'as' operator??

[Ian Branch 121]

Hi Team,

I have never knowingly used the 'as' operator and have no idea what it is about and how to use it.  I looked at the EMB description of the operator but was none the wiser.

So,

1. can somebody point me to a clear explanation of the use of the 'as' operator, preferably with examples, and

2. I have the following code..

pptr := PaPInAddr(phe^.h_addr_list);

Which also uses something that I never caught on to as well, pointers.

Using Pascal Analyzer, it suggests I should use 'as' somewhere in there.

Could someone enlighten me where and why it may be better.  Although that insight may come from 1. above.

 

Regards & TIA,

Ian

[Lars Fosdal 1732]

Ref.1:
The as operator is a checked typecast.

Let's say that you have a method that takes a TObject as parameter aInstance.

The difference between a simple typecast and a checked typecase is that

var Unchecked := TMyClass(aInstance); 

will never complain, while 

var Unchecked := aInstance as TMyClass; 

will raise an exception if aInstance is not a TMyClass

 

Also see the is operator which can be used to validate the type before the cast.

if aInstance is TMyClass
 then (aInstance as TMyClass).SomeTMyClassMethod;

 

[Fr0sT.Brutal 897]

5 hours ago, Ian Branch said:

2. I have the following code..

pptr := PaPInAddr(phe^.h_addr_list);

Which also uses something that I never caught on to as well, pointers. 

Using Pascal Analyzer, it suggests I should use 'as' somewhere in there.

PA is wrong here. "AS" only deals with classes (you can try it yourself)

[Anders Melander 1639]

5 hours ago, Ian Branch said:

2. I have the following code..

pptr := PaPInAddr(phe^.h_addr_list);

Which also uses something that I never caught on to as well, pointers.

Using Pascal Analyzer, it suggests I should use 'as' somewhere in there.

I think Pascal Analyser is wrong in this case. If PaPInAddr is a pointer type then 'is' and 'as' can't be used since these only operate on object types (i.e. object references/pointers).