Why would one use nested classes in C

Question

Can someone please point me towards some nice resources for understanding and using nested classes  I have some material like Programming Principles and things like this IBM Knowledge Center - Nested Classes  But I m still having trouble understanding their purpose  Could someone please help me

User · Answer

I don t use nested classes much  but I do use them now and then   Especially when I define some kind of data type  and I then want to define a STL functor designed for that data type   For example  consider a generic Field class that has an ID number  a type code and a field name   If I want to search a vector of these Fields by either ID number or name  I might construct a functor to do so   class Field   public    unsigned id     string name     unsigned type      class match   public std  unary function lt bool  Field gt        public      match const string amp  name    name  name   has name  true          match unsigned id    id  id   has id  true          bool operator   const Field amp  rhs  const             bool ret   true        if  ret  amp  amp  has id    ret   id     rhs id         if  ret  amp  amp  has name    ret   name     rhs name         return ret             private        unsigned id         bool has id         string name         bool has name             Then code that needs to search for these Fields can use the match scoped within the Field class itself   vector lt Field gt   const iterator it   find if fields begin    fields end    Field  match  FieldName

User · Answer

Nested classes are cool for hiding implementation details   List   class List       public          List    head nullptr   tail nullptr         private          class Node                         public                    int   data                    Node  next                    Node  prev                 private          Node      head          Node      tail       Here I don t want to expose Node as other people may decide to use the class and that would hinder me from updating my class as anything exposed is part of the public API and must be maintained forever  By making the class private  I not only hide the implementation I am also saying this is mine and I may change it at any time so you can not use it   Look at std  list or std  map they all contain hidden classes  or do they    The point is they may or may not  but because the implementation is private and hidden the builders of the STL were able to update the code without affecting how you used the code  or leaving a lot of old baggage laying around the STL because they need to maintain backwards compatibility with some fool who decided they wanted to use the Node class that was hidden inside list

User · Answer

One can implement a Builder pattern with nested class  Especially in C    personally I find it semantically cleaner  For example   class Product      public          class Builder    class Product  Builder          Builder Implementation     Rather than   class Product    class ProductBuilder

User · Answer

Nested classes are just like regular classes  but    they have additional access restriction  as all definitions inside a class definition do   they don t pollute the given namespace  e g  global namespace  If you feel that class B is so deeply connected to class A  but the objects of A and B are not necessarily related  then you might want the class B to be only accessible via scoping the A class  it would be referred to as A  Class     Some examples   Publicly nesting class to put it in a scope of relevant class    Assume you want to have a class SomeSpecificCollection which would aggregate objects of class Element  You can then either    declare two classes  SomeSpecificCollection and Element - bad  because the name  Element  is general enough in order to cause a possible name clash introduce a namespace someSpecificCollection and declare classes someSpecificCollection  Collection and someSpecificCollection  Element  No risk of name clash  but can it get any more verbose  declare two global classes SomeSpecificCollection and SomeSpecificCollectionElement - which has minor drawbacks  but is probably OK  declare global class SomeSpecificCollection and class Element as its nested class  Then    you don t risk any name clashes as Element is not in the global namespace  in implementation of SomeSpecificCollection you refer to just Element  and everywhere else as SomeSpecificCollection  Element - which looks  - the same as 3   but more clear it gets plain simple that it s  an element of a specific collection   not  a specific element of a collection  it is visible that SomeSpecificCollection is also a class     In my opinion  the last variant is definitely the most intuitive and hence best design   Let me stress - It s not a big difference from making two global classes with more verbose names  It just a tiny little detail  but imho it makes the code more clear   Introducing another scope inside a class scope    This is especially useful for introducing typedefs or enums  I ll just post a code example here   class Product   public      enum ProductType           FANCY  AWESOME  USEFUL            enum ProductBoxType           BOX  BAG  CRATE            Product ProductType t  ProductBoxType b  String name           the rest of the class  fields  methods      One then will call   Product p Product  FANCY  Product  BOX     But when looking at code completion proposals for Product    one will often get all the possible enum values  BOX  FANCY  CRATE  listed and it s easy to make a mistake here  C  0x s strongly typed enums kind of solve that  but never mind    But if you introduce additional scope for those enums using nested classes  things could look like   class Product   public      struct ProductType           enum Enum   FANCY  AWESOME  USEFUL               struct ProductBoxType           enum Enum   BOX  BAG  CRATE               Product ProductType  Enum t  ProductBoxType  Enum b  String name           the rest of the class  fields  methods      Then the call looks like   Product p Product  ProductType  FANCY  Product  ProductBoxType  BOX     Then by typing Product  ProductType   in an IDE  one will get only the enums from the desired scope suggested  This also reduces the risk of making a mistake   Of course this may not be needed for small classes  but if one has a lot of enums  then it makes things easier for the client programmers   In the same way  you could  organise  a big bunch of typedefs in a template  if you ever had the need to  It s a useful pattern sometimes   The PIMPL idiom    The PIMPL  short for Pointer to IMPLementation  is an idiom useful to remove the implementation details of a class from the header  This reduces the need of recompiling classes depending on the class  header whenever the  implementation  part of the header changes   It s usually implemented using a nested class   X h   class X   public      X        virtual  X        void publicInterface        void publicInterface2    private      struct Impl      std  unique ptr lt Impl gt  impl      X cpp    include  X h   include  lt windows h gt   struct X  Impl       HWND hWnd     this field is a part of the class  but no need to include windows h in header        all private fields  methods go here      void privateMethod HWND wnd       void privateMethod        X  X     impl new Impl                       and the rest of definitions go here   This is particularly useful if the full class definition needs the definition of types from some external library which has a heavy or just ugly header file  take WinAPI   If you use PIMPL  then you can enclose any WinAPI-specific functionality only in  cpp and never include it in  h

[c++] Why would one use nested classes in C++?

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