What are the differences between struct and class in C

Question

This question was already asked in the context of C   Net   Now I d like to learn the differences between a struct and a class in C    Please discuss the technical differences as well as reasons for choosing one or the other in OO design   I ll start with an obvious difference    If you don t specify public  or private   members of a struct are public by default  members of a class are private by default    I m sure there are other differences to be found in the obscure corners of the C   specification

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Not in the specification  no  The main difference is in programmer expectations when they read your code in 2 years  structs are often assumed to be POD  Structs are also used in template metaprogramming when you re defining a type for purposes other than defining objects

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Class  members are private by default  Struct s members are public by default  Besides that there are no other differences  Also see this question

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Here is a good explanation  http   carcino gen nz tech cpp struct vs class php     So  one more time  in C    a struct is identical to a class except that the members of a struct have public visibility by default  but the members of a class have private visibility by default

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According to Stroustrup in the C   Programming Language      Which style you use depends on circumstances and taste  I usually prefer to use struct for classes that have all data public  I think of such classes as  not quite proper types  just data structures     Functionally  there is no difference other than the public   private

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STRUCT is a type of Abstract Data Type that divides up a given chunk of memory according to the structure specification  Structs are particularly useful in file serialization deserialization as the structure can often be written to the file verbatim   i e  Obtain a pointer to the struct  use the SIZE macro to compute the number of bytes to copy  then move the data in or out of the struct    Classes are a different type of abstract data type that attempt to ensure information hiding  Internally  there can be a variety of machinations  methods  temp variables  state variables  etc  that are all used to present a consistent API to any code which wishes to use the class    In effect  structs are about data  classes are about code   However  you do need to understand that these are merely abstractions  It s perfectly possible to create structs that look a lot like classes and classes that look a lot like structs  In fact  the earliest C   compilers were merely pre-compilers that translates C   code to C  Thus these abstractions are a benefit to logical thinking  not necessarily an asset to the computer itself   Beyond the fact that each is a different type of abstraction  Classes provide solutions to the C code naming puzzle  Since you can t have more than one function exposed with the same name  developers used to follow a pattern of      e g  mathlibextreme max    By grouping APIs into classes  similar functions  here we call them  methods   can be grouped together and protected from the naming of methods in other classes  This allows the programmer to organize his code better and increase code reuse   In theory  at least

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It s worth remembering C   s origins in  and compatibility with  C   C has structs  it has no concept of encapsulation  so everything is public   Being public by default is generally considered a bad idea when taking an object-oriented approach  so in making a form of C that is natively conducive to OOP  you can do OO in C  but it won t help you  which was the idea in C    originally  C With Classes    it makes sense to make members private by default   On the other hand  if Stroustrup had changed the semantics of struct so that its members were private by default  it would have broken compatibility  it is no longer as often true as the standards diverged  but all valid C programs were also valid C   programs  which had a big effect on giving C   a foothold    So a new keyword  class was introduced to be exactly like a struct  but private by default   If C   had come from scratch  with no history  then it would probably have only one such keyword  It also probably wouldn t have made the impact it made   In general  people will tend to use struct when they are doing something like how structs are used in C  public members  no constructor  as long as it isn t in a union  you can have constructors in structs  just like with classes  but people tend not to   no virtual methods  etc  Since languages are as much to communicate with people reading the code as to instruct machines  or else we d stick with assembly and raw VM opcodes  it s a good idea to stick with that

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You forget the tricky 2nd difference between classes and structs   Quoth the standard    11 2 2 in C  98 through C  11       In absence of an access-specifier   for a base class  public is assumed   when the derived class is declared   struct and private is assumed when the class is declared class    And just for completeness  sake  the more widely known difference between class and struct is defined in  11 2       Member of a class defined with the   keyword class are private by   default  Members of a class defined   with the keywords struct or union   are public by default    Additional difference  the keyword class can be used to declare template parameters  while the struct keyword cannot be so used

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One other thing to note  if you updated a legacy app that had structs to use classes you might run into the following issue   Old code has structs  code was cleaned up and these changed to classes   A virtual function or two was then added to the new updated class   When virtual functions are in classes then internally the compiler will add extra pointer to the class data to point to the functions   How this would break old legacy code is if in the old code somewhere the struct was cleared using memfill to clear it all to zeros  this would stomp the extra pointer data as well

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The members of a structure are public by default  the members of class are private by default  Default inheritance for Structure from another structure or class is public Default inheritance for class from another structure or class is private      class A      public          int i            class A2 A          struct A3 A           struct abc          int i      struct abc2 abc          class abc3 abc       int  tmain int argc   TCHAR  argv              abc2 objabc      objabc i   10       A3 ob      ob i   10         A2 obja    privately inherited       obja i   10         abc3 obss        obss i   10      This is on VS2005

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Out of all these factors it can be concluded that concept Class is highly suitable to represent real world objects rather than  Structures  Largely because OOP concepts used in class are highly practical in explaining real world scenarios therefore easier to merge them to reality For an example default inheritance is public for structs but if we apply this rule for real world it s ridiculous But in a class default inheritance is private which is more realistic   Anyways what i need to justify is Class is a much broader real world applicable concept whereas Structure is a primitive Concept with poor internal organization Eventhough struct follows OOP concepts they have a poor meaning

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The difference between struct and class keywords in C   is that  when there is no specific specifier on particular composite data type then by default struct or union is the public keywords that merely considers data hiding but class is the private keyword that considers the hiding of program codes or data  Always some programmers use struct for data and class for code sake  For more information contact other sources

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Not in the specification  no  The main difference is in programmer expectations when they read your code in 2 years  structs are often assumed to be POD  Structs are also used in template metaprogramming when you re defining a type for purposes other than defining objects

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According to Stroustrup in the C   Programming Language      Which style you use depends on circumstances and taste  I usually prefer to use struct for classes that have all data public  I think of such classes as  not quite proper types  just data structures     Functionally  there is no difference other than the public   private

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You might consider this for guidelines on when to go for struct or class  https   msdn microsoft com en-us library ms229017 28v vs 110 29 aspx    v CONSIDER defining a struct instead of a class if instances of the type are small and commonly short-lived or are commonly embedded in other objects  X AVOID defining a struct unless the type has all of the following characteristics  It logically represents a single value  similar to primitive types  int  double  etc    It has an instance size under 16 bytes  It is immutable  It will not have to be boxed frequently

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The only other difference is the default inheritance of classes and structs  which  unsurprisingly  is private and public respectively

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Class  members are private by default  Struct s members are public by default  Besides that there are no other differences  Also see this question

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The only other difference is the default inheritance of classes and structs  which  unsurprisingly  is private and public respectively

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ISO IEC 14882-2003      9 Classes        3      A structure is a class defined with   the class-key struct  its members   and base classes  clause 10  are   public by default  clause 11

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The main difference between structure and class keyword in oops is that  no public and private member declaration present in structure and the data member and member function can be defined as public  private as well as protected

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One other thing to note  if you updated a legacy app that had structs to use classes you might run into the following issue   Old code has structs  code was cleaned up and these changed to classes   A virtual function or two was then added to the new updated class   When virtual functions are in classes then internally the compiler will add extra pointer to the class data to point to the functions   How this would break old legacy code is if in the old code somewhere the struct was cleared using memfill to clear it all to zeros  this would stomp the extra pointer data as well

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According to Stroustrup in the C   Programming Language      Which style you use depends on circumstances and taste  I usually prefer to use struct for classes that have all data public  I think of such classes as  not quite proper types  just data structures     Functionally  there is no difference other than the public   private

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It s worth remembering C   s origins in  and compatibility with  C   C has structs  it has no concept of encapsulation  so everything is public   Being public by default is generally considered a bad idea when taking an object-oriented approach  so in making a form of C that is natively conducive to OOP  you can do OO in C  but it won t help you  which was the idea in C    originally  C With Classes    it makes sense to make members private by default   On the other hand  if Stroustrup had changed the semantics of struct so that its members were private by default  it would have broken compatibility  it is no longer as often true as the standards diverged  but all valid C programs were also valid C   programs  which had a big effect on giving C   a foothold    So a new keyword  class was introduced to be exactly like a struct  but private by default   If C   had come from scratch  with no history  then it would probably have only one such keyword  It also probably wouldn t have made the impact it made   In general  people will tend to use struct when they are doing something like how structs are used in C  public members  no constructor  as long as it isn t in a union  you can have constructors in structs  just like with classes  but people tend not to   no virtual methods  etc  Since languages are as much to communicate with people reading the code as to instruct machines  or else we d stick with assembly and raw VM opcodes  it s a good idea to stick with that

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Here is a good explanation  http   carcino gen nz tech cpp struct vs class php     So  one more time  in C    a struct is identical to a class except that the members of a struct have public visibility by default  but the members of a class have private visibility by default

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UPDATE     Please ignore this reply  I did not consider the possibility that for the struct  the value was uninitialized but just happened to be 0   There is not an initialization difference between struct and class     I am seeing another different between structs and classes having to do with default initialization     struct Foo       int a      class Bar       int a      class Tester       Foo m Foo   Foo        Bar m Bar   Bar     public      Tester          int main         auto myTester   Tester        Run that code and examine myTester   You ll find that for m Foo  the struct  m Foo a has been initialized to 0  but for m Bar  the class  m Bar a is uninitialized   So there does appear to be a difference in what the default constructor does for struct vs  class   I m seeing this with Visual Studio

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One other thing to note  if you updated a legacy app that had structs to use classes you might run into the following issue   Old code has structs  code was cleaned up and these changed to classes   A virtual function or two was then added to the new updated class   When virtual functions are in classes then internally the compiler will add extra pointer to the class data to point to the functions   How this would break old legacy code is if in the old code somewhere the struct was cleared using memfill to clear it all to zeros  this would stomp the extra pointer data as well

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Quoting The C   FAQ       7 8  What s the difference between   the keywords struct and class       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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It s just a convention   Structs can be created to hold simple data but later evolve time with the addition of member functions and constructors   On the other hand it s unusual to see anything other than public  access in a struct

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According to Stroustrup in the C   Programming Language      Which style you use depends on circumstances and taste  I usually prefer to use struct for classes that have all data public  I think of such classes as  not quite proper types  just data structures     Functionally  there is no difference other than the public   private

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Not in the specification  no  The main difference is in programmer expectations when they read your code in 2 years  structs are often assumed to be POD  Structs are also used in template metaprogramming when you re defining a type for purposes other than defining objects

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The difference between struct and class keywords in C   is that  when there is no specific specifier on particular composite data type then by default struct or union is the public keywords that merely considers data hiding but class is the private keyword that considers the hiding of program codes or data  Always some programmers use struct for data and class for code sake  For more information contact other sources

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Quoting The C   FAQ       7 8  What s the difference between   the keywords struct and class       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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Member of a class defined with the keyword class are private by default  Members of a class defined with the keywords struct  or union  are public by default  In absence of an access-specifier for a base class  public is assumed when the derived class is declared struct and private is assumed when the class is declared class  You can declare an enum class but not an enum struct  You can use template lt class T gt  but not template lt struct T gt     Note also that the C   standard allows you to forward-declare a type as a struct  and then use class when declaring the type and vice-versa  Also  std  is class lt Y gt   value is true for Y being a struct and a class  but is false for an enum class

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Class is only meaningful in the context of software engineering  In the context of data structures and algorithms  class and struct are not that different  There s no any rule restricted that class s member must be referenced   When developing large project with tons of people without class  you may finally get complicated coupled code because everybody use whatever functions and data they want  class provides permission controls and inherents to enhance decoupling and reusing codes   If you read some software engineering principles  you ll find most standards can not be implemented easily without class  for example  http   en wikipedia org wiki SOLID  28object-oriented design 29  BTW  When a struct allocates a crunch of memory and includes several variables  value type variables indicates that values are embbeded in where struct is allocated  In contrast  reference type variable s values are external and reference by a pointer which is also embedded in where struct is allocated

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Classes are Reference types and Structures are Values types  When I say Classes are reference types  basically they will contain the address of an instance variables   For example   Class MyClass       Public Int DataMember     By default  accessibility of class data members                                will be private  So I am making it as Public which                                can be accessed outside of the class      In main method   I can create an instance of this class using new operator that allocates memory for this class  and stores the base address of that into MyClass type variable  myClassObject2     Static Public void Main  string   arg        MyClass  myClassObject1   new MyClass         myClassObject1 DataMember   10       MyClass  myClassObject2    myClassObject1       myClassObject2 DataMember 20      In the above program       MyClass  myClassObject2    myClassObject1  instruction indicates that both variables of type MyClass        myClassObject1     myClassObject2   and  will point to the same memory location   It basically assigns the same memory location into another variable of same type   So if any changes that we make in any one of the objects type MyClass will have an effect on another since both are pointing to the same memory location     myClassObject1 DataMember   10   at this line both the object   s data members will contain the value of 10     myClassObject2 DataMember   20   at this line both the object   s data member will contains the value of 20   Eventually  we are accessing datamembers of an object through pointers   Unlike classes  structures are value types   For example    Structure MyStructure       Public Int DataMember     By default  accessibility of Structure data                                members will be private  So I am making it as                                Public which can be accessed out side of the structure     Static Public void Main  string   arg        MyStructure  myStructObject1   new MyStructure         myStructObject1 DataMember   10       MyStructure  myStructObject2    myStructObject1       myStructObject2 DataMember   20      In the above program   instantiating the object of MyStructure type using new operator and   storing address into  myStructObject variable of type MyStructure and   assigning value 10 to data member of the structure using   myStructObject1 DataMember   10     In the next line   I am declaring another variable  myStructObject2 of type MyStructure and assigning  myStructObject1 into that   Here  NET C  compiler creates another copy of  myStructureObject1 object and  assigns that memory location into MyStructure variable  myStructObject2   So whatever change we make on  myStructObject1 will never have an effect on another variable  myStructObject2 of type MyStructrue   That   s why we are saying Structures are value types   So the immediate Base class for class is Object and immediate Base class for Structure is ValueType which inherits from Object  Classes will support an Inheritance whereas Structures won   t   How are we saying that   And what is the reason behind that    The answer is Classes    It can be abstract  sealed  static  and partial and can   t be Private  Protected and protected internal

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While implied by other answers  it s not explicitly mentioned - that structs are C compatible  depending on usage  classes are not   This means if you re writing a header that you want to be C compatible then you ve no option other than struct  which in the C world can t have functions  but can have function pointers

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Class  members are private by default  Struct s members are public by default  Besides that there are no other differences  Also see this question

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The main difference between structure and class keyword in oops is that  no public and private member declaration present in structure and the data member and member function can be defined as public  private as well as protected

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In classes all the members by default are private but in  structure members are public by default    There is no term like constructor and destructor for  structs  but for class compiler creates default if you  don t provide  Sizeof empty structure is 0 Bytes wer as Sizeof empty class is 1 Byte  The struct default access type is public  A struct should typically be used for grouping data     The class default access type is private  and the default mode for inheritance is private  A class should be used for grouping data and methods that operate on that data    In short  the convention is to use struct when the purpose is to group data  and use classes when we require data abstraction and  perhaps inheritance    In C   structures and classes are passed by value  unless explicitly de-referenced  In other languages classes and structures may have distinct semantics - ie  objects  instances of classes  may be passed by reference and structures may be passed by value  Note  There are comments associated with this question  See the discussion page to add to the conversation

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You forget the tricky 2nd difference between classes and structs   Quoth the standard    11 2 2 in C  98 through C  11       In absence of an access-specifier   for a base class  public is assumed   when the derived class is declared   struct and private is assumed when the class is declared class    And just for completeness  sake  the more widely known difference between class and struct is defined in  11 2       Member of a class defined with the   keyword class are private by   default  Members of a class defined   with the keywords struct or union   are public by default    Additional difference  the keyword class can be used to declare template parameters  while the struct keyword cannot be so used

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It s just a convention   Structs can be created to hold simple data but later evolve time with the addition of member functions and constructors   On the other hand it s unusual to see anything other than public  access in a struct

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The difference between class and struct is a difference between keywords  not between data types  This two  struct foo   foo base   int x    class bar   bar base   int x       both define a class type  The difference of the keywords in this context is the different default access    foo  x is public and foo base is inherited publicly bar  x is private and bar base is inherited privately

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Not in the specification  no  The main difference is in programmer expectations when they read your code in 2 years  structs are often assumed to be POD  Structs are also used in template metaprogramming when you re defining a type for purposes other than defining objects

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The other answers have mentioned the private public defaults   but note that a struct is a class is a struct  they are not two different items  just two ways of defining the same item    What might be interesting to note  particularly since the asker is likely to be using MSVC   since he mentions  unmanaged  C    is that Visual C   complains under certain circumstances if a class is declared with class and then defined with struct  or possibly the other way round   although the standard says that is perfectly legal

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Classes are Reference types and Structures are Values types  When I say Classes are reference types  basically they will contain the address of an instance variables   For example   Class MyClass       Public Int DataMember     By default  accessibility of class data members                                will be private  So I am making it as Public which                                can be accessed outside of the class      In main method   I can create an instance of this class using new operator that allocates memory for this class  and stores the base address of that into MyClass type variable  myClassObject2     Static Public void Main  string   arg        MyClass  myClassObject1   new MyClass         myClassObject1 DataMember   10       MyClass  myClassObject2    myClassObject1       myClassObject2 DataMember 20      In the above program       MyClass  myClassObject2    myClassObject1  instruction indicates that both variables of type MyClass        myClassObject1     myClassObject2   and  will point to the same memory location   It basically assigns the same memory location into another variable of same type   So if any changes that we make in any one of the objects type MyClass will have an effect on another since both are pointing to the same memory location     myClassObject1 DataMember   10   at this line both the object   s data members will contain the value of 10     myClassObject2 DataMember   20   at this line both the object   s data member will contains the value of 20   Eventually  we are accessing datamembers of an object through pointers   Unlike classes  structures are value types   For example    Structure MyStructure       Public Int DataMember     By default  accessibility of Structure data                                members will be private  So I am making it as                                Public which can be accessed out side of the structure     Static Public void Main  string   arg        MyStructure  myStructObject1   new MyStructure         myStructObject1 DataMember   10       MyStructure  myStructObject2    myStructObject1       myStructObject2 DataMember   20      In the above program   instantiating the object of MyStructure type using new operator and   storing address into  myStructObject variable of type MyStructure and   assigning value 10 to data member of the structure using   myStructObject1 DataMember   10     In the next line   I am declaring another variable  myStructObject2 of type MyStructure and assigning  myStructObject1 into that   Here  NET C  compiler creates another copy of  myStructureObject1 object and  assigns that memory location into MyStructure variable  myStructObject2   So whatever change we make on  myStructObject1 will never have an effect on another variable  myStructObject2 of type MyStructrue   That   s why we are saying Structures are value types   So the immediate Base class for class is Object and immediate Base class for Structure is ValueType which inherits from Object  Classes will support an Inheritance whereas Structures won   t   How are we saying that   And what is the reason behind that    The answer is Classes    It can be abstract  sealed  static  and partial and can   t be Private  Protected and protected internal

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The main difference between struct and class is that in struct you can only declare data variables of different data types while in class you can declare data variables member functions and thus you can manipulate data variables through functions   -  another handy thing that i find in class vs struct is that while implementing files in a program if you want to make some operations of a struct again and again on every new set of operations you need to make a separate function and you need to pass object of struct after reading it from the file so as to make some operations on it      while in class if you make a function that does some operations on the data needed everytime  its easy you just have to read object from file and call the function    But it depennds on the programmer which way he she finds suitable   according to me i prefer class everytime just because it supports OOPs and thats the reason it is implemented in almost every languages and its the wonderful feature of all time programming  -   And yeah the most unforgotten difference i forgot to mention is that class supports data hiding and also supports operations that are performed on built in data types while struct doesnt

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STRUCT is a type of Abstract Data Type that divides up a given chunk of memory according to the structure specification  Structs are particularly useful in file serialization deserialization as the structure can often be written to the file verbatim   i e  Obtain a pointer to the struct  use the SIZE macro to compute the number of bytes to copy  then move the data in or out of the struct    Classes are a different type of abstract data type that attempt to ensure information hiding  Internally  there can be a variety of machinations  methods  temp variables  state variables  etc  that are all used to present a consistent API to any code which wishes to use the class    In effect  structs are about data  classes are about code   However  you do need to understand that these are merely abstractions  It s perfectly possible to create structs that look a lot like classes and classes that look a lot like structs  In fact  the earliest C   compilers were merely pre-compilers that translates C   code to C  Thus these abstractions are a benefit to logical thinking  not necessarily an asset to the computer itself   Beyond the fact that each is a different type of abstraction  Classes provide solutions to the C code naming puzzle  Since you can t have more than one function exposed with the same name  developers used to follow a pattern of      e g  mathlibextreme max    By grouping APIs into classes  similar functions  here we call them  methods   can be grouped together and protected from the naming of methods in other classes  This allows the programmer to organize his code better and increase code reuse   In theory  at least

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Out of all these factors it can be concluded that concept Class is highly suitable to represent real world objects rather than  Structures  Largely because OOP concepts used in class are highly practical in explaining real world scenarios therefore easier to merge them to reality For an example default inheritance is public for structs but if we apply this rule for real world it s ridiculous But in a class default inheritance is private which is more realistic   Anyways what i need to justify is Class is a much broader real world applicable concept whereas Structure is a primitive Concept with poor internal organization Eventhough struct follows OOP concepts they have a poor meaning

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While implied by other answers  it s not explicitly mentioned - that structs are C compatible  depending on usage  classes are not   This means if you re writing a header that you want to be C compatible then you ve no option other than struct  which in the C world can t have functions  but can have function pointers

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ISO IEC 14882-2003      9 Classes        3      A structure is a class defined with   the class-key struct  its members   and base classes  clause 10  are   public by default  clause 11

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I found an other difference  if you do not define a constructor in a class  the compiler will define one  but in a struct if you do not define a constructor  the compiler do not define a constructor too  so in some cases that we really do not need a constructor  struct is a better choice  performance tip   and sorry for my bad English

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One other thing to note  if you updated a legacy app that had structs to use classes you might run into the following issue   Old code has structs  code was cleaned up and these changed to classes   A virtual function or two was then added to the new updated class   When virtual functions are in classes then internally the compiler will add extra pointer to the class data to point to the functions   How this would break old legacy code is if in the old code somewhere the struct was cleared using memfill to clear it all to zeros  this would stomp the extra pointer data as well

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Quoting The C   FAQ       7 8  What s the difference between   the keywords struct and class       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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Another main difference is when it comes to Templates  As far as I know  you may use a class when you define a template but NOT a struct   template lt class T gt     OK template lt struct T gt     ERROR  struct not allowed here

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In classes all the members by default are private but in  structure members are public by default    There is no term like constructor and destructor for  structs  but for class compiler creates default if you  don t provide  Sizeof empty structure is 0 Bytes wer as Sizeof empty class is 1 Byte  The struct default access type is public  A struct should typically be used for grouping data     The class default access type is private  and the default mode for inheritance is private  A class should be used for grouping data and methods that operate on that data    In short  the convention is to use struct when the purpose is to group data  and use classes when we require data abstraction and  perhaps inheritance    In C   structures and classes are passed by value  unless explicitly de-referenced  In other languages classes and structures may have distinct semantics - ie  objects  instances of classes  may be passed by reference and structures may be passed by value  Note  There are comments associated with this question  See the discussion page to add to the conversation

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The members of a structure are public by default  the members of class are private by default  Default inheritance for Structure from another structure or class is public Default inheritance for class from another structure or class is private      class A      public          int i            class A2 A          struct A3 A           struct abc          int i      struct abc2 abc          class abc3 abc       int  tmain int argc   TCHAR  argv              abc2 objabc      objabc i   10       A3 ob      ob i   10         A2 obja    privately inherited       obja i   10         abc3 obss        obss i   10      This is on VS2005

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Class  members are private by default  Struct s members are public by default  Besides that there are no other differences  Also see this question

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Here is a good explanation  http   carcino gen nz tech cpp struct vs class php     So  one more time  in C    a struct is identical to a class except that the members of a struct have public visibility by default  but the members of a class have private visibility by default

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Member of a class defined with the keyword class are private by default  Members of a class defined with the keywords struct  or union  are public by default  In absence of an access-specifier for a base class  public is assumed when the derived class is declared struct and private is assumed when the class is declared class  You can declare an enum class but not an enum struct  You can use template lt class T gt  but not template lt struct T gt     Note also that the C   standard allows you to forward-declare a type as a struct  and then use class when declaring the type and vice-versa  Also  std  is class lt Y gt   value is true for Y being a struct and a class  but is false for an enum class

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There are 3 basic difference between structure and class   1St- memory are reserved for structure in stack memory  which is near to programming language  whether for class in stack memory  are reserved for only reffrence and actual memory are reserved in heap memory   2Nd - By default structure treat as a public whether class treat as a private    3Rd- can t re -use code in structure but in class we can re-use same code in many time called inhertence

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STRUCT is a type of Abstract Data Type that divides up a given chunk of memory according to the structure specification  Structs are particularly useful in file serialization deserialization as the structure can often be written to the file verbatim   i e  Obtain a pointer to the struct  use the SIZE macro to compute the number of bytes to copy  then move the data in or out of the struct    Classes are a different type of abstract data type that attempt to ensure information hiding  Internally  there can be a variety of machinations  methods  temp variables  state variables  etc  that are all used to present a consistent API to any code which wishes to use the class    In effect  structs are about data  classes are about code   However  you do need to understand that these are merely abstractions  It s perfectly possible to create structs that look a lot like classes and classes that look a lot like structs  In fact  the earliest C   compilers were merely pre-compilers that translates C   code to C  Thus these abstractions are a benefit to logical thinking  not necessarily an asset to the computer itself   Beyond the fact that each is a different type of abstraction  Classes provide solutions to the C code naming puzzle  Since you can t have more than one function exposed with the same name  developers used to follow a pattern of      e g  mathlibextreme max    By grouping APIs into classes  similar functions  here we call them  methods   can be grouped together and protected from the naming of methods in other classes  This allows the programmer to organize his code better and increase code reuse   In theory  at least

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The only other difference is the default inheritance of classes and structs  which  unsurprisingly  is private and public respectively

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1  Members of a class are private by default and members of struct are public by default   For example program 1 fails in compilation and program 2 works fine      Program 1  include  lt stdio h gt   class Test       int x     x is private    int main       Test t    t x   20     compiler error because x is private   getchar      return 0    Run on IDE    Program 2  include  lt stdio h gt   struct Test       int x     x is public    int main       Test t    t x   20     works fine because x is public   getchar      return 0      2  When deriving a struct from a class struct  default access-specifier for a base class struct is public  And when deriving a class  default access specifier is private   For example program 3 fails in compilation and program 4 works fine      Program 3  include  lt stdio h gt   class Base   public      int x      class Derived   Base         is equilalent to class Derived   private Base     int main       Derived d    d x   20     compiler error becuase inheritance is private   getchar      return 0    Run on IDE    Program 4  include  lt stdio h gt   class Base   public      int x      struct Derived   Base         is equilalent to struct Derived   public Base     int main       Derived d    d x   20     works fine becuase inheritance is public   getchar      return 0

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You might consider this for guidelines on when to go for struct or class  https   msdn microsoft com en-us library ms229017 28v vs 110 29 aspx    v CONSIDER defining a struct instead of a class if instances of the type are small and commonly short-lived or are commonly embedded in other objects  X AVOID defining a struct unless the type has all of the following characteristics  It logically represents a single value  similar to primitive types  int  double  etc    It has an instance size under 16 bytes  It is immutable  It will not have to be boxed frequently

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There are 3 basic difference between structure and class   1St- memory are reserved for structure in stack memory  which is near to programming language  whether for class in stack memory  are reserved for only reffrence and actual memory are reserved in heap memory   2Nd - By default structure treat as a public whether class treat as a private    3Rd- can t re -use code in structure but in class we can re-use same code in many time called inhertence

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Quoting The C   FAQ       7 8  What s the difference between   the keywords struct and class       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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The only other difference is the default inheritance of classes and structs  which  unsurprisingly  is private and public respectively

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Another main difference is when it comes to Templates  As far as I know  you may use a class when you define a template but NOT a struct   template lt class T gt     OK template lt struct T gt     ERROR  struct not allowed here

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UPDATE     Please ignore this reply  I did not consider the possibility that for the struct  the value was uninitialized but just happened to be 0   There is not an initialization difference between struct and class     I am seeing another different between structs and classes having to do with default initialization     struct Foo       int a      class Bar       int a      class Tester       Foo m Foo   Foo        Bar m Bar   Bar     public      Tester          int main         auto myTester   Tester        Run that code and examine myTester   You ll find that for m Foo  the struct  m Foo a has been initialized to 0  but for m Bar  the class  m Bar a is uninitialized   So there does appear to be a difference in what the default constructor does for struct vs  class   I m seeing this with Visual Studio

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The other answers have mentioned the private public defaults   but note that a struct is a class is a struct  they are not two different items  just two ways of defining the same item    What might be interesting to note  particularly since the asker is likely to be using MSVC   since he mentions  unmanaged  C    is that Visual C   complains under certain circumstances if a class is declared with class and then defined with struct  or possibly the other way round   although the standard says that is perfectly legal

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I found an other difference  if you do not define a constructor in a class  the compiler will define one  but in a struct if you do not define a constructor  the compiler do not define a constructor too  so in some cases that we really do not need a constructor  struct is a better choice  performance tip   and sorry for my bad English

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It s just a convention   Structs can be created to hold simple data but later evolve time with the addition of member functions and constructors   On the other hand it s unusual to see anything other than public  access in a struct

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It s just a convention   Structs can be created to hold simple data but later evolve time with the addition of member functions and constructors   On the other hand it s unusual to see anything other than public  access in a struct

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Class is only meaningful in the context of software engineering  In the context of data structures and algorithms  class and struct are not that different  There s no any rule restricted that class s member must be referenced   When developing large project with tons of people without class  you may finally get complicated coupled code because everybody use whatever functions and data they want  class provides permission controls and inherents to enhance decoupling and reusing codes   If you read some software engineering principles  you ll find most standards can not be implemented easily without class  for example  http   en wikipedia org wiki SOLID  28object-oriented design 29  BTW  When a struct allocates a crunch of memory and includes several variables  value type variables indicates that values are embbeded in where struct is allocated  In contrast  reference type variable s values are external and reference by a pointer which is also embedded in where struct is allocated

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The difference between class and struct is a difference between keywords  not between data types  This two  struct foo   foo base   int x    class bar   bar base   int x       both define a class type  The difference of the keywords in this context is the different default access    foo  x is public and foo base is inherited publicly bar  x is private and bar base is inherited privately

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STRUCT is a type of Abstract Data Type that divides up a given chunk of memory according to the structure specification  Structs are particularly useful in file serialization deserialization as the structure can often be written to the file verbatim   i e  Obtain a pointer to the struct  use the SIZE macro to compute the number of bytes to copy  then move the data in or out of the struct    Classes are a different type of abstract data type that attempt to ensure information hiding  Internally  there can be a variety of machinations  methods  temp variables  state variables  etc  that are all used to present a consistent API to any code which wishes to use the class    In effect  structs are about data  classes are about code   However  you do need to understand that these are merely abstractions  It s perfectly possible to create structs that look a lot like classes and classes that look a lot like structs  In fact  the earliest C   compilers were merely pre-compilers that translates C   code to C  Thus these abstractions are a benefit to logical thinking  not necessarily an asset to the computer itself   Beyond the fact that each is a different type of abstraction  Classes provide solutions to the C code naming puzzle  Since you can t have more than one function exposed with the same name  developers used to follow a pattern of      e g  mathlibextreme max    By grouping APIs into classes  similar functions  here we call them  methods   can be grouped together and protected from the naming of methods in other classes  This allows the programmer to organize his code better and increase code reuse   In theory  at least

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The main difference between struct and class is that in struct you can only declare data variables of different data types while in class you can declare data variables member functions and thus you can manipulate data variables through functions   -  another handy thing that i find in class vs struct is that while implementing files in a program if you want to make some operations of a struct again and again on every new set of operations you need to make a separate function and you need to pass object of struct after reading it from the file so as to make some operations on it      while in class if you make a function that does some operations on the data needed everytime  its easy you just have to read object from file and call the function    But it depennds on the programmer which way he she finds suitable   according to me i prefer class everytime just because it supports OOPs and thats the reason it is implemented in almost every languages and its the wonderful feature of all time programming  -   And yeah the most unforgotten difference i forgot to mention is that class supports data hiding and also supports operations that are performed on built in data types while struct doesnt

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Here is a good explanation  http   carcino gen nz tech cpp struct vs class php     So  one more time  in C    a struct is identical to a class except that the members of a struct have public visibility by default  but the members of a class have private visibility by default

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1  Members of a class are private by default and members of struct are public by default   For example program 1 fails in compilation and program 2 works fine      Program 1  include  lt stdio h gt   class Test       int x     x is private    int main       Test t    t x   20     compiler error because x is private   getchar      return 0    Run on IDE    Program 2  include  lt stdio h gt   struct Test       int x     x is public    int main       Test t    t x   20     works fine because x is public   getchar      return 0      2  When deriving a struct from a class struct  default access-specifier for a base class struct is public  And when deriving a class  default access specifier is private   For example program 3 fails in compilation and program 4 works fine      Program 3  include  lt stdio h gt   class Base   public      int x      class Derived   Base         is equilalent to class Derived   private Base     int main       Derived d    d x   20     compiler error becuase inheritance is private   getchar      return 0    Run on IDE    Program 4  include  lt stdio h gt   class Base   public      int x      struct Derived   Base         is equilalent to struct Derived   public Base     int main       Derived d    d x   20     works fine becuase inheritance is public   getchar      return 0

[c++] What are the differences between struct and class in C++?

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