Setting var to nil
Maybe (maybe) this is a stupid question, but I couldn't find an answer ...
Please check this hypothetical code:
type
TCustomType = (Type1, Type2, Type3);
function CustomTypeToStr(CTp: TCustomType): string;
begin
Result := '';
case CTp of
Type1: Result := 'Type1';
Type2: Result := 'Type2';
Type3: Result := 'Type3';
end;
end;
function StrToCustomType(Str: string): TCustomType;
begin
Result := nil; <--- ERROR (Incompatible types: 'TCustomType' and 'Pointer')
if (Str = 'Type1') then
Result := Type1 else
if (Str = 'Type2') then
Result := Type2 else
if (Str = 'Type3') then
Result := Type3;
end;
Please, how can I set nil / null / empty for this custom var type so I can check the function result and avoid problems?
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1 answer
An enumerated type cannot be nil
. It must be one of the defined enumeration values.
You have several options. You can add another enum:
type
TCustomType = (NoValue, Type1, Type2, Type3);
You can use a nullable type. For example, Spring has Nullable<T>
.
You can throw an exception if no value is found.
function StrToCustomType(Str: string): TCustomType;
begin
if (Str = 'Type1') then
Result := Type1
else if (Str = 'Type2') then
Result := Type2
else if (Str = 'Type3') then
Result := Type3
else
raise EMyException.Create(...);
end;
Or you can use a template TryXXX
.
function TryStrToCustomType(Str: string; out Value: TCustomType): Boolean;
begin
Result := True;
if (Str = 'Type1') then
Value := Type1
else if (Str = 'Type2') then
Value := Type2
else if (Str = 'Type3') then
Value := Type3
else
Result := False;
end;
function StrToCustomType(Str: string): TCustomType;
begin
if not TryStrToCustomType(Str, Result) then
raise EMyException.Create(...);
end;
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