I have a function that returns a handle to an instantiated object. Something like:
function handle = GetHandle()
handle = SomeHandleClass();
end
I would like to be able to use the return value like I would if I was writing a program in C:
foo = GetHandle().property;
However, I get an error from MATLAB when it tries to parse that:
??? Undefined variable "GetHandle" or class "GetHandle".
The only way I can get this to work without an error is to use a temporary variable as an intermediate step:
handle = GetHandle();
foo = handle.property;
Is there a simple and elegant solution to this, or is this simply impossible with MATLAB's syntax?
To define static properties, you can use the CONSTANT keyword (thanks, #Nzbuu)
Here's one example from MathWorks (with some errors fixed):
classdef NamedConst
properties (Constant)
R = pi/180;
D = 1/NamedConst.R;
AccCode = '0145968740001110202NPQ';
RN = rand(5);
end
end
Constant properties are accessed as className.propertyName, e.g. NamedConst.R. The values of the properties are set whenever the class is loaded for the first time (after the start of Matlab, or after clear classes). Thus, NamedConst.RN will remain constant throughout a session as long as you don't call clear classes.
Hmm, I don't like to disagree with Jonas and his 21.7k points, but I think you can do this using the hgsetget handle class instead of the normal handle class, and then using the get function.
function handle = GetHandle()
handle = employee();
end
classdef employee < hgsetget
properties
Name = ''
end
methods
function e = employee()
e.Name = 'Ghaul';
end
end
end
Then you can use the get function to get the property:
foo = get(GetHandle,'Name')
foo =
Ghaul
EDIT: It is not excactly like C, but pretty close.
The only way to have a static property in MATLAB is as a constant:
classdef someHandleClass < handle
properties (Constant)
myProperty = 3
end
end
then someHandleClass.myProperty will return 3.
Related
I have a method that accepts a string as a parameter.
The string I need to pass is the property of an instantiated object.
I don't need the object to stick around once I get the value of that property.
I know I can do this like so:
Dim x As New myClass1
foo.thing1 = MyMethod(x.Name)
x = New MyClass2
foo.thing2 = MyMethod(x.Name)
'etc...
But I would prefer to do this inline if possible, since I have to do this several times in a row with different MyClass types.
EDIT:
Figured it out:
foo.thing = MyMethod(new MyClass().Name)
Try
foo.thing1 = MyMethod((New myClass1).Name)
foo.thing2 = MyMethod((New MyClass2).Name)
You need the braces around New myClass1, otherwise VB thinks you are trying to create an object of type myClass1.Name, which, of course, does not exist.
foo.thing = MyMethod(new MyClass().Name)
I have a MATLAB class that contains a method that employs a persistent variable. When certain conditions are met I need to clear the persistent variable without clearing the object to which the method belongs. I've been able to do this, but only by employing clear functions which has excessively broad scope for my purposes.
The classdef .m file for this problem:
classdef testMe
properties
keepMe
end
methods
function obj = hasPersistent(obj)
persistent foo
if isempty(foo)
foo = 1;
disp(['set foo: ' num2str(foo)]);
return
end
foo = foo + 1;
disp(['increment foo: ' num2str(foo)]);
end
function obj = resetFoo(obj)
%%%%%%%%%%%%%%%%%%%%%%%%%
% this is unacceptably broad
clear functions
%%%%%%%%%%%%%%%%%%%%%%%%%
obj = obj.hasPersistent;
end
end
end
A script that employs this class:
test = testMe();
test.keepMe = 'Don''t clear me bro';
test = test.hasPersistent;
test = test.hasPersistent;
test = test.hasPersistent;
%% Need to clear the persistent variable foo without clearing test.keepMe
test = test.resetFoo;
%%
test = test.hasPersistent;
test
The output from this is:
>> testFooClear
set foo: 1
increment foo: 2
increment foo: 3
increment foo: 4
set foo: 1
test =
testMe
Properties:
keepMe: 'Don't clear me bro'
Methods
which is the desired output. The problem is that the line clear functions in the classdef file clears all functions in memory. I need a way to clear with a much smaller scope. For example, if hasPersistent' was a function instead of a method, the appropriately scoped clear statement would beclear hasPersistent`.
I know that clear obj.hasPersistent and clear testMe.hasPersistent both fail to clear the persistent variable. clear obj is similarly a bad idea.
Following the discussion in the comments, I think you want to use make foo a private property, accompanied with appropriate increment/reset public functions.
You definitely don't need a persistent variable to implement what you want. But, in any case, to remove a persistent variable from a class method you have to clear the corresponding class. In your case, clear testMe should do what you want.
A related issue is how to clear a persistent variable in a package function. To remove persistent variable myVar from function foo within package foo_pkg you have to do this:
clear +foo_pkg/foo
This should work as long as the parent folder of folder +foo_pkg is in the MATLAB path.
I'm trying to grab a method handle from within an object in MATLAB, yet something in the sort of str2func('obj.MethodName') is not working
The answer is to get a function handle as #Pablo has shown.
Note that your class should be derived from the handle class for this to work correctly (so that the object is passed by reference).
Consider the following example:
Hello.m
classdef hello < handle
properties
name = '';
end
methods
function this = hello()
this.name = 'world';
end
function say(this)
fprintf('Hello %s!\n', this.name);
end
end
end
Now we get a handle to the member function, and use it:
obj = hello(); %# create object
f = #obj.say; %# get handle to function
obj.name = 'there'; %# change object state
obj.say()
f()
The output:
Hello there!
Hello there!
However if we define it as a Value Class instead (change first line to classdef hello), the output would be different:
Hello there!
Hello world!
One could also write
fstr = 'say';
obj.(fstr)();
This has the advantage that it does not require a handle class to work if the object (obj) is modified.
Use #. The following code works for me:
f = #obj.MethodName
No other answer mimics str2func('obj.MethodName'). Actually, this one doesn't either, not exactly. But you can define an auxillary function like so:
function handle = method_handle(obj, mstr)
handle = #(varargin) obj.(mstr)(varargin{:});
end
Then method_handle(obj, 'MethodName') returns a handle to obj.MethodName. Unfortunately, you cannot pass the variable name of obj as a string - eval("obj") will be undefined in the function's scope.
Given a structure, is there a way to create a class in MATLAB? Take for instance
>> p = struct(); p.x = 0; p.y = 0;
>> p
p =
x: 0
y: 0
>> name = 'Point'
name =
Point
What I would like to do, is given a string containing the name of the class and a struct with containing the fields I would like to create a class without having to write a file explicitly writing the definition.
Right now if we use class(p) we will obtain struct. What I want to do is create an object of the type Point so that when I do class(obj) then I get Point.
Any ideas how to accomplish this besides writing a file in MATLAB with the class definition and then executing it?
Either you have specific functionality (methods) associated with the Point class as opposed to e.g. the Line class, in which case you should write out the classes by hand, anyway, or you can make a single dynamicprops class that can have dynamically created properties, and unless you really need to call a method named class, you much simplify your life by calling classname instead.
classdef myDynamicClass < dynamicprops
properties (Hidden)
myClass %# this stores the class name
end
methods
function obj = myDynamicClass(myClassName,varargin)
%# synopsis: obj = myDynamicClass(myClassName,propertyName,propertyValue,...)
%# myClassName is the name of the class that is returned by 'classname(obj)'
%# propertyName/propertyValue define the dynamic properties
obj.myClass = myClassName;
for i=1:2:length(varargin)
addprop(obj,varargin{i})
obj.(varargin{i}) = varargin{i+1};
end
end
function out = classname(obj)
out = obj.myClass;
end
end
end
I don't know of any way of creating objects dynamically, so I'd say the answer to your question is no. However, to solve your problem, I would propose something very similar to what Mikhail said:
Work with a struct with fields x, y and classname:
p.x=0;
p.y=0;
p.classname='Point';
and then write a function myclass(x) which returns x.classname. If for some reason you need to use class() you could even overload it with your own function which checks if x is one of your special structs and calls builtin('class', x) otherwise:
function out=class(varargin)
if nargin==1 && isstruct(varargin{1}) ... #check if we were given a struct
&& isfield(varargin{1}, 'classname') ... #...which contains a field classname
&& ischar(varargin{1}.classname) %# ... which is a string
out=varargin{1}.classname; %# ok, our special case :-)
else
out=builtin('class',varargin{:}); %# normal case - call builtin class()
end
One solution that I've used in the past is to write a MATLAB function that takes this information (i.e. the class name and fields) and writes an M-file containing the required classdef construct.
This works well if you're using this information to describe a prototype that you want to expand later.
Consider the following Matlab (2009a) classes:
classdef BigDeal < handle
properties
hugeness = magic(2000);
end
methods
function self = BigDeal()
end
end
end
classdef BigProxy < handle
properties(Dependent)
hugeness
end
properties(Access = private)
bigDeal
end
methods
function self = BigProxy(bigDeal)
self.bigDeal = bigDeal;
end
function value = get.hugeness(self)
value = self.bigDeal.hugeness;
end
end
end
Now consider the following use of them:
Setup:
>> b = BigDeal
b =
BigDeal handle
Properties:
hugeness: [10x10 double]
Methods, Events, Superclasses
OneLayer:
>> pb = BigProxy(b)
pb =
BigProxy handle
Properties:
hugeness: [10x10 double]
Methods, Events, Superclasses
TwoLayers:
>> ppb = BigProxy(pb)
ppb =
BigProxy handle with no properties.
Methods, Events, Superclasses
Question: Why is my double layered proxy unable to see the hugeness when the single layer can? Dependent properties can be calculated - but does this go only one layer deep for some reason?
Update: See my answer below for workaround.
The problem here is two-fold:
The BigProxy object constructor is designed to accept an input object that has a (non-dependent) hugeness property (like a BigDeal object), which the BigProxy object can then access to compute the value for its own dependent hugeness property.
You are passing a BigProxy object to the BigProxy constructor when you create ppb, and you apparently can't compute a dependent property from another dependent property. For example, this is the error that is thrown when I try to access ppb.hugeness:
??? In class 'BigProxy', the get method for Dependent property 'hugeness'
attempts to access the stored property value. Dependent properties don't
store a value and can't be accessed from their get method.
In other words, the outer BigProxy object tries to compute the value for its dependent hugeness property by accessing the stored value of hugeness for the inner BigProxy object, but there is no stored value since it's a dependent property.
I think the fix for a situation like this would be for the BigProxy constructor to check the type of its input argument to make sure it is a BigDeal object, and throw an error otherwise.
Gnovice provided a good answer to the question "why," thus I awarded him the green check of glory. However, for those wanting a work around, you can do something like this:
classdef BigDeal < handle
properties
hugeness = magic(10000);
end
methods
function self = BigDeal()
end
end
end
classdef BigProxy < handle
properties(Dependent)
hugeness
end
properties(Access = private)
bigDeal
end
methods
function self = BigProxy(bigDeal)
self.bigDeal = bigDeal;
end
function value = get.hugeness(self)
value = self.getHugeness;
end
end
methods(Access = private)
function value = getHugeness(self)
if isa(self.bigDeal,'BigProxy')
value = self.bigDeal.getHugeness;
else
value = self.bigDeal.hugeness;
end
end
end
end
which will allow you to do the following
>> b = BigDeal;
>> pb = BigProxy(b);
>> ppb = BigProxy(pb);
And each of {b,pb,ppb} will all have the same public methods and properties. The only drawback is that I have had to (unnecessarily IMHO) clutter up BigProxy with a new private getter.