How to use Matlab's unit test framework component verifyError to verify an error from a MEX function

Let's say I've written a MEX function, foo, that when given the number 5 throws an error.

Inside the mex function, the code would look something like:

  if (inputValue == 5)
  {
    char errMsg[250];
    sprintf_s(errMsg, "ERROR: input was %d, and so I'll throw an error", inputValue);
    mexErrMsgTxt(errMsg);
  }

Now, if I call foo(5), this error will be thrown.

I cannot seem to figure out how to write a unit test to verify this!

        function Verify_Foo_Throws_Error(testCase)
           err_msg = 'ERROR';
           testCase.verifyError(@()foo(5), err_msg);            
        end % Verify_Foo_Throws_Error()

I cannot seem to substitute anything in for the 'err_msg' variable in my example to get the test to pass!

Interestingly enough, when err_msg has some string, the test throws the feedback:

        Actual Identifier:
                 ''
        Expected Identifier:
            ERROR  (<- whatever the value of the variable 'err_msg' is from my example)

Yet when I make err_msg an empty string, the test fails as incomplete with the messages:

================================================================================
Uncaught error occurred in Foo_Test/Verify_Foo_Throws_Error.
The remainder of the test method will not run to completion.

    --------------
    Error Details:
    --------------
    Error using matlab.unittest.constraints.Throws (line 161)
    Expected exception to be a row vector.

    Error in matlab.unittest.internal.qualifications.QualificationDelegate/qualifyError (line 152)
                throws = delegate.decorateConstraintAlias(Throws(errorClassOrID),'Error');

    Error in matlab.unittest.qualifications.Verifiable/verifyError (line 687)
                qualifyError(verifiable.VerificationDelegate, ...

    Error in Foo_Test/Verify_Foo_Throws_Error (line 138)
                testCase.verifyError(@()foo(5), err_msg);

================================================================================

Any assistance would be most appreciated. Thanks!

Answer 1

[Andy Campbell]

Hi Vincent,

The verifyError function requires that the error thrown is an error with an error identifier. The identifier argument described in the verifyError documentation is not the error message but is rather this error identifier.

In order to test against this error, you'll need to change the mex file's implementation to use mexErrMsgIdAndTxt instead of mexErrMsgTxt in order to throw the error with an ID that can be tested against.

Hope that helps!

Andy

[Vincent]

Thanks! Perhaps embarrassingly, I did not know this function existed!

That having been said, I am still getting the same result when I try the verifyError method.

If I put in the error ID (or the message text), I get back that an empty string was expected. If I put in an empty string, it throws the same errors.

EDIT: I actually implemented a simple foo.cpp file and the test for that worked fine. I'll have to dig around to find out why it's failing in my actual code. I'm sure I'm overlooking something simple. Thanks for the help!

EDIT2: Yes, I overlooked something incredibly simple (read: I'm stupid)! Thanks again for adding to my knowledge of the mex library!

[Andy Campbell]

Hey Vincent,

No need to be embarrassed, and we all overlook simple things from time to time. I am glad to have helped and good luck!

Andy

Answer 2

[BdS]

Hi Andy,

perhaps you can help me.

how to only test the error message? I would like to test the following message written in this function:

function [CorrectFields]=FieldValidity(fieldNames,FieldDescrip)
check=FieldDescrip;
[badFld,indBadFld]=ismember('BAD_FLD',check(:,2));
if badFld
    error('Bad field identifier entered: %s not found.',fieldNames{indBadFld})
else
    CorrectFields=fieldNames;
end
end

Thank you in advance for your help.

[Philip Borghesani]

It is best to ask a new question when you have one. However in this case the same answer applies. verifyError requires an error id so add one to the error arguments call error:

error('FieldValidity:BadIdentifier','Bad field identifier entered: %s not found.',fieldNames{indBadFld})

Then test for FieldValidity:BadIdentifier. verify error is written this way to allow the message to be translated to other languages and to avoid difficult string comparisons when the message changes like this one does.

[Paul Wintz]

In R2021a (possibly earlier) you can pass an empty string for the error ID to verifyError of errors that do not have an error ID.

[Steven Lord]

If you want to verify an error that doesn't have an error identifier or where you care that an error was thrown regardless of its identifier (or even if it has an identifier) you can use a metaclass object. For the class that represents an error (like the object you would get if you wrote a try / catch to store the caught exception in a variable) the metaclass object you want to use in the verifyError call is ?MException.

try
    y = ones(2) + ones(3);
catch ME
    classOfME = class(ME)
    idFromCaughtError = ME.identifier
end

classOfME = 'MException'

idFromCaughtError = 'MATLAB:sizeDimensionsMustMatch'

testcase = matlab.unittest.TestCase.forInteractiveUse;
verifyError(testcase, @() ones(2)+ones(3), 'MATLAB:sizeDimensionsMustMatch', ...
    'Verify that a specific error was thrown')

Verification passed.

verifyError(testcase, @() ones(2)+ones(3), ?MException, ...
    'Verify that an error was thrown, regardless of which one')

Verification passed.