重载函数时不接受数组类型参数

Question

在 Delphi 中，我可以这样做：

Type
  TFourCC = Array[0..3] of AnsiChar;

Function Func(Param : TFourCC) : Boolean;
begin
  { ... }
end;

Func('ABCD'); // I can pass this as literal text without problems

现在，我想将此参数设为可选。

Function Func(Param : TFourCC = 'ABCD') : Boolean;
begin
  { ... }
end;

现在，编译器向我抛出一个错误： E2268 Parameters of this type cannot have default values

好的，所以我在想重载函数应该可以解决问题......

Function Func : Boolean; overload;
begin
  { ... }
end;

Function Func(Param : TFourCC) : Boolean; overload;
begin
  { ... }
end;

Func('ABCD'); // This line that worked in first example now gives an error

不幸的是，Delphi 也不喜欢这样。在它首先接受参数作为TFourCC类型变量的地方，它现在给了我E2250 There is no overloaded version of 'Func' that can be called with these arguments.

我不同意这个错误告诉我的，当它没有超载时，同样的事情会起作用。

有人可以向我解释这背后的逻辑，以及可能的解决方案吗？我想保持TFourCC原样（不是字符串类型），这样可以更轻松地处理读取和写入。我宁愿避免在传递它之前先将它分配给一个变量，因为该函数将被大量使用..

Answer 1

不幸的是，这就是当前类型系统的工作方式。

但好消息是，您可以使用记录和运算符重载来变魔术。例如，与

type
  TFourCC = record
  strict private
    function GetChar(Index: Integer): AnsiChar;
    procedure SetChar(Index: Integer; const Value: AnsiChar);
  public
    class operator Implicit(AValue: AnsiString): TFourCC;
    class operator Implicit(AValue: TFourCC): AnsiString;
    class operator Equal(a, b: TFourCC): Boolean;
    class operator NotEqual(a, b: TFourCC): Boolean;
    property Chars[Index: Integer]: AnsiChar read GetChar write SetChar; default;
  case Boolean of
    False: (AnsiChars: array[0..3] of AnsiChar);
    True: (Data: Cardinal)
  end;

implementation

{ TFourCC }

class operator TFourCC.Implicit(AValue: AnsiString): TFourCC;
begin
  if Length(AValue) <> 4 then
    raise Exception.Create('Not a valid TFourCC string.');
  Result.Data := PCardinal(@AValue[1])^;
end;

class operator TFourCC.Implicit(AValue: TFourCC): AnsiString;
begin
  SetLength(Result, 4);
  PCardinal(@Result[1])^ := AValue.Data;
end;

class operator TFourCC.Equal(a, b: TFourCC): Boolean;
begin
  Result := a.Data = b.Data;
end;

class operator TFourCC.NotEqual(a, b: TFourCC): Boolean;
begin
  Result := a.Data <> b.Data;
end;

function TFourCC.GetChar(Index: Integer): AnsiChar;
begin
  Result := AnsiChars[Index];
end;

procedure TFourCC.SetChar(Index: Integer; const Value: AnsiChar);
begin
  AnsiChars[Index] := Value;
end;

您将获得所有这些好处：

procedure TForm1.FormCreate(Sender: TObject);
var
  x: TFourCC;
begin
  x := 'FINE';                     // implicit conversion from string
  ShowMessage(x);                  // implicit conversion to string
  x[0] := 'D';                     // can access parts for writing (without explicit member)
  ShowMessage(x);
  ShowMessage(x[0]);               // can access parts for reading (without explicit member)
  ShowMessage(x.Data.ToString);    // can access underlying storage as a 32-bit integer
end;

而且，你现在可以做

procedure TForm1.FormCreate(Sender: TObject);
var
  x: TFourCC;
begin
  x := 'FINE';                     // implicit conversion from string
  ShowMessage(x);                  // implicit conversion to string
  x[0] := 'D';                     // can access parts for writing (without explicit member)
  ShowMessage(x);
  ShowMessage(x[0]);               // can access parts for reading (without explicit member)
  ShowMessage(x.Data.ToString);    // can access underlying storage as a 32-bit integer
end;

不幸的是，我现在很着急，所以我现在不能仔细检查正确性或进一步评论！