delphi - 帕斯卡指针改变其指向值

Question

我对 Pascal 比较陌生，目前正在使用指针。我有 2 条记录，其中一条包含 2 个指向另一种记录类型的指针。

type
  WaypointRef = ^Waypoint;

  PathRef = ^Path;

  Waypoint = record
    id: integer;
    Name: string;
    pathRefs: array of PathRef;
  end;

  Path = record
    distance: integer;
    WaypointRefA, WaypointRefB: WaypointRef;
  end; 

所有航点都保存在一个数组中。现在，当我尝试读出路径的值时，我得到了神秘的结果:

writeln(waypoints[0].pathRefs[0]^.distance);
writeln(waypoints[1].pathRefs[0]^.distance);

两者应该打印相同的值，但他们没有。然而，更神秘的是，即使我尝试以下操作:

writeln(waypoints[0].pathRefs[0]^.distance);
writeln(waypoints[0].pathRefs[0]^.distance);
writeln(waypoints[0].pathRefs[0]^.distance);

我得到两个不同的值。 (正确的一个 - 173 - 第一个，然后一直是 2。)

waypoints[0].pathRefs[0]^

总是指向同一个地址，因此我很困惑。我希望有人知道这个问题。

编辑:2 似乎是默认值，因为如果我在创建路径时没有将任何值保存到“距离”，它也会返回 2。

EDIT2:这里是航路点和路径创建的代码。我想一定有失败的时候。我现在可能会因为程序里面的程序而设计困惑。我只是在尝试。

procedure buildWaypoint(Name: string);

  procedure addWaypoint(w: Waypoint);
  var
    lngth: integer;
  begin
    lngth := Length(waypoints);
    SetLength(waypoints, lngth + 1);
    waypoints[lngth] := w;
  end;

var
  w: Waypoint;
begin
  w.id := id;
  id := id + 1;

  w.Name := Name;
  addWaypoint(w);
end;

procedure buildPath(waypointRefA, waypointRefB: WaypointRef; distance: integer);

  procedure addPath(pRef: PathRef);

    procedure addPathToWaypoint(pRef: PathRef; wRef: WaypointRef);
    var
      lngth: integer;
    begin
      lngth := length(wRef^.pathRefs);
      SetLength(wRef^.pathRefs, lngth + 1);
      wRef^.pathRefs[lngth] := pRef;
    end;

  begin
    addPathToWaypoint(pRef, pRef^.WaypointRefA);
    addPathToWaypoint(pRef, pRef^.WaypointRefB);
  end;

var
  p: path;
begin
  p.distance := distance;
  p.WaypointRefA := waypointRefA;
  p.WaypointRefB := waypointRefB;

  addPath(@p);
end;                      

Answer 1

有两件事可能会导致这种意外行为:

1. 如果您有由 getter 方法支持的 waypoints[0] 和 pathRefs[0] 的数组类型属性:那么这些方法可能具有侧面- 会导致问题的影响。 (显然这里的情况并非如此)。
2. 如果您的指针引用“无效内存”位置:则其他代码覆盖内存可能会导致值发生变化。 (这就是你的问题。)

您要添加的路径已在堆栈上声明:

var
  p: path;  //<-- Stack variable
begin
  ...    
  addPath(@p);
end; //<-- When you leave the method the stack variable is no longer valid.

• 由于此代码，您的 wRef^.pathRefs[??] 指向调用堆栈上较高的地址。
• 当您调用其他方法时，相同的内存会用于其他目的。
• 并且这些值可以更改。

您需要确保指向堆上的内存。您可以通过使用动态内存分配例程来完成此操作:New、Dispose、GetMem、FreeMem。

编辑

Documentation关于动态内存分配例程。

如何更改代码的示例:

procedure addPathToWaypoint(pRef: PathRef; wRef: WaypointRef);
var
  lngth: integer;
  LpRefOnHeap: PathRef;
begin
  lngth := length(wRef^.pathRefs);
  SetLength(wRef^.pathRefs, lngth + 1);
  New(LpRefOnHeap); //Allocate heap memory
  LpRefOnHeap^ := pRef^; //Copy data pointed to by pRef to heap
  wRef^.pathRefs[lngth] := LpRefOnHeap; //Hold reference to an address that won't
                                        //become invalid when stack unwinds.
end;

注意:您必须弄清楚何时何地处置动态分配的内存。

<小时/>

EDIT2 添加一个简单的控制台应用程序来演示问题。

program InvalidUseOfStackVar;

{$APPTYPE CONSOLE}

type
  PData = ^TData;
  TData = record
    Value: Integer;
  end;

var
  GData: PData;

procedure SetData;
var
  LData: TData; //Stack variable will no longer be valid when routine exits.
begin
  LData.Value := 42; //The initial value pointed to by GData
  GData := @LData; //The global var will continue to point to invalid memory after exit.
end;

procedure ChangeStack;
var
  //This is written to have the same stack layout as the previous routine.
  LData: TData;
begin
  LData.Value := 777; //This unintentionally changes data pointed to by the global var
end;

begin
  SetData;                //Sets GData, but GData points to an address on the call stack
  Writeln(GData^.Value);  //Writes 42 because that's what was on the stack at the time of the method call.
  ChangeStack;            //Updates the stack variable to a different value
  Writeln(GData^.Value);  //Now writes 777 because GData points to the same location in memory, but the
                          //data at that location was changed.
  Writeln(GData^.Value);  //Note: calling the Writeln method above also changes the stack.
                          //The only difference is that it is less predictable for us to determine
                          //how the stack will be changed.
  Readln;
end.