Question

我想编写一些东西，一旦指向动态分配的地址空间的指针离开堆栈，就会自动释放内存。一个例子是：

procedure FillMemory (var mypointer);
begin
  // CopyMemory, Move, etc... with data
end;

procedure MyProcedure;
var
  MyPointer : Pointer;
begin
  MyPointer := VirtualAlloc (NIL, 1024, MEM_COMMIT or MEM_RESERVE, PAGE_EXECUTE_READWRITE);
  FillMemory (MyPointer);
  VirtualFree(MyPointer, 0, MEM_RELEASE); // I would like to avoid this...
end;

我可以使用字符串，但我也想避免它们（我不太确定堆栈中的字符串是否会被释放...）有什么想法吗？

Answer 1

扩展我对Arioch的评论和讨论，

如果您只想要原始内存块，请使用动态array of byte。编译器将生成代码以在方法结束时释放此内存：

type
  TBytes = array of Byte; // omit for newer Delphi versions

procedure FillMemory(var Bytes: TBytes);
begin
  { passing in here will increase the reference count to 2 }
  // CopyMemory, Move, etc... with data
end; // then drop back to 1

procedure MyProcedure;
var
  Buffer : TBytes;
begin
  SetLength(Buffer, 1024);  // buffer has reference count of 1
  FillMemory (Buffer);
end;   // reference count will drop to 0, and Delphi will free memory here

希望这一切都有意义。现在是午夜，所以我感觉不是最清醒......

Answer 2

托管类型对其引用进行计数，当计数降至零时，它们将被最终确定。如果你有一个局部变量，那么当它超出范围时，它的引用计数将降为零。

因此，您可以创建使用接口引用的TInterfacedObject的后代。像这样：

type
  TLifetimeWatcher = class(TInterfacedObject)
  private
    FDestroyProc: TProc;
  public
    constructor Create(const DestroyProc: TProc);
    destructor Destroy; override;
  end;

constructor TLifetimeWatcher.Create(const DestroyProc: TProc);
begin
  inherited Create;
  FDestroyProc := DestroyProc;
end;

destructor TLifetimeWatcher.Destroy;
begin
  if Assigned(FDestroyProc) then
    FDestroyProc();
  inherited;
end;

然后您可以像这样使用它：

procedure MyProcedure;
var
  MyPointer: Pointer;
  LifetimeWatcher: IInterface;
begin
  MyPointer := VirtualAlloc (NIL, 1024, MEM_COMMIT or MEM_RESERVE, 
    PAGE_EXECUTE_READWRITE);
  LifetimeWatcher := TLifetimeWatcher.Create(
    procedure
    begin
      VirtualFree(MyPointer, 0, MEM_RELEASE);
    end;
  )
  FillMemory(MyPointer);
end;

当LifetimeWatcher离开作用域时，销毁实现对象并执行传递给TLifetimeWatcher.Create的过程。

将这个想法专门用于您的用例是很容易的。这将使呼叫站点的代码更加简洁。

看起来像这样：

function VirtualAllocAutoRelease(Size: SIZE_T; Protect: DWORD;
  out LifetimeCookie: IInterface): Pointer;
var
  Ptr: Pointer;
begin
  Ptr := VirtualAlloc(nil, Size, MEM_COMMIT or MEM_RESERVE, Protect);
  Win32Check(Ptr<>nil);
  LifetimeCookie := TLifetimeWatcher.Create(
    procedure
    begin
      VirtualFree(Ptr, 0, MEM_RELEASE);
    end
  );
  Result := Ptr;
end;

你会像这样使用它：

procedure MyProcedure;
var
  MyPointer: Pointer;
  LifetimeWatcher: IInterface;
begin
  MyPointer := VirtualAllocAutoRelease(1024, PAGE_EXECUTE_READWRITE, 
    LifetimeWatcher);
  FillMemory(MyPointer);
end;

离开堆栈时自动释放内存

2 个答案: