When should you use a class vs a struct in C

Question

In what scenarios is it better to use a struct vs a class in C

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I thought that Structs was intended as a Data Structure  like a multi-data type array of information  and classes was inteded for Code Packaging  like collections of subroutines  amp  functions

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I never use  struct  in C      I can t ever imagine a scenario where you would use a struct when you want private members  unless you re willfully trying to be confusing    It seems that using structs is more of a syntactic indication of how the data will be used  but I d rather just make a class and try to make that explicit in the name of the class  or through comments    E g   class PublicInputData         data members

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Technically both are the same in C   - for instance it s possible for a struct to have overloaded operators etc   However    I use structs when I wish to pass information of multiple types simultaneously I use classes when the I m dealing with a  functional  object   Hope it helps    include  lt string gt   include  lt map gt  using namespace std   struct student       int age      string name      map lt string  int gt  grades     class ClassRoom       typedef map lt string  student gt  student map    public       student getStudentByName string name  const        student map  const iterator m it   students find name   return m it- gt second      private       student map students       For instance  I m returning a struct student in the get      methods over here - enjoy

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The only time I use a struct instead of a class is when declaring a functor right before using it in a function call and want to minimize syntax for the sake of clarity  e g    struct Compare   bool operator              std  sort collection begin    collection end    Compare

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Both struct and class are the same under the hood though with different defaults as to visibility  struct default is public and class default is private  You can change either one to be the other with the appropriate use of private and public  They both allow inheritance  methods  constructors  destructors  and all the rest of the goodies of an object oriented language   However one huge difference between the two is that struct as a keyword is supported in C whereas class is not  This means that one can use a struct in an include file that can be  include into either C   or C so long as the struct is a plain C style struct and everything else in the include file is compatible with C  i e  no C   specific keywords such as private  public  no methods  no inheritance  etc  etc  etc   A C style struct can be used with other interfaces which support using C style struct to carry data back and forth over the interface   A C style struct is a kind of template  not a C   template but rather a pattern or stencil  that describes the layout of a memory area  Over the years interfaces usable from C and with C plug-ins  here s looking at you Java and Python and Visual Basic  have been created some of which work with C style struct

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Just to address this from a C  20 Standardese perspective  working from N4860     A class is a type   The keywords  quot class quot  and  quot struct quot   and  quot union quot   are - in the C   grammar - class-keys  and the only functional significance of the choice of class or struct is   The class-key determines whether     access is public or private by default  11 9    Data member default accessibility That the class keyword results in private-by-default members  and  struct keyword results in public-by-default members  is documented by the examples in 11 9 1   class X   int a        X  a is private by default  class used     vs     struct S   int a        S  a is public by default  struct used  Base class default accessibility 1 9 also says   In the absence of an access-specifier for a base class  public is assumed when the derived class is defined with the class-key struct and private is assumed when the class is defined with the class-key class   Circumstances where consistent use of struct or class is required    There s a requirement   In a redeclaration  partial specialization  explicit specialization or explicit instantiation of a class template  the class-key shall agree in kind with the original class template declaration  9 2 8 3        in any elaborated-type-specifier  the enum keyword shall be used to refer to an enumeration  9 7 1   the union class-key shall be used to refer to a union  11 5   and either the class or struct class-key shall be used to refer to a non-union class  11 1    The following example  of when consistency is not required  is provided   struct S     s  class S  p    amp s           OK  Still  some compilers may warn about this   Interestingly  while the types you create with struct  class and union are all termed  quot classes quot   we have     A standard-layout struct is a standard layout class defined with the class-key struct or the class-key class      so in Standardese  when there s talk of a standard-layout struct it s using  quot struct quot  to imply  quot not a union quot s  I m curious if there are similar use of  quot struct quot  in other terminology  but it s too big a job to do an exhaustive search of the Standard   Comments about that welcome

User · Answer

All class members are private by default and all struct members are public by default   Class has default private bases and Struct has default public bases  Struct in case of C cannot have member functions where as in case of C   we can have member functions being added to the struct  Other than these differences  I don t find anything surprising about them

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From the C   FAQ Lite   The members and base classes of a struct are public by default  while in class  they default to private  Note  you should make your base classes explicitly public  private  or protected  rather than relying on the defaults  struct and class are otherwise functionally equivalent  OK  enough of that squeaky clean techno talk  Emotionally  most developers make a strong distinction between a class and a struct  A struct simply feels like an open pile of bits with very little in the way of encapsulation or functionality  A class feels like a living and responsible member of society with intelligent services  a strong encapsulation barrier  and a well defined interface  Since that s the connotation most people already have  you should probably use the struct keyword if you have a class that has very few methods and has public data  such things do exist in well designed systems    but otherwise you should probably use the class keyword

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Class   Class members are private by default   class test one       int main one         Is equivalent to  class test one     private      int main one         So if you try  int two   one main one      We will get an error  main one is private because its not accessible  We can solve it by initializing it by specifying its a public ie  class test one     public      int main one         Struct   A struct is a class where members are public by default   struct test one       int main one       Means main one is private ie  class test one     public      int main one       I use structs for data structures where the members can take any value  it s easier that way

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As every one says  the only real difference is the default access  But I particularly use struct when I don t want any sort of encapsulation with a simple data class  even if I implement some helper methods  For instance  when I need something like this   struct myvec       int x      int y      int z       int length    return x y z

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To answer my own question  shamelessly   As already mentioned  access privileges are the only difference between them in C       I tend to use a struct for data-storage only   I ll allow it to get a few helper functions if it makes working with the data easier   However as soon as the data requires flow control  i e  getters setters that maintain or protect an internal state  or starts acquring any major functionality  basically more object-like   it will get  upgraded  to a class to better communicate intent

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As others have pointed out   both are equivalent apart from default visibility there may be reasons to be forced to use the one or the other for whatever reason   There s a clear recommendation about when to use which from Stroustrup Sutter      Use class if the class has an invariant  use struct if the data members can vary independently   However  keep in mind that it is not wise to forward declare sth  as a class  class X   and define it as struct  struct X           It may work on some linkers  e g   g    and may fail on others  e g   MSVC   so you will find yourself in developer hell

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The differences between a class and a struct in C   is   struct members and base classes structs are public by default  class  members and base classes struts are private by default   Both classes and structs can have a mixture of public  protected and private members  can use inheritance and can have member functions  I would recommend you   use struct for plain-old-data structures without any class-like features  use class when you make use of features such as private or protected members  non-default constructors and operators  etc

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Structs by default have public access and classes by default have private access   Personally I use structs for Data Transfer Objects or as Value Objects  When used as such I declare all members as const to prevent modification by other code

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they re the same thing with different defaults  private by default for class  and public by default for struct   so in theory they re totally interchangeable   so  if I just want to package some info to move around  I use a struct  even if i put a few methods there  but not many    If it s a mostly-opaque thing  where the main use would be via methods  and not directly to the data members  i use a full class

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You can use  struct  in C   if you are writing a library whose internals are C   but the API can be called by either C or C   code   You simply make a single header that contains structs and global API functions that you expose to both C and C   code as this      C access Header to a C   library  ifdef   cpp extern  C     endif     Put your C struct s here struct foo                 NOTE  the typedef is used because C does not automatically generate    a typedef with the same name as a struct like C    typedef struct foo foo      Put your C API functions here void bar foo  fun     ifdef   cpp    endif   Then you can write a function bar   in a C   file using C   code and make it callable from C and the two worlds can share data through the declared struct s   There are other caveats of course when mixing C and C   but this is a simplified example

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For C    there really isn t much of a difference between structs and classes  The main functional difference is that members of a struct are public by default  while they are private by default in classes  Otherwise  as far as the language is concerned  they are equivalent   That said  I tend to use structs in C   like I do in C   similar to what Brian has said  Structs are simple data containers  while classes are used for objects that need to act on the data in addition to just holding on to it

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They are pretty much the same thing   Thanks to the magic of C    a struct can hold functions  use inheritance  created using  new  and so on  just like a class   The only functional difference is that a class begins with private access rights  while a struct begins with public   This is the maintain backwards compatibility with C   In practice  I ve always used structs as data holders and classes as objects

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I use struct only when I need to hold some data without any member functions associated to it  to operate on the member data  and to access the data variables directly   Eg  Reading Writing data from files and socket streams etc  Passing function arguments in a structure where the function arguments are too many and function syntax looks too lengthy   Technically there is no big difference between class and struture except default accessibility  More over it depends on programming style how you use it

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There are lots of misconceptions in the existing answers   Both class and struct declare a class   Yes  you may have to rearrange your access modifying keywords inside the class definition  depending on which keyword you used to declare the class   But  beyond syntax  the only reason to choose one over the other is convention style preference   Some people like to stick with the struct keyword for classes without member functions  because the resulting definition  looks like  a simple structure from C   Similarly  some people like to use the class keyword for classes with member functions and private data  because it says  class  on it and therefore looks like examples from their favourite book on object-oriented programming   The reality is that this completely up to you and your team  and it ll make literally no difference whatsoever to your program   The following two classes are absolutely equivalent in every way except their name   struct Foo      int x      class Bar   public     int x       You can even switch keywords when redeclaring   class Foo  struct Bar     although this breaks Visual Studio builds due to non-conformance  so that compiler will emit a warning when you do this    and the following expressions both evaluate to true   std  is class lt Foo gt   value std  is class lt Bar gt   value   Do note  though  that you can t switch the keywords when redefining  this is only because  per the one-definition rule  duplicate class definitions across translation units must  consist of the same sequence of tokens   This means you can t even exchange const int member  with int const member   and has nothing to do with the semantics of class or struct

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As everyone else notes there are really only two actual language differences    struct defaults to public access and class defaults to private access  When inheriting  struct defaults to public inheritance and class defaults to private inheritance    Ironically  as with so many things in C    the default is backwards  public inheritance is by far the more common choice  but people rarely declare structs just to save on typing the  public  keyword    But the real difference in practice is between a class struct that declares a constructor destructor and one that doesn t   There are certain guarantees to a  plain-old-data  POD type  that no longer apply once you take over the class s construction   To keep this distinction clear  many people deliberately only use structs for POD types  and  if they are going to add any methods at all  use classes   The difference between the two fragments below is otherwise meaningless   class X     public                struct X                  Incidentally  here s a thread with some good explanations about what  POD type  actually means  What are POD types in C

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Structs  PODs  more generally  are handy when you re providing a C-compatible interface with a C   implementation  since they re portable across language borders and linker formats   If that s not a concern to you  then I suppose the use of the  struct  instead of  class  is a good communicator of intent  as  ZeroSignal said above    Structs also have more predictable copying semantics  so they re useful for data you intend to write to external media or send across the wire     Structs are also handy for various metaprogramming tasks  like traits templates that just expose a bunch of dependent typedefs   template  lt typename T gt  struct type traits     typedef T type    typedef T  iterator type iterator type                But that s really just taking advantage of struct s default protection level being public

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An advantage of struct over class is that it save one line of code  if adhering to  first public members  then private   In this light  I find the keyword class useless   Here is another reason for using only struct and never class  Some code style guidelines for C   suggest using small letters for function macros  the rationale being that when the macro is converted to an inline function  the name shouldn t need to be changed  Same here  You have your nice C-style struct and one day  you find out you need to add a constructor  or some convenience method  Do you change it to a class  Everywhere   Distinguishing between structs and classes is just too much hassle  getting into the way of doing what we should be doing - programming  Like so many of C   s problems  it arises out of the strong desire for backwards compatability

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I use structs when I need to create POD type or functor

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One place where a struct has been helpful for me is when I have a system that s receiving fixed format messages  over say  a serial port  from another system   You can cast the stream of bytes into a struct that defines your fields  and then easily access the fields   typedef struct       int messageId      int messageCounter      int messageData    tMessageType   void processMessage unsigned char  rawMessage        tMessageType  messageFields    tMessageType   rawMessage      printf  MessageId is  d n   messageFields- gt messageId       Obviously  this is the same thing you would do in C  but I find that the overhead of having to decode the message into a class is usually not worth it

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When would you choose to use struct   and when to use class in C      I use struct when I define functors and POD  Otherwise I use class           is public by default  struct mycompare   public std  binary function lt int  int  bool gt        bool operator   int first  int second        return first  lt  second        class mycompare   public std  binary function lt int  int  bool gt    public      bool operator   int first  int second        return first  lt  second

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