When do you use the this keyword

Question

I was curious about how other people use the this keyword  I tend to use it in constructors  but I may also use it throughout the class in other methods  Some examples   In a constructor   public Light Vector v        this dir   new Vector v       Elsewhere  public void SomeMethod         Vector vec   new Vector        double d    vec   vec  -  this radius   this radius

User · Answer

Any time you need a reference to the current object.

One particularly handy scenario is when your object is calling a function and wants to pass itself into it.

Example:

void onChange()
{
    screen.draw(this);
}

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C     this is used in the assignment operator where most of the time you have to check and prevent strange  unintentional  dangerous  or just a waste of time for the program  things like   A a  a   a    Your assignment operator will be written   A amp  A  operator  const A amp  a        if  this     amp a  return  this          we know both sides of the   operator are different  do something         return  this

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I can t believe all of the people that say using it always is a  best practice  and such   Use  this  when there is ambiguity  as in Corey s example or when you need to pass the object as a parameter  as in Ryan s example  There is no reason to use it otherwise because being able to resolve a variable based on the scope chain should be clear enough that qualifying variables with it should be unnecessary   EDIT  The C  documentation on  this  indicates one more use  besides the two I mentioned  for the  this  keyword - for declaring indexers  EDIT   Juan  Huh  I don t see any inconsistency in my statements - there are 3 instances when I would use the  this  keyword  as documented in the C  documentation   and those are times when you actually need it  Sticking  this  in front of variables in a constructor when there is no shadowing going on is simply a waste of keystrokes and a waste of my time when reading it  it provides no benefit

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1 - Common Java setter idiom    public void setFoo int foo         this foo   foo       2 - When calling a function with this object as a parameter  notifier addListener this

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this   helps find members on  this  class with a lot of members  usually due to a deep inheritance chain    Hitting CTRL Space doesn t help with this  because it also includes types  where-as  this   includes members ONLY   I usually delete it once I have what I was after  but this is just my style breaking through   In terms of style  if you are a lone-ranger -- you decide  if you work for a company stick to the company policy  look at the stuff in source control and see what other people are doing   In terms of using it to qualify members  neither is right or wrong  The only wrong thing is inconsistency -- that is the golden rule of style  Leave the nit-picking others  Spend your time pondering real coding problems -- and obviously coding -- instead

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There is one use that has not already been mentioned in C    and that is not to refer to the own object or disambiguate a member from a received variable   You can use this to convert a non-dependent name into an argument dependent name inside template classes that inherit from other templates   template  lt typename T gt  struct base      void f          template  lt typename T gt  struct derived   public base lt T gt       void test             f        1  error       base lt T gt   f       quite verbose if there is more than one argument  but valid       this- gt f       f is now an argument dependent symbol          Templates are compiled with a two pass mechanism  During the first pass  only non-argument dependent names are resolved and checked  while dependent names are checked only for coherence  without actually substituting the template arguments    At that step  without actually substituting the type  the compiler has almost no information of what base lt T gt  could be  note that specialization  of the base template can turn it into completely different types  even undefined types   so it just assumes that it is a type  At this stage the non-dependent call f that seems just natural to the programmer is a symbol that the compiler must find as a member of derived or in enclosing namespaces --which does not happen in the example-- and it will complain   The solution is turning the non-dependent name f into a dependent name  This can be done in a couple of ways  by explicitly stating the type where it is implemented  base lt T gt   f --adding the base lt T gt  makes the symbol dependent on T and the compiler will just assume that it will exist and postpones the actual check for the second pass  after argument substitution   The second way  much sorter if you inherit from templates that have more than one argument  or long names  is just adding a this- gt  before the symbol  As the template class you are implementing does depend on an argument  it inherits from base lt T gt   this- gt  is argument dependent  and we get the same result  this- gt f is checked in the second round  after template parameter substitution

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I pretty much only use this when referencing a type property from inside the same type   As another user mentioned  I also underscore local fields so they are noticeable without needing this

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There are several usages of this keyword in C     To qualify members hidden by similar name To have an object pass itself as a parameter to other methods To have an object return itself from a method To declare indexers To declare extension methods To pass parameters between constructors To internally reassign value type  struct  value  To invoke an extension method on the current instance To cast itself to another type To chain constructors defined in the same class   You can avoid the first usage by not having member and local variables with the same name in scope  for example by following common naming conventions and using properties  Pascal case  instead of fields  camel case  to avoid colliding with local variables  also camel case   In C  3 0 fields can be converted to properties easily by using auto-implemented properties

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I use it only when required  except for symmetric operations which due to single argument polymorphism have to be put into methods of one side   boolean sameValue  SomeNum other       return this importantValue    other importantValue

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I don t mean this to sound snarky  but it doesn t matter   Seriously   Look at the things that are important  your project  your code  your job  your personal life  None of them are going to have their success rest on whether or not you use the  this  keyword to qualify access to fields  The this keyword will not help you ship on time  It s not going to reduce bugs  it s not going to have any appreciable effect on code quality or maintainability  It s not going to get you a raise  or allow you to spend less time at the office    It s really just a style issue  If you like  this   then use it  If you don t  then don t  If you need it to get correct semantics then use it  The truth is  every programmer has his own unique programing style  That style reflects that particular programmer s notions of what the  most aesthetically pleasing code  should look like  By definition  any other programmer who reads your code is going to have a different programing style  That means there is always going to be something you did that the other guy doesn t like  or would have done differently  At some point some guy is going to read your code and grumble about something    I wouldn t fret over it  I would just make sure the code is as aesthetically pleasing as possible according to your own tastes  If you ask 10 programmers how to format code  you are going to get about 15 different opinions  A better thing to focus on is how the code is factored  Are things abstracted right  Did I pick meaningful names for things  Is there a lot of code duplication  Are there ways I can simplify stuff  Getting those things right  I think  will have the greatest positive impact on your project  your code  your job  and your life  Coincidentally  it will probably also cause the other guy to grumble the least  If your code works  is easy to read  and is well factored  the other guy isn t going to be scrutinizing how you initialize fields  He s just going to use your code  marvel at it s greatness  and then move on to something else

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Here s when I use it    Accessing Private Methods from within the class  to differentiate  Passing the current object to another method  or as a sender object  in case of an event  When creating extension methods  D   I don t use this for Private fields because I prefix private field variable names with an underscore

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You should always use it  I use it to diferantiate private fields and parameters  because our naming conventions state that we don t use prefixes for member and parameter names  and they are based on information found on the internet  so I consider that a best practice

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I use it whenever StyleCop tells me to  StyleCop must be obeyed  Oh yes

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I tend to underscore fields with   so don t really ever need to use this  Also R  tends to refactor them away anyway

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I only use it when absolutely necessary  ie  when another variable is shadowing another   Such as here   class Vector3       float x      float y      float z       public Vector3 float x  float y  float z                this x   x          this y   y          this z   z             Or as Ryan Fox points out  when you need to pass this as a parameter   Local variables have precedence over member variables

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It depends on the coding standard I m working under   If we are using   to denote an instance variable then  this  becomes redundant  If we are not using   then I tend to use this to denote instance variable

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When you are many developers working on the same code base  you need some code guidelines rules  Where I work we ve desided to use  this  on fields  properties and events   To me it makes good sense to do it like this  it makes the code easier to read when you differentiate between class-variables and method-variables

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In Jakub   turc s answer his  5 about passing data between contructors probably could use a little explanation  This is in overloading constructors and is the one case where use of this is mandatory  In the following example we can call the parameterized constructor from the parameterless constructor with a default parameter   class MyClass       private int  x     public MyClass     this 5         public MyClass int v     x   v       I ve found this to be a particularly useful feature on occasion

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this on a C   compiler  The C   compiler will silently lookup for a symbol if it does not find it immediately  Sometimes  most of the time  it is good    using the mother class  method if you did not overloaded it in the child class  promoting a value of a type into another type   But sometimes  You just don t want the compiler to guess  You want the compiler to pick-up the right symbol and not another   For me  those times are when  within a method  I want to access to a member method or member variable  I just don t want some random symbol picked up just because I wrote printf instead of print  this- printf would not have compiled   The point is that  with C legacy libraries       legacy code written years ago         or whatever could happen in a language where copy pasting is an obsolete but still active feature  sometimes  telling the compiler to not play wits is a great idea   These are the reasons I use this        it s still a kind of mystery to me  but I now wonder if the fact you include the  lt windows h gt  header in your source  is the reason all the legacy C libraries symbols will pollute your global namespace         realizing that  you need to include a header  but that including this header will break  your code because it uses some dumb macro with a generic name  is one of those russian roulette moments of a coder s life

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You should not use  this  unless you absolutely must   There IS a penalty associated with unnecessary verbosity  You should strive for code that is exactly as long as it needs to be  and no longer

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I use it every time I refer to an instance variable  even if I don t need to  I think it makes the code more clear

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Never  Ever  If you have variable shadowing  your naming conventions are on crack  I mean  really  no distinguishing naming for member variables  Facepalm

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I use it when  in a function that accepts a reference to an object of the same type  I want to make it perfectly clear which object I m referring to  where   For example  class AABB            members   bool intersects  AABB other           return other left    lt  this- gt right    amp  amp             this- gt left    lt  other right    amp  amp                  y increases going down            other top    lt  this- gt bottom    amp  amp             this- gt top    lt  other bottom                vs   class AABB     bool intersects  AABB other           return other left    lt  right    amp  amp             left    lt  other right    amp  amp                  y increases going down            other top    lt  bottom    amp  amp             top    lt  other bottom               At a glance which AABB does right   refer to   The this adds a bit of a clarifier

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Personally  I try to always use this when referring to member variables   It helps clarify the code and make it more readable   Even if there is no ambiguity  someone reading through my code for the first time doesn t know that  but if they see this used consistently  they will know if they are looking at a member variable or not

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C     I agree with the  use it when you have to  brigade  Decorating code unnecessarily with this isn t a great idea because the compiler won t warn you when you forget to do it  This introduces potential confusion for people expecting this to always be there  i e  they ll have to think about it   So  when would you use it  I ve just had a look around some random code and found these examples  I m not passing judgement on whether these are good things to do or otherwise     Passing  yourself  to a function  Assigning  yourself  to a pointer or something like that  Casting  i e  up down casting  safe or otherwise   casting away constness  etc  Compiler enforced disambiguation

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I tend to use it everywhere as well  just to make sure that it is clear that it is instance members that we are dealing with

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I use it anywhere there might be ambiguity  obviously   Not just compiler ambiguity  it would be required in that case   but also ambiguity for someone looking at the code

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I use it every time I can   I believe it makes the code more readable  and more readable code equals less bugs and more maintainability

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Another somewhat rare use for the this keyword is when you need to invoke an explicit interface implementation from within the implementing class  Here s a contrived example   class Example   ICloneable       private void CallClone                 object clone     ICloneable this  Clone               object ICloneable Clone                 throw new NotImplementedException

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I use it to invoke Intellisense just like JohnMcG  but I ll go back and erase  this-   when I m done   I follow the Microsoft convention of prefixing member variables with  m    so leaving it as documentation would just be redundant

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I got in the habit of using it liberally in Visual C   since doing so would trigger IntelliSense ones I hit the     key  and I m lazy   and prone to typos   But I ve continued to use it  since I find it handy to see that I m calling a member function rather than a global function

[c#] When do you use the "this" keyword?

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