I've recently been working with someone else's code and I realized that this individual has a very different philosophy regarding private variables and method parameters than I do. I generally feel that private variables should only be used in a case when:
The variable needs to be stored for recall later.
The data stored in the variable is used globally in the class.
When the variable needs to be globally manipulated (something decidedly different from the need to read the variable by every class method).
When it will make programming substantially easier. (Admittedly vague, but one has to be in many circumstances to avoid painting oneself into a corner).
(I admit, that many of the above are slightly repetitive, but they each seem different enough to merit such treatment... )
It just seems that this is the most efficient means of preventing changing a variable by accident. It also seems like following these standards will allow for the eventual manipulation of external references (if the class is eventually modified), thus leaving you with further options in the future. Is this simply a style issue (like one true bracket or Hungarian naming conventions), or do I have justification in this belief? Is there actually a best practice in this case?
edit
I think this needs to be corrected. I used "globally" above where I actually meant, "globally by instance methods" not "globally accessible by anything, anywhere".
edit2
An example was asked for:
class foo
{
private $_my_private_variable;
public function __constructor__()
{
}
public function useFoo( $variable )
{
// This is the line I am wondering about,
// there does not seem to be a need for storing it.
$this->_my_private_variable = $variable;
$this->_doSometing();
}
private function _doSomething()
{
/*
do something with $this->_my_private_variable.
*/
// This is the only place _my_private_variable is used.
echo $this->_my_private_variable;
}
}
This is the way I would have done it:
class foo
{
public function __constructor__()
{
}
public function useFoo( $variable )
{
$this->_doSometing( $variable );
}
private function _doSomething( $passed_variable )
{
/*
do something with the parameter.
*/
echo $passed_variable;
}
}
In general, class members should represent state of the class object.
They are not temporary locations for method parameters (that's what method parameters are for).
I claim that it isn't a style issue but rather a readability/maintainability issue. One variable should have one use, and one use only. “Recycling” variables for different purposes just because they happen to require the same type doesn't make any sense.
From your description it sounds as if the other person's code you worked on does exactly this, since all other uses are basically covered by your list. Put simply, it uses private member variables to act as temporaries depending on situation. Am I right to assume this? If so, the code is horrible.
The smaller the lexical scope and lifetime of any given variable, the less possiblity of erroneous use and the better for resource disposal.
Having a member variable implies that it will be holding state that needs to be held between method calls. If the value doesn't need to live between calls it has no reason to exist outside of the scope of a single call, and thus (if it exists at all) should be a variable within the method itself.
Style is always a hard one, once you develop one you can get stuck in a bit of a rut and it can be difficult to see why what you do may not be the best way.
You should only create variables when and where they are needed, and dispose of them when you are done. If the class doesn't need a class level variable to function, then it just doesn't need one. Creating variables where you don't need them is very bad practice.
Class members should be any of the following:
A dependency of a class
A variable that represents the state of the class
A method of the class
I think the answer is straightforward if you are familiar with C++ destructors. All member variables should be assigned a way to be destructed while function parameters are not. So that's why member variables are usually the states or dependicies of an object having some kind of relation regarding their lifecycle.
I'm not sure there is a stated best-practice for using globally scoped variables versus always passing as method parameters. (By "private variables", I'm assuming you mean globally scoped variables.)
Using a globally scoped variable is the only way to implement properties in .NET (even automatic properties ultimately use a globally scoped variable, just not one you have to declare yourself).
There is a line of arguement for always using method parameters because it makes it completely clear where the value is coming from. I don't think it really helps prevent the method from making changes to the underlying value and it can, in my opinion, make things more difficult to read at times.
I would disagree with implementing it for global access or to make programming easier. By exposing these globally without filtering of any kind make it more difficult to determine access in the future.
Since object properties are meant to hold state, as stated by the others, my policy is to have all of them private by default unless I have a good reason to expose them.
It's much easier to make them public later on, if you have to, simply by writing a getter method for example (which i also don't have to think about right at the beginning of writing a class). But reeling in a public property later on may require a huge amount of code to be re-written.
I like to keep it flexible while not having to think about this more than needed.
Related
This is one of those questions that has been niggling around in the back of my mind since day one of programming in VB in general. I have my own personal ideas about it, but I am wondering what the general consensus is on the use of static variables.
Static Variable:
Public Function Whatever() as Boolean
Static Flag as Boolean
If not Flag then
' do something
Flag = True
end if
Return Something
End sub
VS:
Private Variable:
Private Flag as Boolean
Public Function Whatever() as Boolean
If not Flag then
' do something
Flag = True
end if
Return Something
End sub
Unless someone knows otherwise, the above are functionally equivalent, other than the fact that the "Private" Flag is exposed to be used elsewhere in the class.
Questions begin to arise with Statics.. like..
Where are they stored.. when are they REALLY created and disposed etc.
Obviously, the compiler adds them to the data heap (I know, bad usage of THAT word) for the class somehow... But is there a penalty for that in terms of overhead, garbage collection etc.
Generally I avoid them like the plague because they present more questions than answers.
Is there really any SOLID reason to ever use statics?
PS: Hopefully this passes the SO question test...
Note I am not asking specifics about how statics are created.. I am more asking what if ANYTHING would make using a static worth it.
ADDENDUM....
I did a little more research and found this rather enlightening.
https://weblogs.asp.net/psteele/7717
Is there really any SOLID reason to ever use statics?
Yes. It’s arguably in the first letter of SOLID: S for “single responsibility principle”. In this particular context it’s a slightly different rule:
Objects should have the smallest possible scope.
If an object isn’t needed outside scope X, it should be declared inside scope X. This ensures that it has a single responsibility, and isn’t improperly accessed elsewhere. It also ensures that only one method has the responsibility of accessing this object.
Hence, in your case, the best idea would indeed to make the variable a function-static (= local) variable rather than an object-private variable.
This is indeed fairly uncommon in my experience. But it is best practice.
(In terms of performance/memory these two variants would be exactly identical.)
So from research I have come up with the following.
Static variables are "Lazy" variables, in that they are created when required the first time the function or routine is called. There is an overhead associated with the creation, and more importantly, first initialization in order to make it thread safe.
As Konrad suggests the main reason to use statics is to minimize the scope of the variable to where it is needed.
So... when required
Use SIMPLE static variables (Booleans, Integers etc.) in functions and subroutines that are less commonly used. If you expect the users of the class to ALWAYS call that function, and expect there to be 1,000s of instances of the class... you are adding A LOT of overhead.
Never use static variables of object types that need to be disposed unless you add some functionality (paramaters) to the routine that allow you to call it in a Dispose mode and remember to do so in the classes Dispose function.
Similarily, do not use a static variables as a reference to some other object in your project. Doing so can prevent said object from being garbage collected elsewhere in your code since it is still referenced.
Finally, if you use the Private variable method instead, give it a name that makes it obvious that the variable is being used for that function.. e.g. stat_Half_Time_Score
I am from a C# background and have been doing programming for quite some time now. But only recently i started giving some thoughts on how i program. Apparently, my OOP is very bad.
I have a few questions maybe someone can help me out. They are basic but i want to confirm.
1- In C#, we can declare class properties like
private int _test;
and there setter getters like
public int Test {get; set;}
Now, lets say i have to use this property inside the class. Which one will i use ? the private one or the public one ? or they both are the same ?
2- Lets say that i have to implement a class that does XML Parsing. There can be different things that we can use as input for the class like "FILE PATH". Should i make this a class PROPERTY or should i just pass it as an argument to a public function in the class ? Which approach is better. Check the following
I can create a class property and use like this
public string FilePath {get; set;}
public int Parse()
{
var document = XDocument.Load(this.FilePath);
.........//Remaining code
}
Or
I can pass the filepath as a parameter
public int Parse(string filePath)
On what basis should i make a decision that i should make a property or i should pass something as argument ?
I know the solutions of these questions but i want to know the correct approach. If you can recommend some video lectures or books that will be nice also.
Fields vs Properties
Seems like you've got a few terms confused.
private int _test;
This is an instance field (also called member).
This field will allow direct access to the value from inside the class.
Note that I said "inside the class". Because it is private, it is not accessible from outside the class. This is important to preserve encapsulation, a cornerstone of OOP. Encapsulation basically tells us that instance members can't be accessed directly outside the class.
For this reason we make the member private and provide methods that "set" and "get" the variable (at least: in Java this is the way). These methods are exposed to the outside world and force whoever is using your class to go trough your methods instead of accessing your variable directly.
It should be noted that you also want to use your methods/properties when you're inside the current class. Each time you don't, you risk bypassing validation rules. Play it safe and always use the methods instead of the backing field.
The netto result from this is that you can force your logic to be applied to changes (set) or retrieval (get). The best example is validation: by forcing people to use your method, your validation logic will be applied before (possibly) setting a field to a new value.
public int Test {get; set;}
This is an automatically implemented property. A property is crudely spoken an easier way of using get/set methods.
Behind the scenes, your code translates to
private int _somevariableyoudontknow;
public void setTest(int t){
this._somevariableyoudontknow = t;
}
public int getTest(){
return this._somevariableyoudontknow;
}
So it is really very much alike to getters and setters. What's so nice about properties is that you can define on one line the things you'd do in 7 lines, while still maintaining all the possibilities from explicit getters and setters.
Where is my validation logic, you ask?
In order to add validation logic, you have to create a custom implemented property.
The syntax looks like this:
private int _iChoseThisName;
public int Test {
get {
return _iChoseThisName;
}
set {
if(value > 5) { return _iChoseThisName; }
throw new ArgumentException("Value must be over 5!");
}
}
Basically all we did was provide an implementation for your get and set. Notice the value keyword!
Properties can be used as such:
var result = SomeClass.Test; // returns the value from the 'Test' property
SomeClass.Test = 10; // sets the value of the 'Test' property
Last small note: just because you have a property named Test, does not mean the backing variable is named test or _test. The compiler will generate a variablename for you that serves as the backing field in a manner that you will never have duplication.
XML Parsing
If you want your second answer answered, you're going to have to show how your current architecture looks.
It shouldn't be necessary though: it makes most sense to pass it as a parameter with your constructor. You should just create a new XmlParser (random name) object for each file you want to parse. Once you're parsing, you don't want to change the file location.
If you do want this: create a method that does the parsing and let it take the filename as a parameter, that way you still keep it in one call.
You don't want to create a property for the simple reason that you might forget to both set the property and call the parse method.
There are really two questions wrapped in your first question.
1) Should I use getters and setters (Accessors and Mutators) to access a member variable.
The answer depends on whether the implementation of the variable is likely to change. In some cases, the interface type (the type returned by the getter, and set by the setter) needs to be kept consistent but the underlying mechanism for storing the data may change. For instance, the type of the property may be a String but in fact the data is stored in a portion of a much larger String and the getter extracts that portion of the String and returns it to the user.
2) What visibility should I give a property?
Visibility is entirely dependent on use. If the property needs to be accessible to other classes or to classes that inherit from the base class then the property needs to be public or protected.
I never expose implementation to external concerns. Which is to say I always put a getter and setter on public and protected data because it helps me ensure that I will keep the interface the same even if the underlying implementation changes. Another common issue with external changes is that I want a chance to intercept an outside user's attempt to modify a property, maybe to prevent it, but more likely to keep the objects state in a good or safe state. This is especially important for cached values that may be exposed as properties. Think of a property that sums the contents of an array of values. You don't want to recalculate the value every time it is referenced so you need to be certain that the setter for the elements in the array tells the object that the sum needs to be recalculated. This way you keep the calculation to a minimum.
I think the second question is: When do I make a value that I could pass in to a constructor public?
It depends on what the value is used for. I generally think that there are two distinct types of variables passed in to constructors. Those that assist in the creation of the object (your XML file path is a good example of this) and those that are passed in because the object is going to be responsible for their management. An example of this is in collections which you can often initialize the collection with an array.
I follow these guidelines.
If the value passed in can be changed without damaging the state of the object then it can be made into a property and publicly visible.
If changing the value passed in will damage the state of the object or redefine its identity then it should be left to the constructor to initialize the state and not be accesible again through property methods.
A lot of these terms are confusing because of the many different paradigms and languages in OO Design. The best place to learn about good practices in OO Design is to start with a good book on Patterns. While the so-called Gang of Four Book http://en.wikipedia.org/wiki/Design_Patterns was the standard for many years, there have since been many better books written.
Here are a couple resources on Design Patterns:
http://sourcemaking.com/design_patterns
http://www.oodesign.com/
And a couple on C# specific.
http://msdn.microsoft.com/en-us/magazine/cc301852.aspx
http://www.codeproject.com/Articles/572738/Building-an-application-using-design-patterns-and
I can possibly answer your first question. You asked "I have to use this property inside the class." That sounds to me like you need to use your private variable. The public method which you provided I believe will only do two things: Allow a client to set one of your private variables, or to allow a client to "see" (get) the private variable. But if you want to "use this property inside the class", the private variable is the one that should be your focus while working with the data within the class. Happy holidays :)
The following is my personal opinion based on my personal experience in various programming languages. I do not think that best practices are necessarily static for all projects.
When to use getters, when to use private instance variables directly
it depends.
You probably know that, but let's talk about why we usually want getters and setters instead of public instance variables: it allows us to aquire the full power of OOP.
While an instance variable is just some dump piece of memory (the amount of dumbness surely depends on the language you're working in), a getter is not bound to a specific memory location. The getter allows childs in the OOP hirarchy to override the behaviour of the "instance variable" without being bound to it. Thus, if you have an interface with various implementations, some may use ab instance variable, while others may use IO to fetch data from the network, calculate it from other values, etc.
Thus, getters do not necessarily return the instance variable (in some languages this is more complicated, such as c++ with the virtual keyword, but I'll try to be language-independent here).
Why is that related to the inner class behaviour? If you have a class with a non-final getter, the getter and the inner variable may return different values. Thus, if you need to be sure it is the inner value, use it directly. If you, however, rely on the "real" value, always use the getter.
If the getter is final or the language enforces the getter to be equal (and this case is way more common than the first case), I personally prefer accessing the private field directly; this makes code easy to read (imho) and does not yield any performance penalty (does not apply to all languages).
When to use parameters, when to use instance variables/properties
use parameters whereever possible.
Never use instance variables or properties as parameters. A method should be as self-contained as possible. In the example you stated, the parameterized version is way better imo.
Intance variables (with getters or not) are properties of the instance. As they are part of the instance, they should be logically bound to it.
Have a look at your example. If you hear the word XMLParser, what do you think about it? Do you think that a parser can only parse a single file it is bound to? Or do you think that a parser can parse any files? I tend to the last one (additionally, using an instance variable would additionally kill thread-safety).
Another example: You wish to create an XMLArchiver, taking multiple xml documents into a single archive. When implementing, you'd have the filename as a parameter of the constructor maybe opening an outputstream towards the file and storing a reference to it as an instance variable. Then, you'd call archiver.add(stuff-to-add) multiple times. As you see, the file (thus, the filename) is naturally bound to the XMLArchiver instance, not to the method adding files to it.
I'm a little unclear as to how far to take the idea in making all members within a class private and make public methods to handle mutations. Primitive types are not the issue, it's encapsulated object that I am unclear about. The benefit of making object members private is the ability to hide methods that do not apply to the context of class being built. The downside is that you have to provide public methods to pass parameters to the underlying object (more methods, more work). On the otherside, if you want to have all methods and properties exposed for the underlying object, couldn't you just make the object public? What are the dangers in having objects exposed this way?
For example, I would find it useful to have everything from a vector, or Array List, exposed. The only downside I can think of is that public members could potentially assigned a type that its not via implicit casting (or something to that affect). Would a volitile designation reduce the potential for problems?
Just a side note: I understand that true enapsulation implies that members are private.
What are the dangers in having objects exposed this way?
Changing the type of those objects would require changing the interface to the class. With private objects + public getters/setters, you'd only have to modify the code in the getters and setters, assuming you want to keep the things being returned the same.
Note that this is why properties are useful in languages such as Python, which technically doesn't have private class members, only obscured ones at most.
The problem with making instance variables public is that you can never change your mind later, and make them private, without breaking existing code that relies on directly public access to those instance vars. Some examples:
You decide to later make your class thread-safe by synchronizing all access to instance vars, or maybe by using a ThreadLocal to create a new copy of the value for each thread. Can't do it if any thread can directly access the variables.
Using your example of a vector or array list - at some point, you realize that there is a security flaw in your code because those classes are mutable, so somebody else can replace the contents of the list. If this were only available via an accessor method, you could easily solve the problem by making an immutable copy of the list upon request, but you can't do that with a public variable.
You realize later that one of your instance vars is redundant and can be derived based on other variables. Once again, easy if you're using accessors, impossible with public variables.
I think that it boils down to a practical point - if you know that you're the only one who will be using this code, and it pains you to write accessors (every IDE will do it for you automatically), and you don't mind changing your own code later if you decide to break the API, then go for it. But if other people will be using your class, or if you would like to make it easier to refactor later for your own use, stick with accessors.
Object oriented design is just a guideline. Think about it from the perspective of the person who will be using your class. Balance OOD with making it intuitive and easy to use.
You could run into issues depending on the language you are using and how it treats return statements or assignment operators. In some cases it may give you a reference, or values in other cases.
For example, say you have a PrimeCalculator class that figures out prime numbers, then you have another class that does something with those prime numbers.
public PrimeCalculator calculatorObject = new PrimeCalculator();
Vector<int> primeNumbers = calculatorObject.PrimeNumbersVector;
/* do something complicated here */
primeNumbers.clear(); // free up some memory
When you use this stuff later, possibly in another class, you don't want the overhead of calculating the numbers again so you use the same calculatorObject.
Vector<int> primes = calculatorObject.PrimeNumbersVector;
int tenthPrime = primes.elementAt(9);
It may not exactly be clear at this point whether primes and primeNumbers reference the same Vector. If they do, trying to get the tenth prime from primes would throw an error.
You can do it this way if you're careful and understand what exactly is happening in your situation, but you have a smaller margin of error using functions to return a value rather than assigning the variable directly.
Well you can check the post :
first this
then this
This should solve your confusion . It solved mine ! Thanks to Nicol Bolas.
Also read the comments below the accepted answer (also notice the link given in the second last comment by me ( in the first post) )
Also visit the wikipedia post
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Public Data members vs Getters, Setters
In what cases should public fields be used, instead of properties or getter and setter methods (where there is no support for properties)? Where exactly is their use recommended, and why, or, if it is not, why are they still allowed as a language feature? After all, they break the Object-Oriented principle of encapsulation where getters and setters are allowed and encouraged.
If you have a constant that needs to be public, you might as well make it a public field instead of creating a getter property for it.
Apart from that, I don't see a need, as far as good OOP principles are concerned.
They are there and allowed because sometimes you need the flexibility.
That's hard to tell, but in my opinion public fields are only valid when using structs.
struct Simple
{
public int Position;
public bool Exists;
public double LastValue;
};
But different people have different thoughts about:
http://kristofverbiest.blogspot.com/2007/02/public-fields-and-properties-are-not.html
http://blogs.msdn.com/b/ericgu/archive/2007/02/01/properties-vs-public-fields-redux.aspx
http://www.markhneedham.com/blog/2009/02/04/c-public-fields-vs-automatic-properties/
If your compiler does not optimize getter and setter invocations, the access to your properties might be more expensive than reading and writing fields (call stack). That might be relevant if you perform many, many invocations.
But, to be honest, I know no language where this is true. At least in both .NET and Java this is optimized well.
From a design point of view I know no case where using fields is recommended...
Cheers
Matthias
Let's first look at the question why we need accessors (getters/setters)? You need them to be able to override the behaviour when assigning a new value/reading a value. You might want to add caching or return a calculated value instead of a property.
Your question can now be formed as do I always want this behaviour? I can think of cases where this is not useful at all: structures (what were structs in C). Passing a parameter object or a class wrapping multiple values to be inserted into a Collection are cases where one actually does not need accessors: The object is merely a container for variables.
There is one single reason(*) why to use get instead of public field: lazy evaluation. I.e. the value you want may be stored in a database, or may be long to compute, and don't want your program to initialize it at startup, but only when needed.
There is one single reason(*) why to use set instead of public field: other fields modifications. I.e. you change the value of other fields when you the value of the target field changes.
Forcing to use get and set on every field is in contradiction with the YAGNI principle.
If you want to expose the value of a field from an object, then expose it! It is completely pointless to create an object with four independent fields and mandating that all of them uses get/set or properties access.
*: Other reasons such as possible data type change are pointless. In fact, wherever you use a = o.get_value() instead of a = o.value, if you change the type returned by get_value() you have to change at every use, just as if you would have changed the type of value.
The main reason is nothing to do with OOP encapsulation (though people often say it is), and everything to do with versioning.
Indeed from the OOP position one could argue that fields are better than "blind" properties, as a lack of encapsulation is clearer than something that pretends to encapsulation and then blows it away. If encapsulation is important, then it should be good to see when it isn't there.
A property called Foo will not be treated the same from the outside as a public field called Foo. In some languages this is explicit (the language doesn't directly support properties, so you've got a getFoo and a setFoo) and in some it is implicit (C# and VB.NET directly support properties, but they are not binary-compatible with fields and code compiled to use a field will break if it's changed to a property, and vice-versa).
If your Foo just does a "blind" set and write of an underlying field, then there is currently no encapsulation advantage to this over exposing the field.
However, if there is a later requirement to take advantage of encapsulation to prevent invalid values (you should always prevent invalid values, but maybe you didn't realise some where invalid when you first wrote the class, or maybe "valid" has changed with a scope change), to wrap memoised evaluation, to trigger other changes in the object, to trigger an on-change event, to prevent expensive needless equivalent sets, and so on, then you can't make that change without breaking running code.
If the class is internal to the component in question, this isn't a concern, and I'd say use fields if fields read sensibly under the general YAGNI principle. However, YAGNI doesn't play quite so well across component boundaries (if I did need my component to work today, I certainly am probably going to need that it works tomorrow after you've changed your component that mine depends on), so it can make sense to pre-emptively use properties.
Example:
// access fields directly
private void doThis()
{
return doSomeWork(this.data);
}
// receive data as an argument
private void doThis(data)
{
return doSomeWork(data);
}
The first option is coupled to the value in this.data while the second option avoids this coupling. I feel like the second option is always better. It promotes loose coupling WITHIN the class. Accessing global class data willy-nilly throughout just seems like a bad idea. Obviously this class data needs to be accessed directly at some point. However, if accesses, to this global class data can be eliminated by parameter passing, it seems that this is always preferable.
The second example has the advantage of working with any data of the proper type, whereas the first is bound to working with the just class data. Even if you don't NEED the additional flexibility, it seems nice to leave it as an option.
I just don't see any advantage in accessing member data directly from private methods as in the first example. Whats the best practice here? I've referenced code complete, but was not able to find anything on this particular issue.
if the data is part of the object's state, private/protected is just fine. option 1 - good.
i noticed some developers like to create private/protected vars just to pass parameters between methods in a class so that they dun have to pass them in the method call. they are not really to store the model/state of an object. ...then, option 1 - NOT good.
Why option 1 not good in this case...
expose only as much as you need (var scoping). so, pass the data in. do not create a private/protected var just to pass data between 2 methods.
private methods that figures out everything internally makes it very easy to understand. keep it this way, unless its unavoidable.
private/protected vars make it harder to refactor as your method is not 'self encompassing', it depends on external vars that might be used elsewhere.
my 2 cents! :-)
In class global data are not a problem IMHO. Classes are used to couple state, behaviour and identity. So such a coupling is not a problem. The argument suggests, that you can call that method with data from other objects, even of other classes and I think that should be more considered than coupling inside class.
They are both instance methods, therefore #1 makes more sense unless you have a situation involving threads (but depending on the language and scenario, even then you can simply lock/mark the data method as syncronized - my Java knowledge is rusty).
The second technique is more reminiscent of procedural programming.