Issue open-source-parsers#731: Provide new JSONCPP_OP_EXPLICIT macro to restore VS 2012 support after recent introduction of explicit conversion function in JSON::Value.

Author: Dani-Hub

include/json/config.h

@@ -84,15 +84,23 @@
 #if __cplusplus >= 201103L
 # define JSONCPP_OVERRIDE override
 # define JSONCPP_NOEXCEPT noexcept
+# define JSONCPP_OP_EXPLICIT explicit
 #elif defined(_MSC_VER) && _MSC_VER > 1600 && _MSC_VER < 1900
 # define JSONCPP_OVERRIDE override
 # define JSONCPP_NOEXCEPT throw()
+# if _MSC_VER >= 1800 // MSVC 2013
+#   define JSONCPP_OP_EXPLICIT explicit
+# else
+#   define JSONCPP_OP_EXPLICIT
+# endif
 #elif defined(_MSC_VER) && _MSC_VER >= 1900
 # define JSONCPP_OVERRIDE override
 # define JSONCPP_NOEXCEPT noexcept
+# define JSONCPP_OP_EXPLICIT explicit
 #else
 # define JSONCPP_OVERRIDE
 # define JSONCPP_NOEXCEPT throw()
+# define JSONCPP_OP_EXPLICIT
 #endif
 
 #ifndef JSON_HAS_RVALUE_REFERENCES

include/json/value.h

@@ -408,7 +408,7 @@ Json::Value obj_value(Json::objectValue); // {}
   bool empty() const;
 
   /// Return !isNull()
-  explicit operator bool() const;
+  JSONCPP_OP_EXPLICIT operator bool() const;
 
   /// Remove all object members and array elements.
   /// \pre type() is arrayValue, objectValue, or nullValue