How do I check if a property exists for an empty object?

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Captain Karnage 2023-10-18

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https://ww2.mathworks.cn/matlabcentral/answers/2035434-how-do-i-check-if-a-property-exists-for-an-empty-object

回答： Captain Karnage 2023-10-18

I have a function that gets passed an object named exObj . The intent is this object can be one of a set of custom class objects that I have created. If the passed object has a property or method (sometimes I can't assign the property and have to use a static method instead) that has a specific name, there is an additional set of commands that are executed. So, I start that block of code with

if ( isprop(exObj, "flagged" ) || ismethod(exObj, "flagged") )

The problem is, the object passed COULD be an empty version of the object. In that case, as described in the help pages here: https://www.mathworks.com/help/matlab/ref/isprop.html isprop returns an empty logical array and I get the error:

Operands to the logical AND (&&) and OR (||) operators must be convertible to logical scalar values. Use the ANY or ALL functions to reduce operands to logical scalar values.

Interestingly, ismethod does not give the same error. I still want to execute the code block if that property exists - so I can't just add if ~isempty(exObj) before this statement. So, how can I check that this property exists on an empty object?

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Les Beckham 2023-10-18

I'm not an OOP expert by any means, but don't you know which classes have that property and which don't? Couldn't you check class(exObj) instead of checking for the presence of a certain property?

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