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function Assigned(var P): Boolean;判斷過程或指針是否存在
Description
? Use Assigned to determine whether the pointer or procedure referenced by P is nil. P must be a variable reference of a pointer or procedural type. Assigned(P) corresponds to the test P<> nil for a pointer variable, and @P <> nil for a procedural variable.
? Assigned returns False if P is nil, True otherwise.