如何测试Delphi中是否释放了一个对象

Question

下面显示的两个程序尝试使用此处描述的技术测试对象是否被释放,对已释放的对象的错误引用.

如果在Delphi 7下编译,下面显示的第一个程序运行正确,但如果在Delphi XE和upper下编译,则错误地运行.也就是说,它输出

 D7          DXE 
True        True
True        True 
True        False
True        True
False       True
False       False

program Project1;

{$APPTYPE CONSOLE}

uses
  SysUtils;

function ValidateObj(Obj: TObject): Pointer;
// see { Virtual method table entries } in System.pas
begin
  Result := Obj;
  if Assigned(Result) then
    try
      if Pointer(PPointer(Obj)^) <> Pointer(Pointer(Cardinal(PPointer(Obj)^) + Cardinal(vmtSelfPtr))^) then
        // object not valid anymore
        Result := nil;
    except
      Result := nil;
    end;
end;

function ValidateObj2(Obj: TObject): Pointer;
type
  PPVmt = ^PVmt;
  PVmt = ^TVmt;
  TVmt = record
    SelfPtr : TClass;
    Other   : array[0..17] of pointer;
  end;
var
  Vmt: PVmt;
begin
  Result := Obj;
  if Assigned(Result) then
    try
      Vmt := PVmt(Obj.ClassType);
      Dec(Vmt);
      if Obj.ClassType <> Vmt.SelfPtr then
        Result := nil;
    except
      Result := nil;
    end;
end;

var
   Obj: TObject;
begin
  Obj := TObject.Create;
  Writeln(BoolToStr(Assigned(Obj), True));
  Writeln(BoolToStr(Assigned(ValidateObj(Obj)), True));
  Writeln(BoolToStr(Assigned(ValidateObj2(Obj)), True));
  Obj.free;
  Writeln(BoolToStr(Assigned(Obj), True));
  Writeln(BoolToStr(Assigned(ValidateObj(Obj)), True));
  Writeln(BoolToStr(Assigned(ValidateObj2(Obj)), True));
  Readln;
end.

第二个程序,显式使用FastMM4,如下所示,在Delphi 7或XE和upper下编译时运行错误.也就是说,它输出

Expected      D7    DXE
  False     False  False
  True      True   True
  True      True   True 
  True      True   False
  True      False  False
  True      True   True
  False     True   True
  False     True   False

program Project2;

{$APPTYPE CONSOLE}

uses
  FastMM4,
  SysUtils;

function ValidateObj(Obj: TObject): Pointer;
// see { Virtual method table entries } in System.pas
begin
  Result := Obj;
  if Assigned(Result) then
    try
      if Pointer(PPointer(Obj)^) <> Pointer(Pointer(Cardinal(PPointer(Obj)^) + Cardinal(vmtSelfPtr))^) then
        // object not valid anymore
        Result := nil;
    except
      Result := nil;
    end;
end;

function ValidateObj2(Obj: TObject): Pointer;
type
  PPVmt = ^PVmt;
  PVmt = ^TVmt;
  TVmt = record
    SelfPtr : TClass;
    Other   : array[0..17] of pointer;
  end;
var
  Vmt: PVmt;
begin
  Result := Obj;
  if Assigned(Result) then
    try
      Vmt := PVmt(Obj.ClassType);
      Dec(Vmt);
      if Obj.ClassType <> Vmt.SelfPtr then
        Result := nil;
    except
      Result := nil;
    end;
end;

var
   Obj: TObject;
begin
  Obj := TObject.Create;        
  Writeln(BoolToStr(Obj is FastMM4.TFreedObject, True));
  Writeln(BoolToStr(Assigned(Obj), True));
  Writeln(BoolToStr(Assigned(ValidateObj(Obj)), True));
  Writeln(BoolToStr(Assigned(ValidateObj2(Obj)), True));
  Obj.free;                                
  Writeln(BoolToStr(Obj is FastMM4.TFreedObject, True));
  Writeln(BoolToStr(Assigned(Obj), True));
  Writeln(BoolToStr(Assigned(ValidateObj(Obj)), True));
  Writeln(BoolToStr(Assigned(ValidateObj2(Obj)), True));
  Readln;
end.

我很困惑如何导致错误的行为,并想知道如何测试一个对象是否被释放为Delphi 7和Delphi XE和鞋面,特别是当使用FastMM4时？

Answer 1

通常,不可能进行强健的测试,指针指的是已释放的实例.程序员的工作是保持对对象生命周期的控制.