Object created in a Try block???

[Ian Branch 127]

Hi Guys,

Is there anything actually wrong with creating an object in a TRY block or is it just not good practice?

Are there Pros & Cons?

I ask because FixInsight flags it with a Warning.

 

Regards & TIA,

Ian 

[Anders Melander 1783]

21 minutes ago, Ian Branch said:

Is there anything actually wrong with creating an object in a TRY block or is it just not good practice?

It depends. Do either this:

Foo := TFooBar.Create;
try
  ...
finally
  Foo.Free;
end;

or this:

Foo := nil;
try
  Foo := TFooBar.Create;
  ...
finally
  Foo.Free;
end;

 

[Ian Branch 127]

Hi Anders,

I was using the second but without the Foo := nil;

Adding the equivalent of Foo := nil; to my code eliminates the FI Warning.

I think I will use the first method to avoid any complications.

Regards & Tks again.

Ian

[David Heffernan 2345]

try
  Foo := TFooBar.Create;
  ...
finally
  Foo.Free;
end;

Consider the above. Suppose that TFooBar.Create raises an exception. In that case the finally block is executed, and `Foo.Free` is called on an uninitialised variable (Foo). That leads to undefined behaviour.

 

So the correct pattern is

 

Foo := TFooBar.Create;
try
  ...
finally
  Foo.Free;
end;

Here, if TFooBar.Create raises an exception, the try is never reached, and so the finally block never executes.

 

Now another pattern was also mentioned

 

Foo := nil;
try
  Foo := TFooBar.Create;
  ...
finally
  Foo.Free;
end;

This is also valid, because Foo is initialised before the try executes. And so in case of constructor exception we would call Free on a nil reference which is fine. However, this pattern is pointless and should be avoided in the scenario here where there is just a single object. It is useful sometimes if there are multiple objects and you want to avoid deep nesting.

 

A better example of using this pattern is if the object is created conditionally:

 

Foo := nil;
try
  if someTest then
  begin
    Foo := TFooBar.Create;
    ...
  end;
  ...
finally
  Foo.Free;
end;

 

Edited November 13, 2020 by David Heffernan