学习 TList 类的实现[5]

先来实现 TMyList.SetCapacity.

马上会想到下面代码:

---

procedure TMyList.SetCapacity(const Value: Integer);
begin
  if FCapacity <> Value then
    FCapacity := Value;
end;

但这样是远远不够的, 关键是需要分配内存, 像这样: ReallocMem(数组的起点指针, 元素个数*元素大小);

数组的起点指针即是 FList; 元素个数就是 SetCapacity 的参数: Value; 元素即是指针, Win32 指针的大小是 4 个字节, 因此可以写为: ReallocMem(FList, Value*4);

在 Win64 下的指针还会是 4 个字节吗? 还是这样写稳妥些: ReallocMem(FList, Value*SizeOf(Pointer));

把方法改写为:

---

procedure TMyList.SetCapacity(const Value: Integer);
begin
  if FCapacity <> Value then
  begin
    ReallocMem(FList, Value * SizeOf(Pointer));
    FCapacity := Value;
  end;
end;

这还不够, 如果用户设置的 Value 值大于了列表的最大许可值(MaxListSize)怎么办? 小于了当前的元素数也不行啊, 再改写:

---

procedure TMyList.SetCapacity(const Value: Integer);
begin
  if (Value < FCount) or (Value > MaxListSize) then
    Exit;

  if FCapacity <> Value then
  begin
    ReallocMem(FList, Value * SizeOf(Pointer));
    FCapacity := Value;
  end;
end;

莫名其妙地 Exit, 会让人摸不着头脑, 抛出个异常吧(使用异常类需要 uses SysUtils 单元), 譬如:

---

if (Value < FCount) or (Value > MaxListSize) then
    raise Exception.Create('非法的数据');

再具体点, 让异常说出非法数据到底是什么:

---

if (Value < FCount) or (Value > MaxListSize) then
    raise Exception.CreateFmt('非法数据:%d', [Value]);

TList 把抛出异常也做成了一个 Error 方法, 我们暂时就这样吧, 再次改写方法为:

---

procedure TMyList.SetCapacity(const Value: Integer);
begin
  if (Value < FCount) or (Value > MaxListSize) then
    raise Exception.CreateFmt('非法数据:%d', [Value]);

  if FCapacity <> Value then
  begin
    ReallocMem(FList, Value * SizeOf(Pointer));
    FCapacity := Value;
  end;
end;

TMyList.SetCapacity 方法完成了, 现在完整的代码是:

---

unit MyList;

interface

uses SysUtils; {异常类 Exception 声明在 SysUtils 单元}

const
  MaxListSize = Maxint div 16;

type
  PPointerList = ^TPointerList;
  TPointerList = array[0..MaxListSize - 1] of Pointer;

  TMyList = class(TObject)
  private
    FList: PPointerList;
    FCount: Integer;
    FCapacity: Integer;
    procedure SetCapacity(const Value: Integer);
    procedure SetCount(const Value: Integer);
  public
    destructor Destroy; override;
    function Add(Item: Pointer): Integer;
    procedure Clear;
    procedure Delete(Index: Integer);
    property Capacity: Integer read FCapacity write SetCapacity;
    property Count: Integer read FCount write SetCount;
    property List: PPointerList read FList;
  end;

implementation

{ TMyList }

function TMyList.Add(Item: Pointer): Integer;
begin

end;

procedure TMyList.Clear;
begin

end;

procedure TMyList.Delete(Index: Integer);
begin

end;

destructor TMyList.Destroy;
begin
  Clear;
  inherited;
end;

procedure TMyList.SetCapacity(const Value: Integer);
begin
  if (Value < FCount) or (Value > MaxListSize) then
    raise Exception.CreateFmt('非法数据:%d', [Value]);

  if FCapacity <> Value then
  begin
    ReallocMem(FList, Value * SizeOf(Pointer));
    FCapacity := Value;
  end;
end;

procedure TMyList.SetCount(const Value: Integer);
begin

end;

end.