Unexpected behavior in classdefs that inherit from handle

I have a simple class, SomeClass, that inherits from handle.

I have another simple class, SomeOtherClass, that has a properity that is a SomeClass object.

When I instantiate two SomeOtherClass and set the value of the SomeClass property then both SomeOtherClass objects receive the value. I assume this is because both SomeOtherClass objects point to the same SomeClass object. But why? The two SomeOtherClass are separate instantiations. Please help me understand. The included code demostrates what I'm talking about.

Thanks, Roger

O1 = SomeClass;
O2 = SomeClass;
O3 = SomeOtherClass;
O4 = SomeOtherClass;
fprintf(1,'\nProperty Values After Instantiation\n');

Property Values After Instantiation

worker(O1,O2,O3,O4);

O1.AProperty = 0.000000
O2.AProperty = 0.000000
O3.PropFromAbove.AProperty = 0.000000
O3.SomeOtherProp = 0.000000
O4.PropFromAbove.AProperty = 0.000000
O4.SomeOtherProp = 0.000000

O2.AProperty = pi;
fprintf(1,'\nProperty Values After Setting O2.AProperity to pi.\n');

Property Values After Setting O2.AProperity to pi.

worker(O1,O2,O3,O4);

O1.AProperty = 0.000000
O2.AProperty = 3.141593
O3.PropFromAbove.AProperty = 0.000000
O3.SomeOtherProp = 0.000000
O4.PropFromAbove.AProperty = 0.000000
O4.SomeOtherProp = 0.000000

O3.PropFromAbove.AProperty = exp(1);
fprintf(1,'\nProperty Values After Setting O3.PropFromAbove.AProperity to exp(1).\n');

Property Values After Setting O3.PropFromAbove.AProperity to exp(1).

worker(O1,O2,O3,O4);

O1.AProperty = 0.000000
O2.AProperty = 3.141593
O3.PropFromAbove.AProperty = 2.718282
O3.SomeOtherProp = 0.000000
O4.PropFromAbove.AProperty = 2.718282
O4.SomeOtherProp = 0.000000

fprintf(1,'\n!!! Why are O3.PropFromAbove and O4.PropFromAbove pointing to the same object!!!\n');

!!! Why are O3.PropFromAbove and O4.PropFromAbove pointing to the same object!!!

function worker(O1,O2,O3,O4)
parm = {'O1','O2'};
objs = {O1,O2};
for k = 1:numel(objs)
    fprintf(1,'%s.AProperty = %f\n',parm{k},objs{k}.AProperty);
end
parm = {'O3','O4'};
objs = {O3,O4};
for k = 1:numel(objs)
    fprintf(1,'%s.PropFromAbove.AProperty = %f\n',parm{k},objs{k}.PropFromAbove.AProperty);
    fprintf(1,'%s.SomeOtherProp = %f\n',parm{k},objs{k}.SomeOtherProp);
end
end

Answer 1

[Walter Roberson]

This is documented.

https://www.mathworks.com/help/matlab/matlab_oop/initialize-property-values.html#brqy3km-10

MATLAB evaluates a default expression when the property value is first needed (for example, when the class is first instantiated). The same default value is then used for all instances of a class. MATLAB does not reevaluate the default expression unless the class definition is cleared from memory.

So when you initialize a property to an expression that yields a handle, then the same handle will be used for all instances of the class.

If this is not what you want the you need to put in a constructor: constructors are run each time an object of the class is created.

[Roger Rouse]

Thank you!