The VFP ASSERT statement
Although Visual FoxPro has a good debugger, there are times when you need a little more
information about what's going on - or perhaps you need to be able to see what's going
on without the intrusive overhead of the debugger. The classic technique from the days
of DOS is to use a WAIT WINDOW command to display values
at the crucial point in the program and this is still a useful technique.
The screen shot shows a Wait window. It's not pretty and it's not part of the regular
Windows user experience but it's still useful. Having said that, it's not the sort of
thing that you want a user to see too often. You can write code to detect whether you're
developing
or running the program and only show the window in the programming environment but
the Assert mechanism is sometimes easier.
Visual FoxPro Asserts
The traditional use of an Assert in more rigorous languages such as C++ or Java is as a
guarantee that the program is indeed in the state which the programmer expects it to be
in. FoxPro too allows an Assert to be used as a tool for Quality Assurance in this way.
This example for instance
shows that the programmer expects the Customer table to be open and to be indexed in
Name order at this point;
Set Asserts On
...
Assert Used("Customer") ;
Message
"The customer table must be open."
Assert Order
("Customer") = "Name";
Message
"Table must be in Name Order."
If the customer table is not open or it's not in the right order then
there's no point in letting program execution continue. The developer needs to find out
why these conditions have not been met. Without the Assert, the program would have
continued to run until it reached the point where it needed to use the Customer table
and that might have been several hundred lines of code away, possibly in another method
or function altogether. The Assert statement means that the program will stop as soon
as we know that something is going wrong, it doesn't allow the error to propagate
across the application.
The general form of the command is the
Assert keyword followed by a boolean expression. If the
expression evaluates as false then FoxPro will pause execution and display a dialog
giving the programmer the options of opening the debugger, cancelling the program or
ignoring the failure. The example here makes use of the optional
Message clause to display some programmer-friendly text.
Without it we'd just get a generic "Assertion failed on 23 of procedure myProg."
Using Assert to debug
The key word in the previous sentence is 'programmer' because the assert mechanism does
not operate at runtime. This means that you should not be tempted to try to use
Asserts as a mechanism for validation during data entry at runtime. The assert will
work for you as a programmer in the development environment but not for the user.
An Assert is meant to be used as an aid to software quality control, to ensure that the
developer has got the program environment set up correctly. The fact that asserts don't
fire at runtime does mean that we can use Assert as a sneaky way of showing us debug
information. We can be certain that the user won't see these messages. All we have to
do is to write an Assert statement with a boolean
expression that will evaluate as false. Something as simple as this:
Set Asserts On
...
Assert .F. ;
Message
"Postcode is " + customer.postalcode
will show the debug message to the developer but not to the user. If you want to
temporarily suppress the messages in the development environment too then you can
just turn asserts off:
Set Asserts Off
Note that the default is for asserts to be off. You must have a
Set Asserts On statement if you want to use asserts at all.
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