Add the ability to use external definitions for monostate and bad variant access (#55, thanks @MostafaNanticock)

* Add support for configurable monostate type

Introduced variant_CONFIG_MONOSTATE that users can set its value if they want variant-lite to use it instead of its own monostate class

* Add support for configurable bad_variant_access type

Introduced variant_CONFIG_BAD_VARIANT_ACCESS that users can set its value if they want variant-lite to use it instead of its own bad_variant_access class

* Update README.md synopsis: Configuration macros

Added `variant_CONFIG_BAD_VARIANT_ACCESS` and `variant_CONFIG_MONOSTATE` to the Synopsis: Configuration macros section

* Updated template/variant.hpp

* Fixed: std::hash support with variant_CONFIG_MONOSTATE

Author: MostafaNanticock

README.md

@@ -248,6 +248,32 @@ Define this to the pod-type you want to align to (no default).
 \-D<b>variant\_CONFIG\_ALIGN\_AS\_FALLBACK</b>=*pod_type*  
 Define this to the pod-type to use for alignment if the algorithm of *variant lite* cannot find a suitable POD type to use for alignment. Default is `double`.
 
+#### Override `bad_variant_access`
+<b>variant_CONFIG_BAD_VARIANT_ACCESS</b>=type 
+
+Define this macro to override the default definition of `bad_variant_access`. 
+
+This is useful when integrating with other compatibility libraries or the standard library to avoid conflicting exception types. 
+
+Example:
+```cpp
+#define variant_CONFIG_BAD_VARIANT_ACCESS std::bad_variant_access
+```
+If not defined, variant-lite uses its own `nonstd::bad_variant_access`.
+
+#### Override `monostate`
+<b>variant_CONFIG_MONOSTATE</b>=type 
+
+Define this macro to override the default `monostate` type used by variant-lite. 
+
+This helps prevent type conflicts when multiple libraries define their own monostate. 
+
+Example:
+```cpp
+#define variant_CONFIG_MONOSTATE std::monostate
+```
+If not defined, variant-lite uses its own `nonstd::monostate`.
+
 ## Reported to work with
 
 The table below mentions the compiler versions *variant lite* is reported to work with.

include/nonstd/variant.hpp

@@ -1212,16 +1212,19 @@ class variant;
 
 // 19.7.8 Class monostate
 
+#ifdef variant_CONFIG_MONOSTATE
+using variant_CONFIG_MONOSTATE;
+#else
 class monostate{};
 
 // 19.7.9 monostate relational operators
-
 inline variant_constexpr bool operator< ( monostate, monostate ) variant_noexcept { return false; }
 inline variant_constexpr bool operator> ( monostate, monostate ) variant_noexcept { return false; }
 inline variant_constexpr bool operator<=( monostate, monostate ) variant_noexcept { return true;  }
 inline variant_constexpr bool operator>=( monostate, monostate ) variant_noexcept { return true;  }
 inline variant_constexpr bool operator==( monostate, monostate ) variant_noexcept { return true;  }
 inline variant_constexpr bool operator!=( monostate, monostate ) variant_noexcept { return false; }
+#endif
 
 // 19.7.4 variant helper classes
 
@@ -1280,6 +1283,9 @@ static const std::size_t variant_npos = static_cast<std::size_t>( -1 );
 
 // 19.7.11 Class bad_variant_access
 
+#ifdef variant_CONFIG_BAD_VARIANT_ACCESS
+using variant_CONFIG_BAD_VARIANT_ACCESS;
+#else
 class variant_nodiscard bad_variant_access : public std::exception
 {
 public:
@@ -1292,6 +1298,7 @@ class variant_nodiscard bad_variant_access : public std::exception
         return "bad variant access";
     }
 };
+#endif
 
 #endif // variant_CONFIG_NO_EXCEPTIONS
 
@@ -2698,7 +2705,7 @@ using namespace variants;
 
 } // namespace nonstd
 
-#if variant_CPP11_OR_GREATER
+#if ! defined (variant_CONFIG_MONOSTATE) && variant_CPP11_OR_GREATER
 
 // 19.7.12 Hash support
 
@@ -2746,7 +2753,7 @@ struct hash< nonstd::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T
 
 } //namespace std
 
-#endif // variant_CPP11_OR_GREATER
+#endif // ! defined (variant_CONFIG_MONOSTATE) && variant_CPP11_OR_GREATER
 
 #if variant_BETWEEN( variant_COMPILER_MSVC_VER, 1300, 1900 )
 # pragma warning( pop )

template/variant.hpp

@@ -1087,16 +1087,19 @@ class variant;
 
 // 19.7.8 Class monostate
 
+#ifdef variant_CONFIG_MONOSTATE
+using variant_CONFIG_MONOSTATE;
+#else
 class monostate{};
 
 // 19.7.9 monostate relational operators
-
 inline variant_constexpr bool operator< ( monostate, monostate ) variant_noexcept { return false; }
 inline variant_constexpr bool operator> ( monostate, monostate ) variant_noexcept { return false; }
 inline variant_constexpr bool operator<=( monostate, monostate ) variant_noexcept { return true;  }
 inline variant_constexpr bool operator>=( monostate, monostate ) variant_noexcept { return true;  }
 inline variant_constexpr bool operator==( monostate, monostate ) variant_noexcept { return true;  }
 inline variant_constexpr bool operator!=( monostate, monostate ) variant_noexcept { return false; }
+#endif // variant_CONFIG_MONOSTATE
 
 // 19.7.4 variant helper classes
 
@@ -1155,6 +1158,9 @@ static const std::size_t variant_npos = static_cast<std::size_t>( -1 );
 
 // 19.7.11 Class bad_variant_access
 
+#ifdef variant_CONFIG_BAD_VARIANT_ACCESS
+using variant_CONFIG_BAD_VARIANT_ACCESS;
+#else
 class variant_nodiscard bad_variant_access : public std::exception
 {
 public:
@@ -1167,6 +1173,7 @@ class variant_nodiscard bad_variant_access : public std::exception
         return "bad variant access";
     }
 };
+#endif // variant_CONFIG_BAD_VARIANT_ACCESS
 
 #endif // variant_CONFIG_NO_EXCEPTIONS
 
@@ -2043,7 +2050,7 @@ using namespace variants;
 
 } // namespace nonstd
 
-#if variant_CPP11_OR_GREATER
+#if ! defined (variant_CONFIG_MONOSTATE) && variant_CPP11_OR_GREATER
 
 // 19.7.12 Hash support
 
@@ -2077,7 +2084,7 @@ struct hash< nonstd::variant<{{TplArgsList}}> >
 
 } //namespace std
 
-#endif // variant_CPP11_OR_GREATER
+#endif // ! defined (variant_CONFIG_MONOSTATE) && variant_CPP11_OR_GREATER
 
 #if variant_BETWEEN( variant_COMPILER_MSVC_VER, 1300, 1900 )
 # pragma warning( pop )