Why does the program crash when using const string arguments?

Question

My program has following code:

function FooBar(const s: string): string;
var
  sa: AnsiString;
begin

  // ..........................

  sa := AnsiString(s);
  sa := AnsiString(StringReplace(string(sa), '*', '=', [rfReplaceAll]));
  sa := AnsiString(StringReplace(string(sa), ' ', '+', [rfReplaceAll]));
  result := string(sa);

  // ..........................

end;

I noticed that the program did crash "somewhere" and FastMM4 said that I had written to a freed object. As soon as I have commented out "const", the program did work.

I have read the Delphi documentation about const arguments, but I can't figure out why the const argument crashes the program. I would love to understand it.

UPDATE: The program does only crash in Delphi 6 and only if optimization is ON. If the optimization is OFF, the program will work normally. Might it be a Delphi bug?

Answer 1

对于本例：

 sa := s;

自动引用计数（ARC）起作用。这是惯用的方式，编译器知道如何处理这些字符串 - 如果 sa 发生变化，它会创建新的副本等等。

对于硬类型转换的情况（尽管类型相同）

sa := AnsiString(s);

您告诉编译器您只想获取指向字符串的指针，并且您知道如何使用此字符串引用。编译器不会干扰和烦扰你，但你要对正确操作负责

PS 我无法用 Delphi XE5 重现问题 - 简单的赋值和类型转换都会导致 LStrLAsg（内部函数）使用 ARC 进行调用。（当然，编译器魔法可以稍微改变一下）