In a script I want to create an object of a class only if all the constructor inputs meet some conditions. All the checks are embedded in the constructor itself. Problem is when:
myObj = myClass(input1,input2,...);
is triggered in the script. The object, even when an input doesn't meet the relative condition, is still created eventually with all empty properties.
Inserting the following code within the constructor:
if nargout == 0
clear obj;
end
prevents the creation of the object, but only when the output is assigned to the ans variable. Otherwise, I get an error.
Is there a way to obtain something like that without adding some code directly into the script (like using try)?
If the constructor throws an error, no object will be created. Use error. You can put a try/catch block around the constructor call if you like.
There is no way AFAIK to have the constructor return normally but produce no object. What would be assigned to its output?
Related
Within a subclass constructor, what is the difference between calling obj#SuperClass(a,b); and obj = obj#SuperClass(a,b);
Both are found in doc, for ex. here
The canonical syntax for calling a superclass constructor is
obj = obj#SuperClass(args)
(see the documentation).
This is comparable to calling any constructor, where the output of the function call is assigned to a variable:
obj = SuperClass(args)
However, because the superclass constructor must initialize fields inside a (larger) derived object, the obj to be modified must be passed in some way to the function call, hence the (awkward) obj# syntax.
But because we pass the object to be initialized, which is modified, we don’t really need to capture that output any more. Hence the other form,
obj#SuperClass(args)
does exactly the same things in all situations I have encountered.
There is no difference that I can see. I would be surprised if the first syntax did any data copying whatsoever, that has most certainly been optimized out, just like obj = obj.method(args) doesn’t copy the object.
I tried to change an attribute value of a class by invoking one of its member function:
p1 = tank();
p1.checkOri(p1);
And in the class definition, I have that static method:
classdef tank
properties
value
...
end
methods
...
methods (Static)
function obj = checkOri(obj)
if (CONDITION) %the thing I want it to do
obj.value = EXPRESSION;
...
Yet this checkOri method is not working. But if I write this method in the main file, or to say altering the value of p1-the instance of class tank-it works perfectly:
p1 = tank();
p1.checkOri(p1);
if (CONDITION) %the thing I want it to do
p1.value = EXPRESSION;
It works perfectly.
I wonder what caused this. From my experience with other programming languages, invoking method should have worked, is it because of some tricks with Matlab syntax or static method? How could I fix it so that this method would work?
So, as #Navan in the comment said, handle class could be a solution.
It appears Matlab has a similar parameter concept with Java and C++, arguments modified in a function/method only remains that modification inside the function/method.
For this class, I simply added < handle in the head of class definition and it worked:
classdef tank < handle
properties
...
But I am not sure if that is the only solution, there might be better ways to do this. So I'll leave that question open, you are more than welcomed to post your opinion:D
In MATLAB, the call
p1.checkOri();
is equivalent to
checkOri(p1);
In both cases, the class method checkOri is called for the class of object p1, passing p1 as first argument to the function by value.
Because p1 is passed by value, any changes made to it inside the function are not seen by the object in the calling workspace. Therefore, one typically does
p1 = checkOri(p1);
This way, the object that was passed by value and modified inside the function is passed back out and assigned to the variable that held the original object.
If the method is written as follows:
function obj = checkOri(obj)
%...
end
then MATLAB will optimize the above function call such that no copy of the object is actually made. Note that both in the function declaration and in the function call, the input and output variable is the same.
As already discovered by OP, the above does not hold for handle classes, classes that inherit from handle. These classes act as if they are always passed by reference, and any change made to them in any workspace will be reflected in all other copies in other workspaces.
Also assigning to a member variable does not follow the above, such that
p1.value = 0;
modifies object p1.
For more information on the difference between value classes and handle classes see this other question.
After evolving my project code for months, I've finally hit a need to define a new class. Having to romp through my previous class definitions as a refresher of the conventions, I noticed that all constructors and property setters all have an output argument, even though nothing is assigned to it, e.g.:
function o = myConstructor( arg1, arg2, ... )
function o = set.SomeProperty( o, arg1 )
I've been looking through the documentation for upward of an hour without finding the explanation for this. It doesn't look like it depends on whether a function is defined in the class definition file or in its own separate m-file.
Can anyone please explain?
The best place to start is the documentation "Comparison of Handle and Value Classes". From the very top:
A value class constructor returns an object that is associated with the variable to which it is assigned. If you reassign this variable, MATLAB® creates an independent copy of the original object. If you pass this variable to a function to modify it, the function must return the modified object as an output argument.
A handle class constructor returns a handle object that is a reference to the object created. You can assign the handle object to multiple variables or pass it to functions without causing MATLAB to make a copy of the original object. A function that modifies a handle object passed as an input argument does not need to return the object.
In other words, value classes need to return a modified object (which is a new object distinct from the original), while handle classes don't. The constructor of either class will always have to return an object, since it is actually constructing it.
Some good additional reading is "Which Kind of Class to Use", which links to a couple helpful examples of each type of class object. Looking at the DocPolynom value class example, you can see that property set methods have to return the modified object, while the dlnode handle class example only requires an output for its constructor. Note that you could still return an object from a handle class method (if desired), but it's not required.
In netbeans I use to call a method that returns a value, but I am directly calling it where I have to pass a parameter to the function i.e. Function(getValue()) where getValuue() returns String. So what I want to know is that what is more efficient way to call this method whether should I assign value to a string first and then pass that value to parameter, as netbeans suggests me and shows a warning there, or calling it directly is good? I know code runs fine but keeping in mind the efficiency or rules of coding should I consider this thing or not? Or how badly can it effect if I ignore it?
If you are only using that value once, then calling it directly where it is used as a parameter is fine.
In Java, this is fine:
MyClass myClass = new MyClass();
myFunction(myClass.getSomeValue());
Whereas in the following case:
MyClass myClass = new MyClass();
MyOtherClass myOtherClass = myClass.someLongComputation();
Int value = myFunction(myOtherClass);
anotherFunction(value, myOtherClass);
It may be better to have a local variable so that you avoid calling a long running calculation twice. However, for simple getValue()s, it really doesn't matter.
I'm using Eclipse JDT AST to parse a given java source code. While parsing the code, when it hits a method invocation, I want to find out whether that particular method returns or sets a value of an instance variable (basically to find out whether the callee method is a getter/setter of the same class of caller method).
E.g.:
public void test(){
//when parsing the following line I want to check whether "getName"
//returns a value of an instance variable.
String x = getName();
//when parsing the following line I want to check whether "setName"
//sets the value of an instance variable.
setName("some-name");
}
I've used the AST plugin also find out a possible path which would help me to refer it from the API, but couldn't.
Please let me know whether this is possible and if so, which approach that would help me to get the required information.
Don't think that there is an api which tells you whether a method is a getter or a setter.
You will have to write code to do this. For a getter, you can probably simply check if the last statement in the method is a return statement which returns an instance variable.