delphi - 将tmemorystream作为var传递

Question

当我尝试将tmemorystream作为var或指向过程的指针传递时，它又损坏了。正确的方法是什么？

例如：

function tform1.downloadmemupdate(url, desc: string; var data: tmemorystream; var msg: string): boolean;
begin
  filelabel.Caption:=desc;
  downloadmemthread:=tdownloadmemthread.create(url);
  dlcancelbtn.Enabled:=true;
  downloadmemthread.dlstart;
  waitforsingleobject(downloadmemthread.Handle, INFINITE);

  downloadmemthread.data.SaveToStream(data); //corrupted
  downloadmemthread.data.SaveToFile('data.zip');  //works

  dlcancelbtn.Enabled:=false;
  result:=not (downloadmemthread.canceled and downloadmemthread.success);
  dlcanceled:=downloadmemthread.canceled;
  msg:=downloadmemthread.msg;
  downloadthread.Free;
end;

Answer 1

您不在此方法中创建data。由于它是var（byref）参数，因此我希望可以在tform1.downloadmemupdate中创建它，即：

data := TMemoryStream.Create;

请注意，如果创建这样的对象，则可能需要在调用代码中将其释放到其他位置。

例如

   Data := nil;
   try
     downloadmemupdate(url, desc, data, msg);
     // do something with data
   finally
     Data.Free;
   end;  

另一种方法（Delphi中的惯用方法）是按值传递对象（不带var）。并留给调用代码来创建和销毁它们。这主要是因为Delphi没有垃圾回收，因此它迫使编写调用代码的人考虑“所有权”。

这将是

function tform1.downloadmemupdate(url, desc: string; data: TStream; var msg: string): boolean;
begin
  filelabel.Caption:=desc;
  downloadmemthread:=tdownloadmemthread.create(url);
  try
    ...
    downloadmemthread.data.SaveToStream(data); //corrupted
    downloadmemthread.data.SaveToFile('data.zip');  //works    
  finally
    downloadmemthread.Free;
  end;

end;

调用代码：

   Data := TMemoryStream.Create;
   try
     downloadmemupdate(url, desc, data, msg);
     // do something with data
   finally
     Data.Free;
   end;