i'm not sure if i'm getting this right, but i still remember java allowing me to cast an object to another type as long as their ancestors are the same?

I can't seem to do this in delphi, the compiler doesn't seem to understand the methods and attributes of each object even after I've casted them. What's wrong?

I'm trying to figure out how to stick two database query objects under the same name as illustrated :

qry : TObject;


if(callType = 'A') then
begin

//casting qry as a TOraQuery object :

qry := TOraQuery.Create(nil);
qry := commonObj.GetConnection('DATABASE_A');

qry.SQL.Add('DELETE FROM TEST1');
qry.ExecSQL;
qry.SQL.Clear;
end;

if(callType = 'B') then
begin

//casting qry as a TADOConnection object :

qry := TADOConnection.Create(nil);
qry := commonObj.GetConnection('DATABASE_B');

qry.SQL.Add('DELETE FROM TEST2');
qry.ExecSQL;
qry.SQL.Clear;
end;

ADrive, it may be possible that you have not fully understood just how Delphi typecasts work. First thing first - though not wrong, it does strike me as odd that you should assign the object you want to typecast as a TObject - I would have used a raw pointer. Secondly, what you have done is not really a typecast at all. I provide a few examples below which might point you in the right direction.

• Imagine that we have a TMaskEdit component on a form
• You want to use the OnClick event handler for this component
  []]The empty template Delphi gives you will go like this

Here we done the typecast as TMaskEdit(Sender). The with block ensures that the properties MaxLength and Text of your TMaskEdit control can be correctly accessed.

Here is another way to do the same thing


Here we dispense with the explicit typecast. Instead we declare a local TMaskEdit variable which sits at the same memory location as the Sender parameter. Note that you have not created a new TMaskEdit object - you are simply referencing the Sender object in a different way.

Now look at something closer to your own example


I am not entirely clear just what you are trying to do but I hope this helps you make sense of Delphi typecasts.

There are a number of miscellaneous points to emphasize

• Code such as this should normally go inside a try...finally block and you must ensure that the object you created is freed up once you are done.
• You can reference multiple objects in a with block. For instance

When you have such code Delphi scoping rules come into play. The first three lines of code raise no issues. The fourth one, Clear, is rather more complicated. Remember that both the TOraQuery and the TStringList classes have a Clear method. So which of the two objects will get cleared? Given the order in which we have specified them in the with block it will be qry. This can lead to very hard to trace bugs so be careful when writing such statements!

thanks explainthat, its really very informative.

but what i really wanted to do was to be able to use one variable to represent different types of database connection, one was TADOConnection, another TOraQuery, both sharing same qry variable.

this is because they both have some similiar method names and thus i could probably reduce my code lines by eliminating the need to do alot of conditional statements and switch between different variable types.

Like I explained, you should be able to do that with a combination of a typecast and a with statement as I have done.


Post your actual code - not working if that happens to be the case - if this does not make complete sense and I'll correct it for you.

Actually you can _cast_ any object to any class, but you cannot _assign_ an ancestor-object to a variable of its descendant class.

Therefore you cannot assign TStrings to the TStringList, only vice-versa.


I cannot see a single casting in your code. Casting looks like something this:




I think TOraQuery and TADOConnection don't have the same ancestor that would have properties like ExecSQL and SQL. So you must use the approach suggested by ExplainThat.



This is not casting. This is merely creating an instance of the TOraQuery and than losing that reference immedietely afterwards (which is a memory leak).



Unfortunately, those similar method names cannot be exploited, you'll have to duplicate your code, or use an alternative connection component.

Hope I've shed some light.

thanks guys, you were really helpful!