Question

本条

procedure MyThreadTestA(const AStr: string);

Is faster than

procedure MyThreadTestB(AStr: string);

在从事同样工作的同时,双方都走了一条路。

However version B correctly updates the referencecount of AStr and makes a copy if I change it.
Version A passes just a pointer and only the compiler prevents me from changing AStr.

Version 如果我在议会中玩s,或者为了绕过编纂者的保护,那是不安全的。

Is passed AStr by reference as a const parameters thread safe?
What happens if AStr s reference count in some other thread goes to zero and the string is destroyed?

Answer 1

无,这种骗子不是read。 <代码>Const 防止添加-ref,这样,另一条面的改动就会以不可预测的方式影响其价值。抽样方案,在<代码>定义中尝试修改<代码>const P :

{$apptype console}
uses SysUtils, Classes, SyncObjs;

type
  TObj = class
  public
    S: string;
  end;

  TWorker = class(TThread)
  public
    procedure Execute; override;
  end;

var
  lock: TCriticalSection;
  obj: TObj;

procedure P(const x: string);
// procedure P(x: string);
begin
  Writeln( P(1): x =  , x);
  Writeln( Releasing obj );
  lock.Release;
  Sleep(10); // give worker a chance to run
  Writeln( P(2): x =  , x);
end;

procedure TWorker.Execute;
begin
  // wait until TMonitor is freed up
  Writeln( Worker started... );
  lock.Acquire;
  Writeln( worker fiddling with obj.S );
  obj.S :=  bar ;
  TMonitor.Exit(obj);
end;

procedure Go;
begin
  lock := TCriticalSection.Create;
  obj := TObj.Create;
  obj.S :=  foo ;
  UniqueString(obj.S);
  lock.Acquire;
  TWorker.Create(False);
  Sleep(10); // give worker a chance to run and block
  P(obj.S);
end;

begin
  Go;
end.

但是,它不仅局限于深层;修改基本变形地点具有类似影响:

{$apptype console}
uses SysUtils, Classes, SyncObjs;

type
  TObj = class
  public
    S: string;
  end;

var
  obj: TObj;

procedure P(const x: string);
begin
  Writeln( P(1): x =  , x);
  obj.S :=  bar ; 
  Writeln( P(2): x =  , x);
end;

procedure Go;
begin
  obj := TObj.Create;
  obj.S :=  foo ;
  UniqueString(obj.S);
  P(obj.S);
end;

begin
  Go;
end.

Answer 2

在Barry的答复中添加: 如果从电离器范围内的一个地方变量中穿透镜,那肯定是read。

In that case that local variable will hold a valid reference and the only way (assuming just valid pascal code, no fiddling around in asm) for that local variable to be changed is if your call returns.

这还包括由职能电话(包括财产存取,例如TStrings)产生的星座变数来源的所有情况。优点在于,在这种情况下,汇编者必须在当地的温变中储存插座。

只有当你直接从一个地点(在同一或另一处)经人叫回之前可以改变这种扼杀,才可能产生表面安全问题。

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