整数为var的枚举类型？

Question

For example I have enum

const
  MY_FIRST = 1;
  MY_SECOND = 2;
  MY_THIRD = 3;
type
  TMyEnum = MY_FIRST .. MY_THIRD;

and I have function like

procedure DoSomething(var _type: TMyEnum);

where I should pass integer as TMyEnum . By doing

var
  some_int_value: integer;
begin

DoSomething(TMyEnum(some_int_value));

I have error [DCC Error] : E2197 Constant object cannot be passed as var parameter . How to solve this?

Answer 1

A var parameter requires a real variable, eg:

var
  some_int_value: integer;
  my_enum: TMyEnum;
begin
  my_enum := TMyEnum(some_int_value);
  DoSomething(my_enum);
  some_int_value := Ord(my_enum);

Answer 2

Since TEnum is a subrange of Integer the compiler only uses the smallest possible type that fits this subrange. In this case it is a Byte .

So if some_int_value were declared as Byte your typecast would succeed. But it actually is declared as Integer , so the typecast introduces a temporary Byte to store the result of the typecast. The compiler doesn't go that far to write any changes to this temprary Byte back to the source of the typecast. That is why you get the described error.