using std::enable_if_t;
 template<bool B, typename T>
 using enable_if_t = typename std::enable_if<B, T>::type;
 
-#endif // __cplusplus >= 2014
+#endif // cplusplus >= 2014
 
 // ---------------------------------------------------------------------------
 // return_type_of_t
 
-#if __cplusplus >= 201703L && defined(__cpp_lib_is_invocable) && __cpp_lib_is_invocable>=201703
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L && defined(__cpp_lib_is_invocable) && __cpp_lib_is_invocable>=201703
 
 template<typename F, typename ... Args>
 using return_type_of_t = std::invoke_result_t<F, Args...>;

toml/utility.hpp

@@ -7,8 +7,9 @@
 #include <utility>
 
 #include "traits.hpp"
+#include "version.hpp"
 
-#if __cplusplus >= 201402L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201402L
 #  define TOML11_MARK_AS_DEPRECATED(msg) [[deprecated(msg)]]
 #elif defined(__GNUC__)
 #  define TOML11_MARK_AS_DEPRECATED(msg) __attribute__((deprecated(msg)))
@@ -21,7 +22,7 @@
 namespace toml
 {
 
-#if __cplusplus >= 201402L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201402L
 
 using std::make_unique;
 
@@ -33,7 +34,7 @@ inline std::unique_ptr<T> make_unique(Ts&& ... args)
     return std::unique_ptr<T>(new T(std::forward<Ts>(args)...));
 }
 
-#endif // __cplusplus >= 2014
+#endif // TOML11_CPLUSPLUS_STANDARD_VERSION >= 2014
 
 namespace detail
 {
@@ -91,7 +92,7 @@ T from_string(const std::string& str, T opt)
 
 namespace detail
 {
-#if __cplusplus >= 201402L
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201402L
 template<typename T>
 decltype(auto) last_one(T&& tail) noexcept
 {

toml/version.hpp

@@ -0,0 +1,42 @@
+#ifndef TOML11_VERSION_HPP
+#define TOML11_VERSION_HPP
+
+// This file checks C++ version.
+
+#ifndef __cplusplus
+#    error "__cplusplus is not defined"
+#endif
+
+// Since MSVC does not define `__cplusplus` correctly unless you pass
+// `/Zc:__cplusplus` when compiling, the workaround macros are added.
+// Those enables you to define version manually or to use MSVC specific
+// version macro automatically.
+//
+// The value of `__cplusplus` macro is defined in the C++ standard spec, but
+// MSVC ignores the value, maybe because of backward compatibility. Instead,
+// MSVC defines _MSVC_LANG that has the same value as __cplusplus defined in
+// the C++ standard. First we check the manual version definition, and then
+// we check if _MSVC_LANG is defined. If neither, use normal `__cplusplus`.
+//
+// FYI: https://docs.microsoft.com/en-us/cpp/build/reference/zc-cplusplus?view=msvc-170
+//      https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros?view=msvc-170
+//
+#if   defined(TOML11_ENFORCE_CXX11)
+#  define TOML11_CPLUSPLUS_STANDARD_VERSION 201103L
+#elif defined(TOML11_ENFORCE_CXX14)
+#  define TOML11_CPLUSPLUS_STANDARD_VERSION 201402L
+#elif defined(TOML11_ENFORCE_CXX17)
+#  define TOML11_CPLUSPLUS_STANDARD_VERSION 201703L
+#elif defined(TOML11_ENFORCE_CXX20)
+#  define TOML11_CPLUSPLUS_STANDARD_VERSION 202002L
+#elif defined(_MSVC_LANG)
+#  define TOML11_CPLUSPLUS_STANDARD_VERSION _MSVC_LANG
+#else
+#  define TOML11_CPLUSPLUS_STANDARD_VERSION __cplusplus
+#endif
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION < 201103L
+#    error "toml11 requires C++11 or later."
+#endif
+
+#endif// TOML11_VERSION_HPP