马上会想到下面代码:

procedure TMyList.SetCapacity(const Value: Integer);
beginif FCapacity <> Value thenFCapacity := 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);
beginif FCapacity <> Value thenbeginReallocMem(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);
beginif (Value < FCount) or (Value > MaxListSize) thenExit;if FCapacity <> Value thenbeginReallocMem(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) thenraise 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) thenraise 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);
beginif (Value < FCount) or (Value > MaxListSize) thenraise Exception.CreateFmt('非法数据:%d', [Value]);if FCapacity <> Value thenbeginReallocMem(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;interfaceuses SysUtils; {异常类 Exception 声明在 SysUtils 单元}constMaxListSize = Maxint div 16;typePPointerList = ^TPointerList;TPointerList = array[0..MaxListSize - 1] of Pointer;TMyList = class(TObject)privateFList: PPointerList;FCount: Integer;FCapacity: Integer;procedure SetCapacity(const Value: Integer);procedure SetCount(const Value: Integer);publicdestructor 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;
beginend;procedure TMyList.Clear;
beginend;procedure TMyList.Delete(Index: Integer);
beginend;destructor TMyList.Destroy;
beginClear;inherited;
end;procedure TMyList.SetCapacity(const Value: Integer);
beginif (Value < FCount) or (Value > MaxListSize) thenraise Exception.CreateFmt('非法数据:%d', [Value]);if FCapacity <> Value thenbeginReallocMem(FList, Value * SizeOf(Pointer));FCapacity := Value;end;
end;procedure TMyList.SetCount(const Value: Integer);
beginend;end.