当AnsiString被转换为PAnsiString时会发生什么？ [英] What happens when AnsiString is cast to PAnsiString?

问题描述

我有方法（Delphi 2009）：

I have the method (Delphi 2009):

procedure TAnsiStringType.SetData(const Value: TBuffer; IsNull: boolean = False);
begin
  if not IsNull then
    FValue:= PAnsiString(Value)^;
  inherited;
end;

这是一个基类的抽象方法，其中Value：Pointer对应数据，如：

This is an abstract method on the base class, where "Value: Pointer" expects the pointer of the corresponding data, as:

String = PString
AnsiString = PAnsiString
Integer = PInteger
Boolean = PBoolean

所以我试着传递这样的值：

So I try to pass the value like this:

var
  S: AnsiString;
begin
  S:= 'New AnsiString Buffer';
  SetBuffer(PAnsiString(S));
end;

但是从AnsiString到PAnsiString的转换不工作，我可以看到为什么，但我想知道铸造的结果是什么。所以我写了一个简单的测试：

But a cast from AnsiString to PAnsiString does NOT work, I can see why, but I want to know what the result of the casting is. So I wrote a simple test:

var
  Buffer: AnsiString;
  P1: Pointer;
  P2: Pointer;
  P3: Pointer;
  P4: Pointer;
begin
  P1:= PAnsiString(Buffer);
  P2:= Addr(Buffer);
  P3:= @Buffer;
  P4:= Pointer(Buffer);
  P5:= PChar(Buffer[1]);

  WriteLn('P1: ' + IntToStr(Integer(P1)));
  WriteLn('P2: ' + IntToStr(Integer(P2)));
  WriteLn('P3: ' + IntToStr(Integer(P3)));
  WriteLn('P4: ' + IntToStr(Integer(P4)));
  WriteLn('P5: ' + IntToStr(Integer(P5)));
end;

结果是：

P1: 5006500
P2: 1242488
P3: 1242488
P4: 5006500
P5: 67

其中：

- P2 and P3, is the address of Buffer: AnsiString 
- P5 is the Char Ord value of Buffer[1] char, in this case "67 = C"
- How about P1 and P4?

P1和P4的意义是什么？ b $ b

推荐答案

很好的拼图，但我有解决方案：

Nice puzzle, but I have the solution:

• P2和P3是指向缓冲区的指针的地址


• P1和P4是缓冲区的地址


• P5是第一个元素缓冲区

我在代码中添加了注释：

I have added comment in the code:

var
  Buffer: AnsiString;
  P1: Pointer;
  P2: Pointer;
  P3: Pointer;
  P4: Pointer;
  P5: Pointer;
begin
  P1:= PAnsiString(Buffer); 
  (* A cast from AnsiString to PAnsiString has no real meaning 
     because both are a pointer to a block of characters ()
  P2:= Addr(Buffer);
  P3:= @Buffer;
  (* Both Addr and @ give the address of a variable. The variable Buffer is 
     a pointer so we get the address of the pointer, not the value of the 
     pointer. *)
  P4:= Pointer(Buffer);
  (* See the remark on P1. Due to the cast both give the same result. *)
  P5:= PChar(Buffer[1]);
  (* This looks like a pointer to the first element. But the cast changes 
     it into the character. *)
  WriteLn('P1: ' + IntToStr(Integer(P1)));
  WriteLn('P2: ' + IntToStr(Integer(P2)));
  WriteLn('P3: ' + IntToStr(Integer(P3)));
  WriteLn('P4: ' + IntToStr(Integer(P4)));
  WriteLn('P5: ' + IntToStr(Integer(P5)));
end;

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