当将一个对象的新实例传递给其类实现的接口具有const接口参数的方法时,编译器是否应提示/警告?
编辑:示例当然很简单,只是为了说明问题。但在现实生活中,它变得更加复杂:如果创建和使用的代码相距甚远(不同的单元、不同的类、不同的项目)怎么办?如果由不同的人维护呢?如果非const参数变成const参数,并且不能检查所有调用代码的情况(因为更改代码的人无法访问所有调用代码)怎么办?
像下面这样的代码会崩溃,而且很难找到原因。
首先看日志:
1.Run begin
1.RunLeakCrash
2.RunLeakCrash begin
NewInstance 1
AfterConstruction 0
3.LeakCrash begin
_AddRef 1
4.Dump begin
4.Dump Reference=10394576
4.Dump end
_Release 0
_Release Destroy
BeforeDestruction 0
3.LeakCrash Reference got destroyed if it had a RefCount of 1 upon entry, so now it can be unsafe to access it
_AddRef 1
4.Dump begin
4.Dump Reference=10394576
4.Dump end
_Release 0
_Release Destroy
BeforeDestruction 0
3.LeakCrash end with exception
1.Run end
EInvalidPointer: Invalid pointer operation
接下来是过早释放实现接口的对象实例的代码:
//{$define all}
program InterfaceConstParmetersAndPrematureFreeingProject;
{$APPTYPE CONSOLE}
uses
SysUtils,
Windows,
MyInterfacedObjectUnit in '..\src\MyInterfacedObjectUnit.pas';
procedure Dump(Reference: IInterface);
begin
Writeln(' 4.Dump begin');
Writeln(' 4.Dump Reference=', Integer(PChar(Reference)));
Writeln(' 4.Dump end');
end;
procedure LeakCrash(const Reference: IInterface);
begin
Writeln(' 3.LeakCrash begin');
try
Dump(Reference); // now we leak because the caller does not keep a reference to us
Writeln(' 3.LeakCrash Reference got destroyed if it had a RefCount of 1 upon entry, so now it can be unsafe to access it');
Dump(Reference); // we might crash here
except
begin
Writeln(' 3.LeakCrash end with exception');
raise;
end;
end;
Writeln(' 3.LeakCrash end');
end;
procedure RunLeakCrash;
begin
Writeln(' 2.RunLeakCrash begin');
LeakCrash(TMyInterfacedObject.Create());
Writeln(' 2.RunLeakCrash end');
end;
procedure Run();
begin
try
Writeln('1.Run begin');
Writeln('');
Writeln('1.RunLeakCrash');
RunLeakCrash();
finally
Writeln('');
Writeln('1.Run end');
end;
end;
begin
try
Run();
except
on E: Exception do
Writeln(E.ClassName, ': ', E.Message);
end;
Readln;
end.
当第二次调用Dump(Reference);
时,EInvalidPointer错误会出现。原因是表露Reference的基础对象的引用计数已经为零,所以基础对象已经被销毁。
关于编译器插入或省略的引用计数代码的一些注释:
- 没有标记为
const
的参数(如procedure Dump(Reference: IInterface);
中)会隐式使用try/finally块执行引用计数。 - 标记为
const
的参数(如procedure LeakCrash(const Reference: IInterface);
)不会得到任何引用计数代码。 - 传递对象实例创建的结果(如
LeakCrash(TMyInterfacedObject.Create());
)不会生成任何引用计数代码。
所有上述编译器行为都非常合乎逻辑,但结合在一起可能会导致EInvalidPointer。
EInvalidPointer只在非常狭窄的使用模式下显现。
该模式很容易由编译器识别,但当你遇到它时,很难进行调试或找到原因。
解决方法非常简单:将TMyInterfacedObject.Create()
的结果缓存到一个中间变量中,然后将其传递给LeakCrash()
。
编译器是否应该提示或警告您有关此使用模式的信息?
最后,以下是我用于跟踪所有_AddRef/_Release/等调用的代码:
unit MyInterfacedObjectUnit;
interface
type
// Adpoted copy of TInterfacedObject for debugging
TMyInterfacedObject = class(TObject, IInterface)
protected
FRefCount: Integer;
function QueryInterface(const IID: TGUID; out Obj): HResult; stdcall;
function _AddRef: Integer; stdcall;
function _Release: Integer; stdcall;
public
procedure AfterConstruction; override;
procedure BeforeDestruction; override;
class function NewInstance: TObject; override;
property RefCount: Integer read FRefCount;
end;
implementation
uses
Windows;
procedure TMyInterfacedObject.AfterConstruction;
begin
InterlockedDecrement(FRefCount);
Writeln(' AfterConstruction ', FRefCount);
end;
procedure TMyInterfacedObject.BeforeDestruction;
begin
Writeln(' BeforeDestruction ', FRefCount);
if RefCount <> 0 then
System.Error(reInvalidPtr);
end;
class function TMyInterfacedObject.NewInstance: TObject;
begin
Result := inherited NewInstance;
TMyInterfacedObject(Result).FRefCount := 1;
Writeln(' NewInstance ', TMyInterfacedObject(Result).FRefCount);
end;
function TMyInterfacedObject.QueryInterface(const IID: TGUID; out Obj): HResult;
begin
Writeln(' QueryInterface ', FRefCount);
if GetInterface(IID, Obj) then
Result := 0
else
Result := E_NOINTERFACE;
end;
function TMyInterfacedObject._AddRef: Integer;
begin
Result := InterlockedIncrement(FRefCount);
Writeln(' _AddRef ', FRefCount);
end;
function TMyInterfacedObject._Release: Integer;
begin
Result := InterlockedDecrement(FRefCount);
Writeln(' _Release ', FRefCount);
if Result = 0 then
begin
Writeln(' _Release Destroy');
Destroy;
end;
end;
end.
--jeroen