default value for struct member in C

Question

Is it possible to set default values for some struct member  I tried the following but  it d cause syntax error   typedef struct     int flag   3    MyStruct    Errors     gcc -o testIt test c  test c 7  error  expected                                    or      attribute      before         token test c  In function    main     test c 17  error     struct  lt anonymous gt     has no member named    flag

User · Answer

You can create a function for it   typedef struct       int id      char name    employee   void set iv employee  em    int main        employee em0  set iv  amp em0      void set iv employee  em         em  id   0        em  name    none

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I agree with Als that you can not initialize at time of defining the structure in C  But you can initialize the structure at time of creating instance shown as below   In C    struct s           int i          int j              struct s s instance     10  20      in C   its possible to give direct value in definition of structure shown as below  struct s       int i       s    i 10

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Structure is a data type  You don t give values to a data type  You give values to instances objects of data types  So no this is not possible in C      Instead you can write a function which does the initialization for structure instance   Alternatively  You could do   struct MyStruct s        int id    MyStruct default    3    typedef struct MyStruct s MyStruct    And then always initialize your new instances as   MyStruct mInstance   MyStruct default

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You can use combination of C preprocessor functions with varargs  compound literals and designated initializers for maximum convenience   typedef struct       int id      char  name    employee    define EMPLOYEE        employee     id   0   name    none       VA ARGS       employee john   EMPLOYEE  name  John        no id initialization employee jane   EMPLOYEE  id 5              no name initialization

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An initialization function to a struct is a good way to grant it default values   Mystruct s  Mystruct init  amp s     Or even shorter   Mystruct s   Mystruct init        this time init returns a struct

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You can implement an initialisation function   employee init employee       empolyee const e    0  none      return e

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I think the following way you can do it   typedef struct     int flag   3    MyStruct

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Another approach  if the struct allows it  is to use a  define with the default values inside    define MYSTRUCT INIT   0  0  true    typedef struct       int id      int flag      bool foo    MyStruct    Use   MyStruct val   MYSTRUCT INIT

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Another approach to default values   Make an initialization function with the same type as the struct   This approach is very useful when splitting large code into separate files    struct structType    int flag      struct structType InitializeMyStruct        struct structType structInitialized      structInitialized flag   3      return structInitialized         int main        struct structType MyStruct   InitializeMyStruct

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Create a default struct as the other answers have mentioned   struct MyStruct       int flag     MyStruct default    3     However  the above code will not work in a header file - you will get error  multiple definition of  MyStruct default   To solve this problem  use extern instead in the header file   struct MyStruct       int flag      extern const struct MyStruct MyStruct default    And in the c file   const struct MyStruct MyStruct default    3     Hope this helps anyone having trouble with the header file

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you can not do it in this way  Use the following instead  typedef struct      int id     char  name   employee   employee emp      id   0    name    none       You can use macro to define and initialize your instances  this will make easiier to you each time you want to define new instance and initialize it   typedef struct      int id     char  name   employee    define INIT EMPLOYEE X  employee X     id   0   name   none     and in your code when you need to define new instance with employee type  you just call this macro like   INIT EMPLOYEE emp

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If you only use this structure for once  i e  create a global static variable  you can remove typedef  and initialized this variable instantly   struct       int id      char  name    employee          id   0       name    none       Then  you can use employee in your code after that

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You can do   struct employee s     int id    char  name    employee default    0   none     typedef struct employee s employee    And then you just have to remember to do the default initialization when you declare a new employee variable   employee foo   employee default    Alternatively  you can just always build your employee struct via a factory function

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Even more so  to add on the existing answers  you may use a macro that hides a struct initializer    define DEFAULT EMPLOYEE   0   none      Then in your code   employee john   DEFAULT EMPLOYEE

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You can use some function to initialize struct as follows   typedef struct       int flag    MyStruct   MyStruct GetMyStruct int value        MyStruct My    0       My flag   value      return My     void main  void        MyStruct temp      temp   GetMyStruct 3       printf   d n   temp flag       EDIT   typedef struct       int flag    MyStruct   MyStruct MyData 20    MyStruct GetMyStruct int value        MyStruct My    0       My flag   value      return My     void main  void        int i      for  i   0  i  lt  20  i             MyData i    GetMyStruct 3        for  i   0  i  lt  20  i             printf   d n   MyData i  flag

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If you are using gcc you can give designated initializers in object creation   typedef struct      int id 0     char  name  none    employee   employee e        id   0    name    none        Or   simply use like array initialization    employee e    0    none

[c] default value for struct member in C

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