Check if class exists

Question

Question

Ruslan K · Jul 23, 2017

#Caché

There is method:

$System.OBJ.New( ClassName As %String = "" )

If class with name ClassName exists - everything is OK.

But, when class does not exist, there is error - <CLASS DOES NOT EXIST>

How can I check before calling $System.OBJ.New() if class witn name ClassName exists?

Discussion (19)1

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Robert Cemper · Jul 23, 2017

set exist=##class(%Dictionary.CompiledClass).%ExistsId(ClassName)

will tell you the status

7 0

Robert Cemper · Jul 25, 2017

so you may use $$$comClassDefined(class) for compile classes

or go for ^oddCOM(....)

2 0

Robert Cemper · Jul 24, 2017

I'm fully with you.
Using hidden %System.whatever Classes is as bad practice as using undocumented $zu(anynumber, , , )

4 0

Robert Cemper · Jul 25, 2017

And if you also want to be sure that also the Method %New exists you my use

write $$$comMemberDefined(ClassName,"m","%New")

[just reading through %occReference.inc]

2 0

Robert Cemper · Aug 14, 2017

Pls. don't forget to mark your question as "answered" on Developer Community,
please click the checkmark alongside the answer you (as author of the question) accept

0 0

score 2 · Answer 1 · 2017-07-24T07:33:48-04:00

Though it works, it's not literal: another programmer could ask himself why you tested the method %New, thus requiring to add a comment for explaining it. However if you use ExistsId, you're literally saying about testing if the class exists.

I know, it's just a detail, but still contributes for clean code directives.

score 1 · Answer 2 · 2017-07-24T08:01:14-04:00

I'm not having much success finding docs for $system.CLS

http://docs.intersystems.com/latest/csp/docbook/DocBook.UI.Page.cls?KEY=... says:

but no mention of CLS. Nor does the %SYSTEM package contain a CLS class in the same way as it contains the ones listed above.

score 0 · Answer 3 · 2017-07-25T08:36:18-04:00

That's $$$defClassDefined(class). It shows that class definition exists (or doesn't), but it doesn't show if a class is compiled.

score 2 · Answer 4 · 2017-08-07T06:55:25-04:00

Robert Cemper · Aug 7, 2017

as suggested earlier

2 0

score 0 · Answer 5 · 2017-08-07T09:52:26-04:00

Luca, any reason for recommending %Exists and having to use $LISTBUILD when you could go direct to %ExistsId ?

score 2 · Answer 6 · 2017-07-24T07:11:37-04:00

Dmitry Maslennikov · Jul 24, 2017

Check if method %New exists with this function

$system.CLS.IsMthd(ClassName, "%New")

2 0

score 0 · Answer 7 · 2017-07-24T08:17:43-04:00

That is because the CLS Package is used internally.
InterSystems uses it to verify if the method is defined or not without throwing exception.

This is probably faster than %ExistsId, because %ExistsId tries to open the instance beforehand instead.

score 1 · Answer 8 · 2017-07-24T12:32:17-04:00

I think it even works without any COS code, just like any other system functions. And they have more than just only IsMthd.

score 0 · Answer 9 · 2017-07-24T15:13:27-04:00

You're right, %Dictionary.CompiledClass actually overwrites the current %Persistent implementation for %Exists (which is called by %ExistsId).

score 3 · Answer 10 · 2017-07-24T15:31:34-04:00

Rubens Silva · Jul 24, 2017

%occReference.inc

3 0

score 0 · Answer 11 · 2017-07-24T15:16:33-04:00

Jeffrey Drumm · Jul 24, 2017

@Eduard, where is the macro $$$comClassDefined defined?

0 0

score 2 · Answer 12 · 2017-07-24T15:00:09-04:00

%ExistsId does not open an object for %Dictionary package. Just checks the globals (see %Dictionary.CompiledClass for example).

The fastest way to check if a class exists would be:

write $$$comClassDefined(class)

score 0 · Answer 13 · 2017-07-25T08:12:45-04:00

mgstat.int uses such approach (in this case, for class %SYSTEM.CPU):

if $D(^oddDEF("%SYSTEM.CPU")) {
...
}

score 1 · Answer 14 · 2017-08-07T06:39:31-04:00

Use the official and documented API.

If pure existence or simply the class is defined (but not compiled)

SAMPLES>w ##class(%Dictionary.ClassDefinition).%Exists($lb("Aviation.Aircraft"))
1

If you're after existing and compiled:

SAMPLES>w ##class(%Dictionary.CompiledClass).%Exists($lb("Aviation.Aircraft"))
1

--

HTH