global variables used in members of class properties

classdef myclass
    properties
        x=X
    end
    methods
    end
end

X is a global variable. I want x to start with a default value of X. Later x will change. What should I do to claim that X is a global variable? I know by using a function in methods, one can construct myclass with a start value of x assigned as a global variable (X putting in this function), but I wonder if there's a way of not using a function.

Answer 1

[Guillaume]

Global variables are usually indication of bad design. Avoid them as much as possible.

In your case, using a global variable to initialise the default property of a class would be an extremely bad idea, fraught with peril. If you're not aware of when and how default values are actually computed, abandon that idea immediately.

When you write:

classdef myclass
    properties
        x = X;
    end
end

X is evaluated only once, when the class is first loaded in memory. If you were to change the global X after that, your default value would stay the same (unless you clear classes). If X has yet to be created when the class is loaded in memory (which may be well before you actually use it), you will get an obscure error as well.

Essential reading:

• Property Default Values, in particular the note there
• How matlab evaluates expressions , which tells you that because the expression x=X is evaluated outside of any workspace.

Saying all that, if you're hell bent on a bad design:

classdef myclass
    properties
        x = getGlobalX;   %Don't go there, you may be eaten by a grue
    end
end

%local function
function x = getGlobalX
    global X;
    x = X;
end

may work, although I haven't actually tested.

edit: Perhaps I should have asked: why are you coming up with this design? There's probably a much better way of achieving whatever you're trying to do which doesn't involve global.

[Stephen23]

I think a grue must have just walked past... something made me shiver.

[a a]

@ Guillaume

Thank you. My real purpose is in a game design, a soldier (class soldier) will, when first created, be endowed with an initial number of bullets x=X, which is a global/constant value. As time goes on, the number of bullets x will decrease. Any better idea?

[Guillaume]

If that initial value is indeed a constant, then just hardcode that constant in the class definition.

classdef myclass
    properties
        x = 10;
    end
end

If that value is not a constant, then pass it to the class constructor:

classdef myclass
    properties
        x;
    end
    methods
        function this = myclass(X)  %constructor
            this.x = X;
        end
    end
end

These are the only safe options. As I've said, due to the way matlab initialise default property values, initialising with a global will lead to unexpected behaviour and lots of puzzling when debugging. For example, changing the global value wouldn't affect the initial value of the class until somebody somewhere happened to issue a clear all or clear classes. At which point, suddenly the x value for all objects (even the ones created previously) may change.

[a a]

@ Guillaume

By doing x=10 in the properties, it works. But it'll be nicer to be able to tune the '10' as a parameter in the main program rather than in the class program, though it's global and constant within 1 game/simulation. Before running every game/simulation, I do 'clear;clc'.

If there's no way around the difficulty, it looks a bit odd, as in functions global variables are definable at will.

[Guillaume]

Really, the safest and simplest is to pass that initial to the constructor. Where you get that initial value is then completely independent of the class. You can use a global if you wish. Again,

classdef myclass
    properties
        x;
    end
    methods
        function this = myclass(X)  %constructor
            this.x = X;
        end
    end
end

%your main program
global X;
%...
obj = myclass(X);  %create object of myclass using global X

If you really want to embed the global in the class, then

classdef myclass
    properties
        x;
    end
    methods
        function this = myclass
            global X;
            this.x = X;           
        end
    end
end

I would really recommend that you use the first option.

[a a]

Thank you. I prefer this answer:

classdef myclass
    properties
        x;
    end
    methods
        function this = myclass
            global X;
            this.x = X;           
        end
    end
end

Though it's not as beautiful as putting the global X directly into properties. But that's how the software was designed.