Check if a nested field exists in a structure OR nested property in an object

I'm working with nested structures with different fields that may hold objects with properties, and any mix of such datatypes. I noticed there are several questions about finding whether nested fields in a struct exist, but couldn't find an answer for finding whether nested properties of objects exist. For example, I have an object with a property, that holds another object with another property, itself being a struct with fields:

obj2.prop2 = struct('field','value');
obj1.prop1 = obj2;

Does "obj1.prop1.prop2.field" exist?

I wrote something that may be useful:

function tf = ischild(o, varargin)
    if isempty(varargin), tf = true; return; end
    if (numel(varargin)==1) && contains(varargin{1},'.')
        varargin = strsplit(varargin{1},'.');
    end
    
    child = varargin{1};
    if (isobject(o) && isprop(o,child)) || (isstruct(o) && isfield(o,child))
        tf = ischild(o.(child),varargin{2:end});
    else
        tf = false;
    end
end

You can use it in two different ways:

ischild(obj1,'prop1.prop2.field')
ischild(obj1,'prop1','prop2','field')

The surprising thing, I noticed, is that MATLAB's "getfield()" works well for nested structs but not for objects, while "setfield()" works for nested objects/struct mix. So at least before calling "getfield" or "setfield", you could check if it exists with "ischild()". Hope this helps.

Answer 1

[Shivam]

Hi @Roys,

I understand that you want to know how to access fields within a nested structure. It appears that you have implemented the "ischild" function to validate the existence of a field within an object.

You can follow the below workaround to resolve the issue.

1. Create two classes, namely “inner” and “outer” where “outer” has the property that holds an object of the “inner” class.

    classdef outer
        properties
            innerObj
        end
        methods
            function obj = outer(field, value)
                obj.innerObj = inner(field, value);
            end
            function innerObjPropFromOuterObj(obj)
                disp(['obj.innerObj.innerProp.field', obj.innerObj.innerProp.field]) % target value
            end
        end
    end
 
    classdef inner
        properties
            innerProp 
        end
        methods
            function obj = inner(field, value)
                obj.innerProp = struct(field, value);  
            end
        end
    end

2. create an object of "outer" class.

    obj = outer("field","value");

Additionally, your implementation and workflow involving the “ischild”, “getfield” and “setfield” functions also appear to be correct.

I hope this helps.