C Structure Initialization

Question

Is it possible to initialize structs in C   as indicated below  struct address       int street no      char  street name      char  city      char  prov      char  postal code     address temp address          city    Hamilton    prov    Ontario       The links here and here mention that it is possible to use this style only in C   If so why is this not possible in C     Is there any underlying technical reason why it is not implemented in C    or is it bad practice to use this style   I like using this way of initializing because my struct is big and this style gives me clear readability of what value is assigned to which member   Please share with me if there are other ways through which we can achieve the same readability   I have referred the following links before posting this question   C C   for AIX C Structure Initialization with Variable Static structure initialization with tags in C   C  11 Proper Structure Initialization

User · Answer

I found this way of doing it for global variables, that does not require to modify the original structure definition :

struct address {
             int street_no;
             char *street_name;
             char *city;
             char *prov;
             char *postal_code;
           };

then declare the variable of a new type inherited from the original struct type and use the constructor for fields initialisation :

struct temp_address : address { temp_address() { 
    city = "Hamilton"; 
    prov = "Ontario"; 
} } temp_address;

Not quite as elegant as the C style though ...

For a local variable it requires an additional memset(this, 0, sizeof(*this)) at the beginning of the constructor, so it's clearly not worse it and @gui13 's answer is more appropriate.

(Note that 'temp_address' is a variable of type 'temp_address', however this new type inherit from 'address' and can be used in every place where 'address' is expected, so it's OK.)

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After my question resulted in no satisfying result  because C   doesn t implement tag-based init for structures   I took the trick I found here  Are members of a C   struct initialized to 0 by default   For you it would amount to do that   address temp address          will zero all fields in C   temp address city    Hamilton   temp address prov    Ontario     This is certainly the closest to what you wanted originally  zero all the fields except those you want to initialize

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It s not implemented in C     also  char  strings  I hope not    Usually if you have so many parameters it is a fairly serious code smell  But instead  why not simply value-initialize the struct and then assign each member

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It is possible  but only if the struct you re initializing is a POD  plain old data  struct  It cannot contain any methods  constructors  or even default values

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In C   the C-style initializers were replaced by constructors which by compile time can ensure that only valid initializations are performed  i e  after initialization the object members are consistent    It is a good practice  but sometimes a pre-initialization is handy  like in your example  OOP solves this by abstract classes or creational design patterns   In my opinion  using this secure way kills the simplicity and sometimes the security trade-off might be too expensive  since simple code does not need sophisticated design to stay maintainable   As an alternative solution  I suggest to define macros using lambdas to simplify the initialization to look almost like C-style   struct address     int street no    const char  street name    const char  city    const char  prov    const char  postal code      define ADDRESS OPEN      address        define ADDRESS CLOSE   return         define ADDRESS x  ADDRESS OPEN x ADDRESS CLOSE   The ADDRESS macro expands to       address          definition         return          which creates and calls the lambda  Macro parameters are also comma separated  so you need to put the initializer into brackets and call like  address temp address   ADDRESS     city    Hamilton     prov    Ontario        You could also write generalized macro initializer   define INIT OPEN type       type        define INIT CLOSE   return         define INIT type x  INIT OPEN type  x INIT CLOSE   but then the call is slightly less beautiful  address temp address   INIT address     city    Hamilton     prov    Ontario        however you can define the ADDRESS macro using general INIT macro easily   define ADDRESS x  INIT address x

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This feature is called designated initializers  It is an addition to the C99 standard  However  this feature was left out of the C  11  According to The C   Programming Language  4th edition  Section 44 3 3 2  C Features Not Adopted by C         A few additions to C99  compared with C89  were deliberately not adopted in C           1  Variable-length arrays  VLAs   use vector or some form of dynamic array        2  Designated initializers  use constructors   The C99 grammar has the designated initializers  See ISO IEC 9899 2011  N1570 Committee Draft - April 12  2011   6 7 9 Initialization  initializer      assignment-expression       initializer-list         initializer-list     initializer-list      designation opt initializer     initializer-list   designationopt initializer designation      designator-list   designator-list      designator     designator-list designator designator        constant-expression         identifier   On the other hand  the C  11 does not have the designated initializers  See ISO IEC 14882 2011  N3690 Committee Draft - May 15  2013   8 5 Initializers  initializer      brace-or-equal-initializer       expression-list   brace-or-equal-initializer        initializer-clause     braced-init-list initializer-clause      assignment-expression     braced-init-list initializer-list      initializer-clause    opt     initializer-list   initializer-clause    opt braced-init-list        initializer-list  opt             In order to achieve the same effect  use constructors or initializer lists

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You can even pack Gui13 s solution into single initialization statement   struct address                    int street no                   char  street name                   char  city                   char  prov                   char  postal code                      address ta    ta   address    ta city    Hamilton   ta prov    Ontario   ta     Disclaimer  I don t recommend this style

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I faced a similar problem today  where I have a struct that I want to fill with test data which will be passed as arguments to a function I m testing   I wanted to have a vector of these structs and was looking for a one-liner method to initialize each struct   I ended up going with a constructor function in the struct  which I believe was also suggested in a few answers to your question   It s probably bad practice to have the arguments to the constructor have the same names as the public member variables  requiring use of the this pointer   Someone can suggest an edit if there is a better way   typedef struct testdatum s       public      std  string argument1      std  string argument2      std  string argument3      std  string argument4      int count       testdatum s           std  string argument1          std  string argument2          std  string argument3          std  string argument4          int count                this- gt rotation   argument1          this- gt tstamp   argument2          this- gt auth   argument3          this- gt answer   argument4          this- gt count   count           testdatum    Which I used in in my test function to call the function being tested with various arguments like this   std  vector lt testdatum gt  testdata   testdata push back testdatum  val11    val12    val13    val14   5    testdata push back testdatum  val21    val22    val23    val24   1    testdata push back testdatum  val31    val32    val33    val34   7     for  std  vector lt testdatum gt   iterator i   testdata begin    i    testdata end      i        function in test i- gt argument1  i- gt argument2  i- gt argument3  i- gt argument4m i- gt count

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The field identifiers are indeed C initializer syntax   In C   just give the values in the correct order without the field names  Unfortunately this means you need to give them all  actually you can omit trailing zero-valued fields and the result will be the same    address temp address     0  0   Hamilton    Ontario   0

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I know this question is quite old  but I found another way of initializing  using constexpr and currying   struct mp struct t       public          constexpr mp struct t int member1    mp struct t member1  0  0             constexpr mp struct t int member1  int member2  int member3    member1 member1   member2 member2   member3 member3             constexpr mp struct t another member int member    return  member1  member      member2             constexpr mp struct t yet another one int member    return  member1  member2  member          int member1  member2  member3      static mp struct t a struct   mp struct t 1                              another member 2                              yet another one 3     This method also works for global static variables and even constexpr ones  The only disadvantage is the bad maintainability  Everytime another member has to be made initializable using this method  all member initialization methods have to be changed

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Inspired by this really neat answer   https   stackoverflow com a 49572324 4808079   You can do lamba closures      Nobody wants to remember the order of these things struct SomeBigStruct     int min   1    int mean   3     int mode   5    int max   10    string name    string nickname           the list goes on        class SomeClass     static const inline SomeBigStruct voiceAmps           ModulationTarget             min   0          nickname    Bobby         bloodtype    O-       return               Or  if you want to be very fancy   define DesignatedInit T              T        VA ARGS    return         class SomeClass     static const inline SomeBigStruct voiceAmps   DesignatedInit      ModulationTarget        min   0        nickname    Bobby         bloodtype    O-            There are some drawbacks involved with this  mostly having to do with uninitialized members   From what the linked answers comments say  it compiles efficiently  though I have not tested it     Overall  I just think it s a neat approach

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I might be missing something here  by why not    include  lt cstdio gt      struct Group       int x      int y      const char  s      int main          Group group        x   1        y   2        s    Hello it works         printf   d   d   s   group x  group y  group s

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As others have mentioned this is designated initializer   This feature is part of C  20

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If you want to make it clear what each initializer value is  just split it up on multiple lines  with a comment on each   address temp addres       0      street no   nullptr      street name    Hamilton       city    Ontario       prov   nullptr      postal code

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You can just initialize via a constructor  struct address     address     city  quot Hamilton quot    prov  quot Ontario quot        int street no    char  street name    char  city    char  prov    char  postal code

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In GNUC    seems to be obsolete since 2 5  a long time ago    See the answers here  C struct initialization using labels  It works  but how    it is possible to initialize a struct like this   struct inventory item       int bananas      int apples      int pineapples      inventory item first item         bananas  2      apples  49      pineapples  4

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