pub type VipsTypeMap2Fn = Option<unsafe extern "C" fn(type_: GType, a: *mut c_void, b: *mut c_void) -> *mut c_void>;
enum VipsTypeMap2Fn { None, Some(unsafe extern "C" fn(u64, *mut c_void, *mut c_void) -> *mut c_void), }
No value.
Some value of type T.
T