How to set a default value to a map container in argument validation?

I'm validating a bunch of arguments in class properties. Seems quite straightfoward to set a deatault for double, integer and string datatypes.

classdef newClass < handle
    
    properties
        someStr (1,:) string = "myStr"
        someNum (1,1) double = 3.1415
    end

But what is the right way to do this for a map container? Because when I add this line for a map container:

    someDict containers.Map

I see the following warning:

Answer 1

[Steven Lord]

Because each function call is independent I'd expect you wouldn't have the same issue with a handle object in a function's arguments block that you would with a class's properties block. Indeed, this little experiment supports my expectation. If the default value for dict was only created once the second call to the function should have listed both oldMissingValue and myKey as keys of the dict variable.

listOfKeys = example1936099_arg(42, "myKey")

listOfKeys = 1×1 cell array
    {'myKey'}

listOfKeys = example1936099_arg(-999, "oldMissingValue")

listOfKeys = 1×1 cell array
    {'oldMissingValue'}

listOfKeys = example1936099_arg()

listOfKeys = 1×1 cell array
    {'myStr'}

function listOfKeys = example1936099_arg(value, name, dict)
arguments
    value (1, 1) double = pi;
    name (1, :) string = "myStr";
    dict containers.Map = containers.Map;
end
dict(name) = value;
listOfKeys = keys(dict);
end

[Anup Kanale]

Thank you Steven, this answers my question! If you could move this to Answer, I'll accept it.

Answer 2

[Walter Roberson]

test = containers.Map

test = 

  Map with properties:

        Count: 0
      KeyType: char
    ValueType: any

isa(test, 'handle')

ans = logical
   1

Therefore if you were to pass in a container.Map to the constructor and copy what was received into a property, then the container would end up being shared with any other uses of the container.

To get around this, to get a private container.Map, you need to create the container.Map in the constructor (and make the property private and do not create any methods that can allow people to get at the container.Map). Or you could somehow clone the incoming container.Map (the class does not provide a method for copying / cloning though.)

If you want the container.Map to be shared then mark the property as Constant .

[Anup Kanale]

Thanks Walter! That does clear up my understanding of the warning.

But I'm still not sure about assigning a default value. I'm looking for a syntax similar to strings and doubles, something along the lines of:

someDict containers.Map = containers.Map()

[Walter Roberson]

Are you talking about setting an initial value for the property, or are you talking about default values in a arguments block for the case where the parameter is not passed to the constructor ?

[Anup Kanale]

I"m referring to the latter- setting a default value when a map container is not passed in the function call.

[Walter Roberson]

I am not clear whether you want to deliberately share the container with the place that supplied the container, or if you want a private copy of the container ?

[Anup Kanale]

I want a private copy of the container. But my bad, your and @Steven Lord's answers made me realize that I am probably mixing up two different issues in my head! The first one is shared vs private copy of the container.

The second issue, and what I originally meant to ask, was how do I go about setting a default value for a container in the argument validation block (it makes sense in my script to move the container downstream from class property to a method)

function returnVar=myFunc(someNum, someStr, someDict)
    arguments
        someNum (1,1) double = 3.1415
        someStr (1,:) string = "myStr"
        someDict containers.Map = %?__default_value__?
    end 
end

PS: For the future viewer's benefit, please let me know if I should modify this question in place, or start a new thread instead.

[Walter Roberson]

Sorry, I do not have experience with using arguments blocks.

Answer 3

[Steven Lord]

Consider the two classes attached to this answer. The class class1936099_A defines someDict to contain a containers.Map in the properties section of the class definition. The class class1936099_B assigns someDict a containers.Map in the constructor.

obj1 = class1936099_A;
obj2 = class1936099_A;

Neither obj1 nor obj2 have 'a' as a key in their someDict property.

[isKey(obj1.someDict, 'a'), isKey(obj2.someDict, 'a')]

ans = 1×2 logical array
   0   0

Let's associate 'a' with 42 in obj1's property. What does obj2's property associate 'a' with now?

obj1.someDict('a') = 42;
obj2.someDict('a')

ans = 42

Let's try the same with class1936099_B.

obj3 = class1936099_B;
obj4 = class1936099_B;
[isKey(obj3.someDict, 'a'), isKey(obj4.someDict, 'a')]

ans = 1×2 logical array
   0   0

obj3.someDict('a') = 42;
obj4.someDict('a')

Error using indexing
The specified key is not present in this container.

obj1 and obj2 have handles to the same containers.Map object. obj3 and obj4 have handles to different objects.