Question

我对Pascal很新，目前正在使用指针。我有2条记录，其中一条包含指向另一条记录类型的2条指针。

type
  WaypointRef = ^Waypoint;

  PathRef = ^Path;

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

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

所有航路点都保存在一个数组中。现在，当我试图读出路径的值时，我得到了神秘的结果：

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

两者都应该打印相同的值，但它们不会。然而，更神秘的是，即使我尝试以下内容：

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

我得到2个不同的值。（正确的 - 先是173 - 然后是2次。）

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，因为如果我没有将任何值保存到＆＃34;距离＆＃34;它也会返回2。在路径创建。

EDIT2：这里是航点和路径创建的代码。我认为一定有失败。我现在可能会因设备内部的程序而混淆设计。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

有两件事可能导致这种意外行为：

如果你有getter方法支持的waypoints[0]和pathRefs[0]的数组类型属性：那么这些方法可能会产生可能导致问题的副作用。（显然，这不是的情况）。
如果您的指针正在引用＆＃34;无效的内存＆＃34;位置：然后由其他代码覆盖内存可能导致值发生变化。（这是你的问题。）

您要添加的路径在堆栈中声明：

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-- When you leave the method the stack variable is no longer valid.

作为此代码的结果，您的wRef^.pathRefs[??]指向调用堆栈中较高的地址。
当您调用其他方法时，相同的内存将用于其他目的。
值可以改变。

您需要确保指向堆上的内存。您可以使用动态内存分配例程执行此操作：New，Dispose，GetMem，FreeMem。

修改

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：您必须弄清楚处理动态分配内存的位置和时间。

EDIT2 添加一个简单的控制台应用来演示此问题。

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.

Pascal指针改变其指向值

1 个答案: