Constant string parameters and the Delphi XE5 compiler for Android

Question

Hopefully I'm just missing something obvious, but I seem to be finding constant string arguments getting corrupted when using the Delphi XE5 Android compiler. Test code: 1) Create a new blank mobile application project. 2) Add a `TButton` to the form, and create an `OnClick` handler for it. 3) Fill out the handler like so: ``` procedure TForm1.Button1Click(Sender: TObject); begin GoToDirectory(PathDelim + 'alpha' + PathDelim + 'beta'); GoToDirectory(FParentDir); end; ``` 4) In the form class declaration, add two fields and one method like this: ``` FCurrentPath, FParentDir: string; procedure GoToDirectory(const Dir: string); ``` 5) Implement `Foo` and `GoToDirectory` like so: ``` function Foo(const S: string): Boolean; begin Result := (Now <> 0); end; procedure TForm1.GoToDirectory(const Dir: string); begin FCurrentPath := IncludeTrailingPathDelimiter(Dir); FParentDir := ExcludeTrailingPathDelimiter(ExtractFilePath(Dir)); ShowMessageFmt('Prior to calling Foo, Dir is "%s"', [Dir]); Foo…

Accepted Answer

We did some internal investigations, and it turns out this depends on the way the code is written and there is nothing the compiler can really do about it. It is slightly complex, but in short the GoToDirectory method receives a const string parameter (Dir) which refers to a string. However, within the method's code replaces the string with a new one (which might be at the same or at a different memory location). Given the const parameter doesn't increase the reference count, if you decrease the ref count of the same string in the code, the string is removed. So you have a parameter pointing to an undefined memory location, and the actual output is kind of random. Same issue happens (might happen) on all platforms, not mobile specific.

There are many workarounds:

1) not have the const param (so the ref count is higher, you change the referenced string but the param is now a reference to a separate string

2) Pass an alias of the string:

```
  Tmp := FParentDir;
  GoToDirectory(Tmp);
```

3) Assign "const String" parameter to temporary local variable:

```
procedure TForm1.GoToDirectory(const Dir: string);
var
  TmpDir: String;
begin
  TmpDir := Dir;
```

I know this is far from a clear description, and I had to red it a few times to grasp it, but it is a scenario the compiler cannot really handle automatically, so we are going to close the bug report "as designed".

Constant string parameters and the Delphi XE5 compiler for Android

Solution

Problem