Struct Constructor in C

Question

Can a struct have a constructor in C     I have been trying to solve this problem but I am not getting the syntax

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In C   the only difference between a class and a struct is that members and base classes are private by default in classes  whereas they are public by default in structs    So structs can have constructors  and the syntax is the same as for classes

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Syntax is as same as of class in C    If you aware of creating constructor in c   then it is same in struct   struct Date       int day       Date int d                day   d             void printDay                 cout  lt  lt   day    lt  lt  day  lt  lt  endl             Struct can have all things as class in c    As earlier said difference is only that by default C   member have private access but in struct it is public But as per programming consideration Use the struct keyword for data-only structures  Use the class keyword for objects that have both data and functions

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In C   both struct  amp  class are equal except struct sdefault member access specifier is public  amp  class has private   The reason for having struct in C   is C   is a superset of C and must have backward compatible with legacy C types   For example if the language user tries to include some C header file legacy-c h in his C    code  amp  it contains struct Test  int x y    Members of struct Test should be accessible as like C

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struct HaveSome      int fun     HaveSome              fun   69            I d rather initialize inside the constructor so I don t need to keep the order

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In c   struct and c   class have only one difference by default struct members are public and class members are private     Here  C   program constructor in struct     include  lt iostream gt  using namespace std   struct hello           public        by default also it is public         hello                 hello            hello  hello             cout lt  lt  calling constructor      lt  lt endl         hello   hello             cout lt  lt  calling destructor      lt  lt endl         int main     hello obj         creating a hello obj  calling hello constructor and destructor   return 0

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Yes  but if you have your structure in a union then you cannot   It is the same as a class   struct Example      unsigned int mTest     Example                  Unions will not allow constructors in the structs   You can make a constructor on the union though  This question relates to non-trivial constructors in unions

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Note that there is one interesting difference  at least with the MS C   compiler       If you have a plain vanilla struct like this  struct MyStruct      int id     double x     double y    MYSTRUCT    then somewhere else you might initialize an array of such objects like this   MYSTRUCT  pointList             1  1 0  1 0          2  1 0  2 0          3  2 0  1 0        however  as soon as you add a user-defined constructor to MyStruct such as the ones discussed above  you d get an error like this         MyStruct    Types with user defined constructors are not aggregate       lt file and line gt    error C2552    pointList    non-aggregates cannot       be initialized with initializer list     So that s at least one other difference between a struct and a class   This kind of initialization may not be good OO practice  but it appears all over the place in the legacy WinSDK c   code that I support   Just so you know

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As the other answers mention  a struct is basically treated as a class in C    This allows you to have a constructor which can be used to initialise the struct with default values  Below  the constructor takes sz and b as arguments  and initializes the other variables to some default values   struct blocknode       unsigned int bsize      bool free      unsigned char  bptr      blocknode  next      blocknode  prev       blocknode unsigned int sz  unsigned char  b  bool f   true                blocknode  p   0  blocknode  n   0                  bsize sz   free f   bptr b   prev p   next n          Usage   unsigned char  bptr   new unsigned char 1024   blocknode  fblock   new blocknode 1024  btpr

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All the above answers technically answer the asker s question  but just thought I d point out a case where you might encounter problems   If you declare your struct like this   typedef struct  int x  foo        foo    You will have problems trying to declare a constructor   This is of course because you haven t actually declared a struct named  foo   you ve created an anonymous struct and assigned it the alias  foo    This also means you will not be able to use  foo  with a scoping operator in a cpp file   foo h   typedef struct  int x  void myFunc int y     foo    foo cpp      lt -- This will not work because the struct  foo  was never declared  void foo  myFunc int y        do something        To fix this  you must either do this   struct foo  int x  foo           or this   typedef struct foo  int x  foo        foo    Where the latter creates a struct called  foo  and gives it the alias  foo  so you don t have to use the struct keyword when referencing it

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Yes  A structure is just like a class  but defaults to public   in the class definition and when inheriting   struct Foo       int bar       Foo void        bar 0                  Considering your other question  I would suggest you read through some tutorials  They will answer your questions faster and more complete than we will

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One more example but using this keyword when setting value in constructor    include  lt iostream gt   using namespace std   struct Node       int value       Node int value            this- gt value   value             void print                 cout  lt  lt  this- gt value  lt  lt  endl            int main         Node n   Node 10       n print         return 0      Compiled with GCC 8 1 0

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Class  Structure and Union is described in below table in short

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In C    we can declare define the structure just like class and have the constructors destructors for the Structures and have variables functions defined in it  The only difference is the default scope of the variables functions defined  Other than the above difference  mostly you should be able to imitate the functionality of class using structs

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Yes structures and classes in C   are the same except that structures members are public by default whereas classes members are private by default   Anything you can do in a class you should be able to do in a structure   struct Foo     Foo              Initialize Foo

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struct TestStruct           int id          TestStruct     id 42

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Yes it possible to have constructor in structure here is one example    include lt iostream h gt   struct a     int x    a   x 100       int main       struct a a1    getch

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