What are POD types in C

Question

I ve come across this term POD-type a few times  What does it mean

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As I understand POD  PlainOldData  is just a raw data - it does not need    to be constructed  to be destroyed  to have custom operators  Must not have virtual functions  and must not override operators    How to check if something is a POD  Well  there is a struct for that called std  is pod   namespace std      Could use is standard layout  amp  amp  is trivial instead of the builtin  template lt typename  Tp gt    struct is pod     public integral constant lt bool    is pod  Tp  gt              From header type traits   Reference    http   en cppreference com w cpp types is pod http   en wikipedia org wiki Plain old data structure http   en wikipedia org wiki Plain Old C   Object File type traits

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Why do we need to differentiate between POD s and non-POD s at all   C   started its life as an extension of C  While modern C   is no longer a strict superset of C  people still expect a high level of compatibility between the two  The  quot C ABI quot  of a platform also frequently acts as a de-facto standard inter-language ABI for other languages on the platform  Roughly speaking  a POD type is a type that is compatible with C and perhaps equally importantly is compatible with certain ABI optimisations  To be compatible with C  we need to satisfy two constraints   The layout must be the same as the corresponding C type  The type must be passed to and returned from functions in the same way as the corresponding C type   Certain C   features are incompatible with this  Virtual methods require the compiler to insert one or more pointers to virtual method tables  something that doesn t exist in C  User-defined copy constructors  move constructors  copy assignments and destructors have implications for parameter passing and returning  Many C ABIs pass and return small parameters in registers  but the references passed to the user defined constructor assigment destructor can only work with memory locations  So there is a need to define what types can be expected to be  quot C compatible quot  and what types cannot  C  03 was somewhat over-strict in this regard  any user-defined constructor would disable the built-in constructors and any attempt to add them back in would result in them being user-defined and hence the type being non-pod  C  11 opened things up quite a bit  by allowing the user to re-introduce the built-in constructors

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Examples of all non-POD cases with static assert from C  11 to C  17 and POD effects  std  is pod was added in C  11  so let s consider that standard onwards for now    std  is pod will be removed from C  20 as mentioned at https   stackoverflow com a 48435532 895245   let s update this as support arrives for the replacements   POD restrictions have become more and more relaxed as the standard evolved  I aim to cover all relaxations in the example through ifdefs   libstdc   has at tiny bit of testing at  https   github com gcc-mirror gcc blob gcc-8 2 0-release libstdc 2B 2B-v3 testsuite 20 util is pod value cc but it just too little  Maintainers  please merge this if you read this post  I m lazy to check out all the C   testsuite projects mentioned at  https   softwareengineering stackexchange com questions 199708 is-there-a-compliance-test-for-c-compilers   include  lt type traits gt   include  lt array gt   include  lt vector gt   int main      if   cplusplus  gt   201103L          Not POD               Non-POD examples  Let s just walk all non-recursive non-POD branches of cppreference                   Non-trivial implies non-POD             https   en cppreference com w cpp named req TrivialType                          Has one or more default constructors  all of which are either                trivial or deleted  and at least one of which is not deleted                                   Not trivial because we removed the default constructor                    by using our own custom non-default constructor                                        struct C                           C int                                                static assert std  is trivially copyable lt C gt                              static assert  std  is trivial lt C gt                              static assert  std  is pod lt C gt                                                No  this is not a default trivial constructor either                     https   en cppreference com w cpp language default constructor                                       The constructor is not user-provided  i e   is implicitly-defined or                    defaulted on its first declaration                                        struct C                           C                                                 static assert std  is trivially copyable lt C gt                              static assert  std  is trivial lt C gt                              static assert  std  is pod lt C gt                                                          Not trivial because not trivially copyable                                struct C                       C C amp                                         static assert  std  is trivially copyable lt C gt                          static assert  std  is trivial lt C gt                          static assert  std  is pod lt C gt                                              Non-standard layout implies non-POD             https   en cppreference com w cpp named req StandardLayoutType                          Non static members with different access control                                   i is public and j is private                                        struct C                           public                              int i                          private                              int j                                             static assert  std  is standard layout lt C gt                              static assert  std  is pod lt C gt                                                These have the same access control                                        struct C                           private                              int i                              int j                                             static assert std  is standard layout lt C gt                              static assert std  is pod lt C gt                               struct D                           public                              int i                              int j                                             static assert std  is standard layout lt D gt                              static assert std  is pod lt D gt                                                          Virtual function                                struct C                       virtual void f     0                                     static assert  std  is standard layout lt C gt                          static assert  std  is pod lt C gt                                        Non-static member that is reference                                struct C                       int  amp i                                     static assert  std  is standard layout lt C gt                          static assert  std  is pod lt C gt                                        Neither                                - has no base classes with non-static data members  or                - has no non-static data members in the most derived class                  and at most one base class with non-static data members                                  Non POD because has two base classes with non-static data members                                        struct Base1                           int i                                             struct Base2                           int j                                             struct C   Base1  Base2                         static assert  std  is standard layout lt C gt                              static assert  std  is pod lt C gt                                                POD  has just one base class with non-static member                                        struct Base1                           int i                                             struct C   Base1                         static assert std  is standard layout lt C gt                              static assert std  is pod lt C gt                                                Just one base class with non-static member  Base1  Base2 has none                                        struct Base1                           int i                                             struct Base2                         struct C   Base1  Base2                         static assert std  is standard layout lt C gt                              static assert std  is pod lt C gt                                                          Base classes of the same type as the first non-static data member                 TODO failing on GCC 8 1 -std c  11  14 and 17                                struct C                     struct D   C                       C c                                       static assert  std  is standard layout lt C gt                            static assert  std  is pod lt C gt                                         C  14 standard layout new rules  yay                                   Has two  possibly indirect  base class subobjects of the same type                     Here C has two base classes which are indirectly  Base                                         TODO failing on GCC 8 1 -std c  11  14 and 17                     even though the example was copy pasted from cppreference                                        struct Q                         struct S   Q                          struct T   Q                          struct U   S  T          not a standard-layout class  two base class subobjects of type Q                       static assert  std  is standard layout lt U gt                                static assert  std  is pod lt U gt                                                Has all non-static data members and bit-fields declared in the same class                     either all in the derived or all in some base                                         struct Base   int i                         struct Middle   Base                         struct C   Middle   int j                         static assert  std  is standard layout lt C gt                              static assert  std  is pod lt C gt                                                None of the base class subobjects has the same type as                    for non-union types  as the first non-static data member                                       TODO  similar to the C  11 for which we could not make a proper example                     but with recursivity added                      TODO come up with an example that is POD in C  14 but not in C  11                                          POD               POD examples  Everything that does not fall neatly in the non-POD examples                   Can t get more POD than this                        struct C                 static assert std  is pod lt C gt                      static assert std  is pod lt int gt                                Array of POD is POD                        struct C                 static assert std  is pod lt C gt                      static assert std  is pod lt C   gt                                Private member  became POD in C  11            https   stackoverflow com questions 4762788 can-a-class-with-all-private-members-be-a-pod-class 4762944 4762944                       struct C                   private                      int i                  if   cplusplus  gt   201103L             static assert std  is pod lt C gt           else             static assert  std  is pod lt C gt           endif                       Most standard library containers are not POD because they are not trivial             which can be seen directly from their interface definition in the standard             https   stackoverflow com questions 27165436 pod-implications-for-a-struct-which-holds-an-standard-library-container                       static assert  std  is pod lt std  vector lt int gt  gt                      static assert  std  is trivially copyable lt std  vector lt int gt  gt                         Some might be though                 https   stackoverflow com questions 3674247 is-stdarrayt-s-guaranteed-to-be-pod-if-t-is-pod             static assert std  is pod lt std  array lt int  1 gt  gt                                    POD effects               Now let s verify what effects does PODness have                Note that this is not easy to do automatically  since many of the        failures are undefined behaviour                A good initial list can be found at         https   stackoverflow com questions 4178175 what-are-aggregates-and-pods-and-how-why-are-they-special 4178176 4178176               struct Pod               uint32 t i              uint64 t j                     static assert std  is pod lt Pod gt                   struct NotPod               NotPod uint32 t i  uint64 t j    i i   j j                 uint32 t i              uint64 t j                     static assert  std  is pod lt NotPod gt                        attribute    packed   only works for POD  and is ignored for non-POD  and emits a warning            https   stackoverflow com questions 35152877 ignoring-packed-attribute-because-of-unpacked-non-pod-field 52986680 52986680                       struct C                   int i                              struct D   C                   int j                              struct E                   D d                    attribute    packed                   static assert std  is pod lt C gt                      static assert  std  is pod lt D gt                      static assert  std  is pod lt E gt                           endif     GitHub upstream   Tested with   for std in 11 14 17  do echo  std  g  -8 -Wall -Werror -Wextra -pedantic -std c   std pod cpp  done   on Ubuntu 18 04  GCC 8 2 0

User · Answer

POD stands for Plain Old Data - that is  a class  whether defined with the keyword struct or the keyword class  without constructors  destructors and virtual members functions  Wikipedia s article on POD goes into a bit more detail and defines it as      A Plain Old Data Structure in C   is an aggregate class that contains only PODS as members  has no user-defined destructor  no user-defined copy assignment operator  and no nonstatic members of pointer-to-member type    Greater detail can be found in this answer for C  98 03  C  11 changed the rules surrounding POD  relaxing them greatly  thus necessitating a follow-up answer here

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Very informally  A POD is a type  including classes  where the C   compiler guarantees that there will be no  quot magic quot  going on in the structure  for example hidden pointers to vtables  offsets that get applied to the address when it is cast to other types  at least if the target s POD too   constructors  or destructors  Roughly speaking  a type is a POD when the only things in it are built-in types and combinations of them  The result is something that  quot acts like quot  a C type  Less informally   int  char  wchar t  bool  float  double are PODs  as are long short and signed unsigned versions of them  pointers  including pointer-to-function and pointer-to-member  are PODs  enums are PODs a const or volatile POD is a POD  a class  struct or union of PODs is a POD provided that all non-static data members are public  and it has no base class and no constructors  destructors  or virtual methods  Static members don t stop something being a POD under this rule  This rule has changed in C  11 and certain private members are allowed  Can a class with all private members be a POD class  Wikipedia is wrong to say that a POD cannot have members of type pointer-to-member  Or rather  it s correct for the C  98 wording  but TC1 made explicit that pointers-to-member are POD   Formally  C  03 Standard    3 9 10    quot Arithmetic types  3 9 1   enumeration types  pointer types  and pointer to member types  3 9 2  and cv-qualified versions of these types  3 9 3  are collectively caller scalar types  Scalar types  POD-struct types  POD-union types  clause 9   arrays of such types and cv-qualified versions of these types  3 9 3  are collectively called POD types quot  9 4    quot A POD-struct is an aggregate class that has no non-static data members of type non-POD-struct  non-POD-union  or array of such types  or reference  and has no user-define copy operator and no user-defined destructor  Similarly a POD-union is an aggregate union that has no non-static data members of type non-POD-struct  non-POD-union  or array of such types  or reference  and has no user-define copy operator and no user-defined destructor  8 5 1 1    quot An aggregate is an array or class  clause 9  with no user-declared constructors  12 1   no private or protected non-static data members  clause 11   no base classes  clause 10  and no virtual functions  10 3   quot

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A POD  plain old data  object has one of these data types--a fundamental type  pointer  union  struct  array  or class--with no constructor  Conversely  a non-POD object is one for which a constructor exists  A POD object begins its lifetime when it obtains storage with the proper size for its type and its lifetime ends when the storage for the object is either reused or deallocated   PlainOldData types also must not have any of    Virtual functions  either their own  or inherited   Virtual base classes  direct or indirect     A looser definition of PlainOldData includes objects with constructors  but excludes those with virtual anything  The important issue with PlainOldData types is that they are non-polymorphic  Inheritance can be done with POD types  however it should only be done for ImplementationInheritance  code reuse  and not polymorphism subtyping   A common  though not strictly correct  definition is that a PlainOldData type is anything that doesn t have a VeeTable

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Plain Old Data   In short  it is all built-in data types  e g  int  char  float  long  unsigned char  double  etc   and all aggregation of POD data  Yes  it s a recursive definition      To be more clear  a POD is what we call  a struct   a unit or a group of units that just store data

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The concept of POD and the type trait std  is pod will be deprecated in C  20  See this question for further information

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With C    Plain Old Data doesn t just mean that things like int  char  etc are the only types used  Plain Old Data really means in practice that you can take a struct memcpy it from one location in memory to another and things will work exactly like you would expect  i e  not blow up   This breaks if your class  or any class your class contains  has as a member that is a pointer or a reference or a class that has a virtual function  Essentially  if pointers have to be involved somewhere  its not Plain Old Data

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