6

编译器允许我执行以下操作:

procedure MyProc(const ADynData: array of string);

或者

procedure MyProc(const ADynData: TStringDynArray);

并像这样传递任意数据:

MyProc(['Data1', 'Data2']);

但是,不允许

function MyFunc: TStringDynArray;
....
function MyFunc: TStringDynArray;
begin
    Result := ['Data1', 'Data2'];
end;

或者

function MyFunc: TStringDynArray;
const
    CDynData: array[0..1] of string = ('Data1', 'Data2');
begin
    Result := CDynData;
end;

为什么是这样?这在技术上不是一样的吗?

对于这些特定场景,返回任意字符串数组的推荐(也是最有效)方法是什么?

4

3 回答 3

10

不,这不是一回事。在

procedure MyProc(const ADynData: array of string);

该参数是一个开放数组参数,它与“普通”动态数组不同。该[..]语法只能用于在函数的开放数组参数中创建开放数组。(否则,[..]用于在代码中指定集合,例如Font.Style := [fsBold, fsItalic]。但是集合只能将序数类型作为其“基本类型”,因此仍然没有“字符串集合”之类的东西。)

换句话说,不可能像您在第二个代码片段中尝试的那样在代码中编写动态数组,

function MyFunc: TStringDynArray;
begin
  result := ['Data1', 'Data2']; // Won't work.
end;

但是,在新版本的 Delphi 中,几乎可以:

type
  TStringDynArray = array of string;

function MyFunc: TStringDynArray;
begin
  result := TStringDynArray.Create('A', 'B');
end;

最后,

function MyFunc: TStringDynArray;
const
  CDynData: array[0..1] of string = ('Data1', 'Data2');
begin
  result := CDynData;
end;

不起作用,因为TStringDynArrayis a dynamic array,而CDynDatais a static array,它们是两种不同的基本类型。

于 2013-04-10T10:51:55.670 回答
6

这种构造

['string1', 'string2'] 

被称为开放数组构造函数。从文档中:

开放数组构造函数允许您直接在函数和过程调用中构造数组。

它们只能作为开放数组参数或变体开放数组参数传递。

因此,您不能使用开放数组构造函数来创建函数返回值。


如果数组中有固定数量的元素需要返回,则可以使用动态数组构造函数:

Result := TStringDynArray.Create('string1', 'string2');

但是,这不适用于可变数量的元素。现在,我知道您问题中的示例在数组中只有恒定数量的元素。但我确信您会遇到需要比动态数组构造函数提供更多灵活性的情况。

如果您希望创建现有动态数组的副本并将其返回,请使用Copy.

Result := Copy(SomeOtherDynamicArray);

当你手头有一个开放的数组时,这就会崩溃。您不能将开放数组传递给Copy. 就我个人而言,我认为这是一种耻辱,因为开放数组参数非常灵活和有用,我希望看到对它们的尽可能多的 RTL 支持。

因此,您最终不得不为这些情况编写辅助函数。您可以为每种数组类型编写一个专用的帮助程序,但这有点令人厌烦。这就是泛型派上用场的地方。为此,我有一个助手类。这是相关的摘录:

type
  TArray = class(Generics.Collections.TArray)
    ....
    class function Copy<T>(const Source: array of T): TArray<T>; overload; static;
    ....
  end;

class function TArray.Copy<T>(const Source: array of T): TArray<T>;
var
  i: Integer;
begin
  SetLength(Result, Length(Source));
  for i := 0 to high(Result) do begin
    Result[i] := Source[i];
  end;
end;

现在,这适用于您的字符串数组,也适用于任何其他类型。像这样称呼它:

Result := TArray.Copy<string>(SomeStringOpenArray);

需要指出的一个关键点是,我们使用的是动态数组的通用版本,TArray<string>而不是TStringDynArray. 如果您想认真使用泛型,那么您必须这样做。那是因为TStringDynArray赋值与TArray<string>声明为array of string. 更改您的代码库以供TArray<T>始终使用是有好处的。

以防万一有人对这个助手类的其余部分感兴趣,这里是:

type
  TArray = class(Generics.Collections.TArray)
  private
    class function Comparison<T>(SortType: TSortType): TComparison<T>; static;
    class function Comparer<T>(const Comparison: TComparison<T>): IComparer<T>; static;
  public
    class procedure Swap<T>(var Left, Right: T); static;
    class procedure Reverse<T>(var Values: array of T); static;
    class function Reversed<T>(const Values: array of T): TArray<T>; static;
    class function Contains<T>(const Values: array of T; const Item: T; out ItemIndex: Integer): Boolean; overload; static;
    class function Contains<T>(const Values: array of T; const Item: T): Boolean; overload; static;
    class function IndexOf<T>(const Values: array of T; const Item: T): Integer; static;
    class function Sorted<T>(var Values: array of T; SortType: TSortType; Index, Count: Integer): Boolean; overload; static;
    class function Sorted<T>(var Values: array of T; SortType: TSortType): Boolean; overload; static;
    class function Sorted<T>(var Values: array of T; const Comparison: TComparison<T>; Index, Count: Integer): Boolean; overload; static;
    class function Sorted<T>(var Values: array of T; const Comparison: TComparison<T>): Boolean; overload; static;
    class function Sorted<T>(GetValue: TFunc<Integer,T>; const Comparison: TComparison<T>; Index, Count: Integer): Boolean; overload; static;
    class procedure Sort<T>(var Values: array of T; SortType: TSortType; Index, Count: Integer); overload; static;
    class procedure Sort<T>(var Values: array of T; SortType: TSortType); overload; static;
    class procedure Sort<T>(var Values: array of T; const Comparison: TComparison<T>; Index, Count: Integer); overload; static;
    class procedure Sort<T>(var Values: array of T; const Comparison: TComparison<T>); overload; static;
    class function Copy<T>(const Source: array of T; Index, Count: Integer): TArray<T>; overload; static;
    class function Copy<T>(const Source: array of T): TArray<T>; overload; static;
    class procedure Move<T>(const Source: array of T; var Dest: array of T; Index, Count: Integer); overload; static;
    class procedure Move<T>(const Source: array of T; var Dest: array of T); overload; static;
    class function Concatenated<T>(const Source1, Source2: array of T): TArray<T>; overload; static;
    class function Concatenated<T>(const Source: array of TArray<T>): TArray<T>; overload; static;
    class procedure Initialise<T>(var Values: array of T; const Value: T); static;
    class procedure Zeroise<T>(var Values: array of T); static;
    class function GetHashCode<T>(const Values: array of T): Integer; overload; static;
    class function GetHashCode<T>(Values: Pointer; Count: Integer): Integer; overload; static;
  end;

class function TArray.Comparison<T>(SortType: TSortType): TComparison<T>;
var
  DefaultComparer: IComparer<T>;
begin
  DefaultComparer := TComparer<T>.Default;
  Result :=
    function(const Left, Right: T): Integer
    begin
      case SortType of
      stIncreasing:
        Result := DefaultComparer.Compare(Left, Right);
      stDecreasing:
        Result := -DefaultComparer.Compare(Left, Right);
      else
        RaiseAssertionFailed(Result);
      end;
    end;
end;

class function TArray.Comparer<T>(const Comparison: TComparison<T>): IComparer<T>;
begin
  Result := TComparer<T>.Construct(Comparison);
end;

class procedure TArray.Swap<T>(var Left, Right: T);
var
  temp: T;
begin
  temp := Left;
  Left := Right;
  Right := temp;
end;

class procedure TArray.Reverse<T>(var Values: array of T);
var
  bottom, top: Integer;
begin
  bottom := 0;
  top := high(Values);
  while top>bottom do begin
    Swap<T>(Values[bottom], Values[top]);
    inc(bottom);
    dec(top);
  end;
end;

class function TArray.Reversed<T>(const Values: array of T): TArray<T>;
var
  i, j, Count: Integer;
begin
  Count := Length(Values);
  SetLength(Result, Count);
  j := Count-1;
  for i := 0 to Count-1 do begin
    Result[i] := Values[j];
    dec(j);
  end;
end;

class function TArray.Contains<T>(const Values: array of T; const Item: T; out ItemIndex: Integer): Boolean;
var
  DefaultComparer: IEqualityComparer<T>;
  Index: Integer;
begin
  DefaultComparer := TEqualityComparer<T>.Default;
  for Index := 0 to high(Values) do begin
    if DefaultComparer.Equals(Values[Index], Item) then begin
      ItemIndex := Index;
      Result := True;
      exit;
    end;
  end;
  ItemIndex := -1;
  Result := False;
end;

class function TArray.Contains<T>(const Values: array of T; const Item: T): Boolean;
var
  ItemIndex: Integer;
begin
  Result := Contains<T>(Values, Item, ItemIndex);
end;

class function TArray.IndexOf<T>(const Values: array of T; const Item: T): Integer;
begin
  Contains<T>(Values, Item, Result);
end;

class function TArray.Sorted<T>(var Values: array of T; SortType: TSortType; Index, Count: Integer): Boolean;
begin
  Result := Sorted<T>(Values, Comparison<T>(SortType), Index, Count);
end;

class function TArray.Sorted<T>(var Values: array of T; SortType: TSortType): Boolean;
begin
  Result := Sorted<T>(Values, Comparison<T>(SortType));
end;

class function TArray.Sorted<T>(var Values: array of T; const Comparison: TComparison<T>; Index, Count: Integer): Boolean;
var
  i: Integer;
begin
  for i := Index+1 to Index+Count-1 do begin
    if Comparison(Values[i-1], Values[i])>0 then begin
      Result := False;
      exit;
    end;
  end;
  Result := True;
end;

class function TArray.Sorted<T>(var Values: array of T; const Comparison: TComparison<T>): Boolean;
begin
  Result := Sorted<T>(Values, Comparison, 0, Length(Values));
end;

class function TArray.Sorted<T>(GetValue: TFunc<Integer, T>; const Comparison: TComparison<T>; Index, Count: Integer): Boolean;
var
  i: Integer;
begin
  for i := Index+1 to Index+Count-1 do begin
    if Comparison(GetValue(i-1), GetValue(i))>0 then begin
      Result := False;
      exit;
    end;
  end;
  Result := True;
end;

class procedure TArray.Sort<T>(var Values: array of T; SortType: TSortType; Index, Count: Integer);
begin
  Sort<T>(Values, Comparison<T>(SortType), Index, Count);
end;

class procedure TArray.Sort<T>(var Values: array of T; SortType: TSortType);
begin
  Sort<T>(Values, SortType, 0, Length(Values));
end;

class procedure TArray.Sort<T>(var Values: array of T; const Comparison: TComparison<T>; Index, Count: Integer);
begin
  if not Sorted<T>(Values, Comparison, Index, Count) then begin
    Sort<T>(Values, Comparer<T>(Comparison), Index, Count);
  end;
end;

class procedure TArray.Sort<T>(var Values: array of T; const Comparison: TComparison<T>);
begin
  Sort<T>(Values, Comparison, 0, Length(Values));
end;

class function TArray.Copy<T>(const Source: array of T; Index, Count: Integer): TArray<T>;
var
  i: Integer;
begin
  SetLength(Result, Count);
  for i := 0 to high(Result) do begin
    Result[i] := Source[i+Index];
  end;
end;

class function TArray.Copy<T>(const Source: array of T): TArray<T>;
var
  i: Integer;
begin
  SetLength(Result, Length(Source));
  for i := 0 to high(Result) do begin
    Result[i] := Source[i];
  end;
end;

class procedure TArray.Move<T>(const Source: array of T; var Dest: array of T; Index, Count: Integer);
var
  i: Integer;
begin
  for i := 0 to Count-1 do begin
    Dest[i] := Source[i+Index];
  end;
end;

class procedure TArray.Move<T>(const Source: array of T; var Dest: array of T);
var
  i: Integer;
begin
  for i := 0 to high(Source) do begin
    Dest[i] := Source[i];
  end;
end;

class function TArray.Concatenated<T>(const Source1, Source2: array of T): TArray<T>;
var
  i, Index: Integer;
begin
  SetLength(Result, Length(Source1)+Length(Source2));
  Index := 0;
  for i := low(Source1) to high(Source1) do begin
    Result[Index] := Source1[i];
    inc(Index);
  end;
  for i := low(Source2) to high(Source2) do begin
    Result[Index] := Source2[i];
    inc(Index);
  end;
end;

class function TArray.Concatenated<T>(const Source: array of TArray<T>): TArray<T>;
var
  i, j, Index, Count: Integer;
begin
  Count := 0;
  for i := 0 to high(Source) do begin
    inc(Count, Length(Source[i]));
  end;
  SetLength(Result, Count);
  Index := 0;
  for i := 0 to high(Source) do begin
    for j := 0 to high(Source[i]) do begin
      Result[Index] := Source[i][j];
      inc(Index);
    end;
  end;
end;

class procedure TArray.Initialise<T>(var Values: array of T; const Value: T);
var
  i: Integer;
begin
  for i := 0 to high(Values) do begin
    Values[i] := Value;
  end;
end;

class procedure TArray.Zeroise<T>(var Values: array of T);
begin
  Initialise<T>(Values, Default(T));
end;

{$IFOPT Q+}
  {$DEFINE OverflowChecksEnabled}
  {$Q-}
{$ENDIF}
class function TArray.GetHashCode<T>(const Values: array of T): Integer;
// see http://stackoverflow.com/questions/1646807 and http://stackoverflow.com/questions/11294686
var
  Value: T;
  EqualityComparer: IEqualityComparer<T>;
begin
  EqualityComparer := TEqualityComparer<T>.Default;
  Result := 17;
  for Value in Values do begin
    Result := Result*37 + EqualityComparer.GetHashCode(Value);
  end;
end;

class function TArray.GetHashCode<T>(Values: Pointer; Count: Integer): Integer;
// see http://stackoverflow.com/questions/1646807 and http://stackoverflow.com/questions/11294686
var
  Value: ^T;
  EqualityComparer: IEqualityComparer<T>;
begin
  EqualityComparer := TEqualityComparer<T>.Default;
  Result := 17;
  Value := Values;
  while Count>0 do begin
    Result := Result*37 + EqualityComparer.GetHashCode(Value^);
    inc(Value);
    dec(Count);
  end;
end;
{$IFDEF OverflowChecksEnabled}
  {$Q+}
{$ENDIF}
于 2013-04-10T11:23:45.103 回答
2

问题与

function MyFunc: TStringDynArray;
begin
  Result := ['Data1', 'Data2'];
end;

['Data1', 'Data2']被解释为一个集合

我有时会使用以下便利功能(但通常不在性能关键部分):

function MakeStringArray(const Strings: array of string): TStringDynArray;
var
  i: Integer;
begin
  SetLength(Result, Length(Strings));
  for i := Low(Strings) to High(Strings) do
    Result[i] := Strings[i];
end {MakeStringArray};

使用它,您可以如下重写您的第一个示例:

function MyFunc: TStringDynArray;
....
function MyFunc: TStringDynArray;
begin
    Result := MakeStringArray(['Data1', 'Data2']);
end;

但是,由于您使用的是 XE3,因此最好使用TStringDynArray.Create,就像 Andreas Rejbrand建议的那样。

于 2013-04-10T11:02:31.437 回答