我的记录看起来像：
TBigint = record
PtrDigits: Pointer; <-- The data is somewhere else.
Size: Byte;
MSB: Byte;
Sign: Shortint;
...
class operator Implicit(a: TBigint): TBigint; <<-- is this allowed?
....
我知道数据应该存储在一个动态的byte数组中,但我不想更改代码,因为所有的东西都在x86-assembly中.
procedure test(a: TBignum); var b: TBignum; begin b:= a; <<-- naive copy will tangle up the `PtrDigit` pointers. ....
class operator TBigint.Implicit(a: TBigint): TBigint; begin sdpBigint.CreateBigint(Result,a.Size); sdpBigint.CopyBigint(a,Result); end;
(如果按预期工作,将测试并添加答案).
解决方法
我的
first answer试图阻止重写赋值运算符的想法.我仍然坚持这个答案,因为很多问题都可以用对象更好地解决.
但是,David非常正确地指出TBigInt是作为利用运算符重载的记录实现的.即a：= b c;.这是坚持基于记录的实现的一个非常好的理由.
因此,我提出了这种替代解决方案,可以一石二鸟：
>它消除了我在其他答案中解释的内存管理风险.
>并提供了一种实现Copy-on-Write语义的简单机制.
(我仍然建议除非有充分的理由保留基于记录的解决方案,否则请考虑切换到基于对象的解决方案.)
总体思路如下：
>定义一个表示BigInt数据的接口. (这最初可以是极简主义的,并且仅支持指针的控制 – 如我的示例所示.这将使现有代码的初始转换更容易.)
>定义将由TBigInt记录使用的上述接口的实现.
>接口解决了第一个问题,因为接口是托管类型;当记录超出范围时,Delphi将取消引用该接口.因此,当不再需要时,底层对象将自我毁灭.
>界面还提供了解决第二个问题的机会,因为我们可以检查RefCount以了解我们是否应该写入Copy-On.
>请注意,从长期来看,将一些BigInt实现从记录转移到类&接口.
下面的代码是精简的“大型int”实现,纯粹是为了说明这些概念. (即,“大”整数仅限于常规的32位数,并且仅实现了添加.)
type
IBigInt = interface
['{1628BA6F-FA21-41B5-81C7-71C336B80A6B}']
function GetData: Pointer;
function GetSize: Integer;
procedure Realloc(ASize: Integer);
function RefCount: Integer;
end;
type
TBigIntImpl = class(TInterfacedObject,IBigInt)
private
FData: Pointer;
FSize: Integer;
protected
{IBigInt}
function GetData: Pointer;
function GetSize: Integer;
procedure Realloc(ASize: Integer);
function RefCount: Integer;
public
constructor CreateCopy(ASource: IBigInt);
destructor Destroy; override;
end;
type
TBigInt = record
PtrDigits: IBigInt;
constructor CreateFromInt(AValue: Integer);
class operator Implicit(AValue: TBigInt): Integer;
class operator Add(AValue1,AValue2: TBigInt): TBigInt;
procedure Add(AValue: Integer);
strict private
procedure CopyOnWriteSharedData;
end;
{ TBigIntImpl }
constructor TBigIntImpl.CreateCopy(ASource: IBigInt);
begin
Realloc(ASource.GetSize);
Move(ASource.GetData^,FData^,FSize);
end;
destructor TBigIntImpl.Destroy;
begin
FreeMem(FData);
inherited;
end;
function TBigIntImpl.GetData: Pointer;
begin
Result := FData;
end;
function TBigIntImpl.GetSize: Integer;
begin
Result := FSize;
end;
procedure TBigIntImpl.Realloc(ASize: Integer);
begin
ReallocMem(FData,ASize);
FSize := ASize;
end;
function TBigIntImpl.RefCount: Integer;
begin
Result := FRefCount;
end;
{ TBigInt }
class operator TBigInt.Add(AValue1,AValue2: TBigInt): TBigInt;
var
LSum: Integer;
begin
LSum := Integer(AValue1) + Integer(AValue2);
Result.CreateFromInt(LSum);
end;
procedure TBigInt.Add(AValue: Integer);
begin
CopyOnWriteSharedData;
PInteger(PtrDigits.GetData)^ := PInteger(PtrDigits.GetData)^ + AValue;
end;
procedure TBigInt.CopyOnWriteSharedData;
begin
if PtrDigits.RefCount > 1 then
begin
PtrDigits := TBigIntImpl.CreateCopy(PtrDigits);
end;
end;
constructor TBigInt.CreateFromInt(AValue: Integer);
begin
PtrDigits := TBigIntImpl.Create;
PtrDigits.Realloc(SizeOf(Integer));
PInteger(PtrDigits.GetData)^ := AValue;
end;
class operator TBigInt.Implicit(AValue: TBigInt): Integer;
begin
Result := PInteger(AValue.PtrDigits.GetData)^;
end;
在构建提出的解决方案时,编写了以下测试.他们证明：一些基本功能,即写时复制按预期工作,并且没有内存泄漏.
procedure TTestCopyOnWrite.TestCreateFromInt;
var
LBigInt: TBigInt;
begin
LBigInt.CreateFromInt(123);
CheckEquals(123,LBigInt);
//Dispose(PInteger(LBigInt.PtrDigits)); //I only needed this until I
//started using the interface
end;
procedure TTestCopyOnWrite.TestAssignment;
var
LValue1: TBigInt;
LValue2: TBigInt;
begin
LValue1.CreateFromInt(123);
LValue2 := LValue1;
CheckEquals(123,LValue2);
end;
procedure TTestCopyOnWrite.TestAddMethod;
var
LValue1: TBigInt;
begin
LValue1.CreateFromInt(123);
LValue1.Add(111);
CheckEquals(234,LValue1);
end;
procedure TTestCopyOnWrite.TestOperatorAdd;
var
LValue1: TBigInt;
LValue2: TBigInt;
LActualResult: TBigInt;
begin
LValue1.CreateFromInt(123);
LValue2.CreateFromInt(111);
LActualResult := LValue1 + LValue2;
CheckEquals(234,LActualResult);
end;
procedure TTestCopyOnWrite.TestCopyOnWrite;
var
LValue1: TBigInt;
LValue2: TBigInt;
begin
LValue1.CreateFromInt(123);
LValue2 := LValue1;
LValue1.Add(111); { If CopyOnWrite,then LValue2 should not change }
CheckEquals(234,LValue1);
CheckEquals(123,LValue2);
end;
编辑
添加了一个测试,证明使用TBigInt作为过程的值参数.
procedure TTestCopyOnWrite.TestValueParameter;
procedure CheckValueParameter(ABigInt: TBigInt);
begin
CheckEquals(2,ABigInt.PtrDigits.RefCount);
CheckEquals(123,ABigInt);
ABigInt.Add(111);
CheckEquals(234,ABigInt);
CheckEquals(1,ABigInt.PtrDigits.RefCount);
end;
var
LValue: TBigInt;
begin
LValue.CreateFromInt(123);
CheckValueParameter(LValue);
end;