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It contains the id of the null-object (e.g. \"GROUP_NULL\" or \"mon_null\"). Code that used the literal string refers to that constant instead.\r\n\r\nIt adds functions to `string_id` that check for the id being the null id (`string_id::is_null`) and, to make it even easier, an explicit conversion to bool (which just return `!is_null`, similar to how a `unique_ptr` or a raw pointer would act). This allows for simpler code:\r\n```C++\r\n// current:\r\nif( type->upgrade_into != mtype_id( \"mon_null\" ) ) {\r\n    poly( type->upgrade_into );\r\n}\r\n// same, but without the mon_null string\r\nif( !type->upgrade_into.is_null() ) {\r\n    poly( type->upgrade_into );\r\n}\r\n// shorter, with operator bool:\r\nif( type->upgrade_into ) {\r\n    poly( type->upgrade_into );\r\n}\r\n```\r\n\r\n----\r\n\r\nTo make the usage of `NULL_ID` easier, there is a global `NULL_ID` object. It is a const instance of an effectively empty struct, which only has a conversion to `string_id<T>` operator. It will automatically transform into a const reference to `string_id<T>::NULL_ID`, where `T` is an arbitrary type. Example:\r\n```C++\r\n// note: the NULL_ID here is not the same as string_id<...>::NULL_ID\r\nstring_id<Bar> bar_id( NULL_ID );\r\nstring_id<Foo> foo_id( NULL_ID );\r\nstring_id<X> x_id = NULL_ID;\r\nconst string_id<Y> &get_id() { return NULL_ID; }\r\n\r\n// alternatively, but this would require repeating the type:\r\nstring_id<Bar> bar_id( string_id<Bar>::NULL_ID );\r\nstring_id<Foo> foo_id( string_id<Foo>::NULL_ID );\r\nstring_id<X> x_id = string_id<X>::NULL_ID;\r\nstring_id<Y> get_id() { return string_id<Y>::NULL_ID; }\r\n```\r\n\r\nThis will come handy when the skill pointers in recipes/books/gun get replaced with skill ids, which will allow to load skill definitions in any order (especially *after* the recipe/book/gun definition has already been loaded). 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