I am attempting to combine an enumeration class with non-static methods in Matlab. I wish to create a 'LogEvent' class, which has the function 'log' which takes as an input argument an enumeration member (such as LogEvent.INFO, LogEvent.ERROR, or LogEvent.WARNING) and a string, for the purpose of appending this message in a file. I wish to use this LogEvent class repeatedly, for different programs, and as such the class has the property 'fileName' which is specified during construction and refers to the output file. Below is the code forming my classdef file:
classdef LogEvent
%Class definition for logging events.
properties
fileName;
end
methods
function obj = LogEvent(outFile)
obj.fileName = outFile;
end
function log(obj,type,logStr)
switch (type)
case LogEvent.INFO
typeStr = 'INFO';
case LogEvent.WARNING
typeStr = 'WARNING';
case LogEvent.ERROR
typeStr = 'ERROR';
end
FID = fopen(obj.fileName,'a');
Str = sprintf('%s - %s: %s\n',datestr(now),typeStr,logStr);
fprintf(FID,Str);
fclose(FID);
end
end
enumeration
INFO,
WARNING,
ERROR
end
end
Now admittedly I don't have a lot of experience programming so I may be approaching this the completely wrong way, though I have tried googling this problem but with little result - I may not know some particular keywords which would 'hit the nail on the head'. It is my belief though because multiple instances of this class need to be created (to refer to different files), the 'log' function needs to be non-static. I get this error message attempting to create an instance of this class though:
Error using LogEvent
While creating an instance of class 'LogEvent':
No value has been provided for the enumeration member named 'INFO'. For an
enumeration derived from a built-in class, a value must be provided for each
enumeration member.
Error in ZOHB (line 10)
obj.Log = LogEvent('ZOHB.log');
Inside the 'ZOHB' class, I attempt to create an instance of the LogEvent class, and assign it as a property of the ZOHB class.
In Matlab's enumeration scheme, the enumerated values must be instances of the class containing the enum. So e.g. WARNING would have to a certain LogEventinstance.
E.g. like in this example from the docs:
classdef Bearing < uint32
enumeration
North (0)
East (90)
South (180)
West (270)
end
end
Which means in your case, you'd have to specify arguments which would fit your LogEvent-constructor - this is what the error message says, basically.
Which is of course totally nonsense in your use-case.
In your special case, you'd better make ERROR, WARNING and INFO constant properties:
properties (Constant)
INFO = 1;
WARNING = 2;
ERROR = 3;
end
You can access constants in a static manner, so your remaining code should pretty much work with this version.
Related
I'm trying to learn how to use classes in Matlab, having never used them in any language before, so apologies if this is a bit basic.
I have defined a class called car, with the properties of colour, type, serial number and speed, with a function to change the speed.
classdef car <handle
properties
colour
type
speed
end
properties (SetAccess = private)
SerialNumber
end
methods
function faster(obj, v)
obj.speed = obj.speed + v;
end
end
end
In another script I can type
car.colour = "red", and when I disp(car), the class has the property colour with label "red". When I call faster(100) however, instead of setting car.speed=100, it throws the error
Check for missing argument or incorrect argument data type in call to function 'faster'.
I built the class and method using the same sort of code structure as in this question:
https://www.mathworks.com/matlabcentral/answers/395472-how-to-call-a-method-from-a-class-called-a-within-another-method-from-a
where the user seemed to not have the issue I do. I'm not sure where I'm going wrong - my function seems like it should work. Can anybody point me in the right direction?
You need to call the function on the class
myCar = car();
myCar.faster( 10 ); % equivalent to 'faster( myCar, 10 )'
If you hadn't specified the < handle type, you would also need to assign it back to the class, i.e.
myCar = myCar.faster( 10 );
But you don't need this with a handle class.
Constant properties are static properties(belongs to classes, not instances) in Matlab, like many other OOP languages. And natural way to access them is ClassName.PropName as in Matlab documentation.
However, I couldn't find a way to do ClassName.PropName from a superclass, in a scenario like this:
classdef (Abstract) Superclass < handle
properties(Dependent)
dependentProperty
end
properties (Abstract, Constant)
constantProperty
end
methods
function result = get.dependentProperty(obj)
c = class(obj); % Here I have to get class of obj
result = length(c.constantProperty); % to use here as `ClassName.PropName`
end
end
end
classdef Subclass < Superclass
properties (Constant)
constantProperty = [cellstr('a'); cellstr('b')];
end
end
so that following commands results following outputs this:(expected output)
>> subclassInstance = Subclass()
subclassInstance =
Subclass with properties:
constantProperty: {2×1 cell}
dependentProperty: 2
>> subclassInstance.dependentProperty
ans =
2
>>
But instead, I get following this:(actual output)
>> subclassInstance = Subclass()
subclassInstance =
Subclass with properties:
constantProperty: {2×1 cell}
>> subclassInstance.dependentProperty
Struct contents reference from a non-struct array object.
Error in Superclass/get.dependentProperty (line 13)
result = length(c.constantProperty);
>>
Also tried: c = metaclass(obj) which gives "No appropriate method, property, or field 'constantProperty' for
class 'meta.class'."
Question: Is there any way to obtain class of an object from superclass, to be able write a statement like ClassName.PropName?
EDIT:
I know I can reach from object reference like this:
function result = get.dependentProperty(obj)
result = length(obj.constantProperty);
end
But this is not what I want as it makes reader to think constantProperty is an instance property. Also this is not documented in Matlab, instead documentation says ClassName.PropName and this makes me think that there must be a way.
The right way to do this in matlab is through the instance, as per the part of my previous answer you have now incorporated in your question. This is because matlab's object-orientation model is "instance" based.
The constant property is an instance property; it just happens to be the same (i.e. constant) in all instances. Presumably, this is why it's called "constant", not "static": it does not refer to a single static item in memory, like in c; instead every instance is instantiated with that same constant value.
You gain nothing by going out of your way to call it via a "class reference" (no such thing exists btw; unlike python and julia, class prototypes are not objects that can be referred to, nor do they have a type themselves).
However, if you insist, there does happen to be a way to do this using metaclasses, since a constant property set from within the constructor will have a default value named in its metaclass profile
subclassInstance = Subclass();
m = metaclass(subclassInstance);
mp = findobj (m.PropertyList, 'Name', 'constantProperty');
mp.DefaultValue
Also, to address why class(subclassInstance).constantProperty doesn't work, this is simply because the result of class(subclassInstance) is a string (whose value happens to be the classname), not a "reference" to a class (like I said, such a thing doesn't exist in matlab).
However, if you wanted to, obviously you could use such a classname string within an eval statement, to evaluate it as if you were typing it directly in the terminal to access the constant property. So this is another way of achieving what you're after:
eval([class(subclassInstance) '.constantProperty'])
but in theory eval statements should generally be avoided unless there's no alternative.
Short note:
in Java this is possible by this.getClass()
In java this is called reflection, and it's java's own mechanism for 'inspecting' objects. When you do something like myObject.getClass(), what you're returning is still not a "reference to a class prototype". It's an instance of type Class. I.e. even in java, you can't do myObject.getClass().aStaticProperty. But you can use the getFields method provided by the Class class to obtain Field objects, and inspect their value with respect to specific object instances; for static fields, this instance simply becomes the null object.
Is there a way to define static member variables in MATLAB classes?
This doesn't work:
classdef A
properties ( Static )
m = 0;
end
end
It suggests to use keyword "Constant" instead of "Static", the constant properties cannot be modified. I want a variable common to all objects of class A and I want to be able to modify that variable in methods of class A.
So what I need is a private static member variable. Is there a way to obtain it in MATLAB?
Found out that a workaround can be done using persistent variables in static member functions.
In this case you should inherit all your classes from a base class like the following.
classdef object < handle
properties ( GetAccess = 'public', SetAccess = 'private' )
id
end
methods ( Access = 'protected' )
function obj = object()
obj.id = object.increment();
end
end
methods ( Static, Access = 'private' )
function result = increment()
persistent stamp;
if isempty( stamp )
stamp = 0;
end
stamp = stamp + uint32(1);
result = stamp;
end
end
end
You can not, it is by design. You should use a persistent variable (technique from the MATLAB as 1980 applied in year 2011)!
For completeness I should mention that actually there is as of 2010b an undocumented and probably not longer supported static property modifier.
For background see here the answer of Dave Foti, MATLAB OO group manager:
In MATLAB, classes can define Constant
properties, but not "static"
properties in the sense of other
languages like C++. There were beta
releases that experimented with
"Static" properties and the
undocumented attribute remains from
then. However, the Static attribute is
undocumented, should not be used, and
will likely be removed in a future
MATLAB release. R2008a implements it
as a synonym for Constant and provides
no additional functionality beyond
the documented behavior of Constant
properties.
Constant properties may not be changed
from the initial value specified in
the property declaration. There are a
couple of reasons why MATLAB works
the way it does. First, MATLAB has
longstanding rules that variables
always take precedent over the names
of functions and classes and that
assignment statements introduce a
variable if one doesn't already exist.
Thus, any expression of the form "A.B
= C" will introduce a new variable A that is a struct array containing a
field B whose value is C. If "A.B = C"
could refer to a static property of
class A, then class A would take
precedent over variable A and this
would be a very significant
incompatibility with prior releases
of MATLAB. It would mean that an
m-file containing the assignment
statement "A.B = C" could have its
meaning changed by the introduction
of a class named A somewhere on the
MATLAB path. MATLAB programmers have
always been able to rely on assignment
statements introducing variables that
shadow any other use of the same name.
Second, we have observed that static
data is rarely used in other classes
except as private data within the
class or as public constants. For
example, a survey of several Java
class libraries found that all public
static fields were also final. In
MATLAB, Constant properties can be
used like "public final static"
fields in Java. For data internal to a
class, MATLAB already has persistent
variables that can be created inside
of private or protected methods or
local functions privately used by a
class. There are also good reasons to
avoid static data in MATLAB where
possible. If a class has static data,
it can be difficult to use the same
class in multiple applications
because the static data can be a
source of conflicts among
applications. In some other languages,
this is less of an issue because
different applications are separately
compiled into executables running in
different processes with different
copies of class static data. In
MATLAB, frequently many different
applications may be running in the
same process and environment with a
single copy of each class.
Here's a direct way to create a static property in Matlab. The only difference between this implementation and a hypothetical (but impossible; see Mikhail's answer) true static property is the syntax for setting the member variable.
classdef StaticVarClass
methods (Static = true)
function val = staticVar(newval)
persistent currentval;
if nargin >= 1
currentval = newval;
end
val = currentval;
end
end
end
Now the static property staticVar can be read via:
StaticVarClass.staticVar
...and be set via:
StaticVarClass.staticVar(newval);
So, for instance, this is the expected output from a test of this functionality:
>> StaticVarClass.staticVar
ans =
[]
>> StaticVarClass.staticVar('foobar')
ans =
foobar
>> StaticVarClass.staticVar
ans =
foobar
>>
This approach works just as well for private static properties like you requested, but the demo code is a little longer. Note that this is not a handle class (though it would work perfectly well on a handle class as well).
classdef StaticVarClass
methods (Access = private, Static = true)
function val = staticVar(newval)
persistent currentval;
if nargin >= 1
currentval = newval;
end
val = currentval;
end
end
methods
function this = setStatic(this, newval)
StaticVarClass.staticVar(newval);
end
function v = getStatic(this)
v = StaticVarClass.staticVar;
end
end
end
...and the test:
>> x = StaticVarClass
x =
StaticVarClass with no properties.
Methods
>> x.getStatic
ans =
[]
>> x.setStatic('foobar')
ans =
StaticVarClass with no properties.
Methods
>> x.getStatic
ans =
foobar
>>
(just to inform)
there is (another?) way to create static-like data in matlab
suppose that you have a "handle" class which its name is "car"
if you want the car class to have static data, you could construct another handle class and use it in car class throw composition, the latter class works as a static data for car class
classdef car<handle
properties
static_data:STATIC_DATA_HOLDER;
end
end
classdef STATIC_DATA_HOLDER<handle
properties
data
end
end
this way when you create first instance of a car class, an instance of STATIC_DATA_HOLDER will be created and when you create second instance of car class it uses previously created STATIC_DATA_HOLDER class.
these code tested with "MATLAB 2013b"
Another workaround to get something like static properties is to use the fact that initialisation code for member variables is only executed once when the class file is loaded. That means, if you have a definition like
classdef foo
properties
stuff = some_function()
end
end
then some_function is invoked only once, and if it returns an object of class type, this will be shared by all instances. I've added a sample implementation that shows how that can be used:
classdef ClassWithStaticMembers
properties
classvars = StaticVarContainer('foo', 0, 'bar', 2);
othervar
end
methods
function obj=ClassWithStaticMembers(var)
obj.othervar = var;
end
end
end
classdef StaticVarContainer < dynamicprops
methods
function obj=StaticVarContainer(varargin)
for i=1:2:numel(varargin)
obj.addprop(varargin{i});
obj.(varargin{i}) = varargin{i+1};
end
end
end
end
If you run this sample code
obj1 = ClassWithStaticMembers(3);
obj2 = ClassWithStaticMembers(5);
obj1.classvars.foo = [2,3];
obj1.othervar
obj1.classvars
obj2.othervar
obj2.classvars
you'll see, that classvars is indeed shared. I think this solution is much nicer than using persistent variables in functions, since you can reuse the StaticVarContainer as often as you want, it's easier to use, and furthermore, you directly see the initialisation of the static variables in the properties section.
To get the result, that is desired in the OP's question (i.e. implementing an object counter) the shared property can be made Constant, so that it can be referenced without an instance at hand:
classdef ClassWithCounter
properties (Constant)
static = StaticVarContainer('counter', 0);
end
methods
function obj=ClassWithCounter()
obj.static.counter = obj.static.counter + 1;
end
end
end
clear all
obj1 = ClassWithCounter();
obj2 = ClassWithCounter();
obj3 = ClassWithCounter();
ClassWithCounter.static.counter
Note, that the Constant attribute only means that, e.g. obj1.static cannot be changed, but it does not affect obj1.static.counter which is not constant, and can be set to heart's desire.
I come from a Java background. I am having issues with classes in Matlab particularly getters and setters. getting a message saying conflict between handle and value class I'm a little lost with what to do so any help for lack of a better word will be helpful.
classdef Person
properties(Access = private)
name;
age;
end
methods
% class constructor
function obj = Person(age,name)
obj.age = age;
obj.name = name;
end
%getters
function name = get.name(obj)
end
function age = get.age(obj)
end
%setters
function value = set.name(obj,name)
end
function value = set.age(obj,age)
end
end
end
Implementation
Since your class is currently a subclass of the default Value class, your setters need to return the modified object:
function obj = set.name(obj,name)
end
function obj = set.age(obj,age)
end
From the documention: "If you pass [a value class] to a function, the function must return the modified object." And in particular: "In value classes, methods ... that modify the object must return a modified object to copy over the existing object variable".
Handle classes (classdef Person < handle) do not need to return the modified object (like returning void):
function [] = set.name(obj,name)
end
function [] = set.age(obj,age)
end
Value vs. Handle
Going a bit deeper, the difference between a Value class and a Handle class lies mostly in assignment:
Assigning a Value class instance to a variable creates a copy of that class.
Assigning a Handle class instance to a variable create a reference (alias) to that instance.
The Mathworks has a good rundown on this topic.
To paraphrase their illustration, the behavior of a Value class is
% p is an instance of Polynomial
p = Polynomial();
% p2 is also an instance of Polynomial with p's state at assignment
p2 = p;
and of a Handle class is
% db is an instance of Database
db = Database();
% db2 is a reference to the db instance
db2 = db;
Quick'n Dirty from the Java perspective:
- "handle" classes are what your mind is set to. proper object instances with pointers to them. use them.
- "value" classes are always returning a full clone of whatever object (which has been modified by what you just did, e.g. setting a name).
the reason they have both in Matlab is that in Matlab you would expect the "value" behaviour natively. Imagine you have a matrix A = [1 2; 3 4], then assign that via B = A. if you now set B(1) = -1 you'd hope that A(1) is still 1, right? this is because matlab keeps track of "copies" and truly creates them as you modify different variables initially set to the same matrix. in OOP you'd have A(1)=-1 now as everythings an object reference.
furthermore, "native" matlab routines dont have a "this/self/me" variable that contains the instance reference to access from within functions. instead, the convention is that the class instance will be prepended to the function's argument list.
so for a function call myclass.mymethod(arg1,arg1), the declaration must be
function mymethod(this, arg1, arg2)
% Note that the name you choose for "this" is arbitrary!
end
mind you, this is the java-perspective (and also my favourite one), the above function call is equivalent to mymethod(myclass,arg1,arg1). this is more native to matlab-style, but somehow makes it harder to see you're calling an objects method.
now, regarding setters/getters: for handle classes, everything feels java-ish now:
classdef MyClass < handle
properties
MyProp;
end
methods
function set.MyProp(this, value) %Note: setMyProp is also valid!
... % do checks etc, trigger calls,
this.MyProp = value;
end
function value = get.MyProp(this)
... % notify, update, triggers etc
value = this.MyProp;
end
end
Of course it goes without saying that you dont need to define a getter if you just want to return the value, i.e. myclassinstance.MyProp will work without any just as well.
Finally, getters/setters for value classes are something that [never encountered me/i never needed] in my 7 years of matlab oop, so my advise would be to go with handle classes and enjoy happy matlab coding :-)
otherwise, the above explanation & official matlab docs is doing the job for value class getter/setters.
Is there a way to define static member variables in MATLAB classes?
This doesn't work:
classdef A
properties ( Static )
m = 0;
end
end
It suggests to use keyword "Constant" instead of "Static", the constant properties cannot be modified. I want a variable common to all objects of class A and I want to be able to modify that variable in methods of class A.
So what I need is a private static member variable. Is there a way to obtain it in MATLAB?
Found out that a workaround can be done using persistent variables in static member functions.
In this case you should inherit all your classes from a base class like the following.
classdef object < handle
properties ( GetAccess = 'public', SetAccess = 'private' )
id
end
methods ( Access = 'protected' )
function obj = object()
obj.id = object.increment();
end
end
methods ( Static, Access = 'private' )
function result = increment()
persistent stamp;
if isempty( stamp )
stamp = 0;
end
stamp = stamp + uint32(1);
result = stamp;
end
end
end
You can not, it is by design. You should use a persistent variable (technique from the MATLAB as 1980 applied in year 2011)!
For completeness I should mention that actually there is as of 2010b an undocumented and probably not longer supported static property modifier.
For background see here the answer of Dave Foti, MATLAB OO group manager:
In MATLAB, classes can define Constant
properties, but not "static"
properties in the sense of other
languages like C++. There were beta
releases that experimented with
"Static" properties and the
undocumented attribute remains from
then. However, the Static attribute is
undocumented, should not be used, and
will likely be removed in a future
MATLAB release. R2008a implements it
as a synonym for Constant and provides
no additional functionality beyond
the documented behavior of Constant
properties.
Constant properties may not be changed
from the initial value specified in
the property declaration. There are a
couple of reasons why MATLAB works
the way it does. First, MATLAB has
longstanding rules that variables
always take precedent over the names
of functions and classes and that
assignment statements introduce a
variable if one doesn't already exist.
Thus, any expression of the form "A.B
= C" will introduce a new variable A that is a struct array containing a
field B whose value is C. If "A.B = C"
could refer to a static property of
class A, then class A would take
precedent over variable A and this
would be a very significant
incompatibility with prior releases
of MATLAB. It would mean that an
m-file containing the assignment
statement "A.B = C" could have its
meaning changed by the introduction
of a class named A somewhere on the
MATLAB path. MATLAB programmers have
always been able to rely on assignment
statements introducing variables that
shadow any other use of the same name.
Second, we have observed that static
data is rarely used in other classes
except as private data within the
class or as public constants. For
example, a survey of several Java
class libraries found that all public
static fields were also final. In
MATLAB, Constant properties can be
used like "public final static"
fields in Java. For data internal to a
class, MATLAB already has persistent
variables that can be created inside
of private or protected methods or
local functions privately used by a
class. There are also good reasons to
avoid static data in MATLAB where
possible. If a class has static data,
it can be difficult to use the same
class in multiple applications
because the static data can be a
source of conflicts among
applications. In some other languages,
this is less of an issue because
different applications are separately
compiled into executables running in
different processes with different
copies of class static data. In
MATLAB, frequently many different
applications may be running in the
same process and environment with a
single copy of each class.
Here's a direct way to create a static property in Matlab. The only difference between this implementation and a hypothetical (but impossible; see Mikhail's answer) true static property is the syntax for setting the member variable.
classdef StaticVarClass
methods (Static = true)
function val = staticVar(newval)
persistent currentval;
if nargin >= 1
currentval = newval;
end
val = currentval;
end
end
end
Now the static property staticVar can be read via:
StaticVarClass.staticVar
...and be set via:
StaticVarClass.staticVar(newval);
So, for instance, this is the expected output from a test of this functionality:
>> StaticVarClass.staticVar
ans =
[]
>> StaticVarClass.staticVar('foobar')
ans =
foobar
>> StaticVarClass.staticVar
ans =
foobar
>>
This approach works just as well for private static properties like you requested, but the demo code is a little longer. Note that this is not a handle class (though it would work perfectly well on a handle class as well).
classdef StaticVarClass
methods (Access = private, Static = true)
function val = staticVar(newval)
persistent currentval;
if nargin >= 1
currentval = newval;
end
val = currentval;
end
end
methods
function this = setStatic(this, newval)
StaticVarClass.staticVar(newval);
end
function v = getStatic(this)
v = StaticVarClass.staticVar;
end
end
end
...and the test:
>> x = StaticVarClass
x =
StaticVarClass with no properties.
Methods
>> x.getStatic
ans =
[]
>> x.setStatic('foobar')
ans =
StaticVarClass with no properties.
Methods
>> x.getStatic
ans =
foobar
>>
(just to inform)
there is (another?) way to create static-like data in matlab
suppose that you have a "handle" class which its name is "car"
if you want the car class to have static data, you could construct another handle class and use it in car class throw composition, the latter class works as a static data for car class
classdef car<handle
properties
static_data:STATIC_DATA_HOLDER;
end
end
classdef STATIC_DATA_HOLDER<handle
properties
data
end
end
this way when you create first instance of a car class, an instance of STATIC_DATA_HOLDER will be created and when you create second instance of car class it uses previously created STATIC_DATA_HOLDER class.
these code tested with "MATLAB 2013b"
Another workaround to get something like static properties is to use the fact that initialisation code for member variables is only executed once when the class file is loaded. That means, if you have a definition like
classdef foo
properties
stuff = some_function()
end
end
then some_function is invoked only once, and if it returns an object of class type, this will be shared by all instances. I've added a sample implementation that shows how that can be used:
classdef ClassWithStaticMembers
properties
classvars = StaticVarContainer('foo', 0, 'bar', 2);
othervar
end
methods
function obj=ClassWithStaticMembers(var)
obj.othervar = var;
end
end
end
classdef StaticVarContainer < dynamicprops
methods
function obj=StaticVarContainer(varargin)
for i=1:2:numel(varargin)
obj.addprop(varargin{i});
obj.(varargin{i}) = varargin{i+1};
end
end
end
end
If you run this sample code
obj1 = ClassWithStaticMembers(3);
obj2 = ClassWithStaticMembers(5);
obj1.classvars.foo = [2,3];
obj1.othervar
obj1.classvars
obj2.othervar
obj2.classvars
you'll see, that classvars is indeed shared. I think this solution is much nicer than using persistent variables in functions, since you can reuse the StaticVarContainer as often as you want, it's easier to use, and furthermore, you directly see the initialisation of the static variables in the properties section.
To get the result, that is desired in the OP's question (i.e. implementing an object counter) the shared property can be made Constant, so that it can be referenced without an instance at hand:
classdef ClassWithCounter
properties (Constant)
static = StaticVarContainer('counter', 0);
end
methods
function obj=ClassWithCounter()
obj.static.counter = obj.static.counter + 1;
end
end
end
clear all
obj1 = ClassWithCounter();
obj2 = ClassWithCounter();
obj3 = ClassWithCounter();
ClassWithCounter.static.counter
Note, that the Constant attribute only means that, e.g. obj1.static cannot be changed, but it does not affect obj1.static.counter which is not constant, and can be set to heart's desire.