Default values in a C Struct

Question

I have a data structure like this        struct foo           int id          int route          int backup route          int current route          and a function called update   that is used to request changes in it       update 42  dont care  dont care  new route     this is really long and if I add something to the structure I have to add a  dont care  to EVERY call to update           I am thinking about passing it a struct instead but filling in the struct with  dont care  beforehand is even more tedious than just spelling it out in the function call  Can I create the struct somewhere with default values of dont care and just set the fields I care about after I declare it as a local variable        struct foo bar      id   42   current route   new route        update  bar     What is the most elegant way to pass just the information I wish to express to the update function    and I want everything else to default to -1  the secret code for  dont care

User · Answer

lt stdarg h gt  allows you to define variadic functions  which accept an indefinite number of arguments  like printf      I would define a function which took an arbitrary number of pairs of arguments  one which specifies the property to be updated  and one which specifies the value   Use an enum or a string to specify the name of the property

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While macros and or functions  as already suggested  will work  and might have other positive effects  i e  debug hooks    they are more complex than needed  The simplest and possibly most elegant solution is to just define a constant that you use for variable initialisation   const struct foo FOO DONT CARE        or maybe FOO DEFAULT or something     dont care  dont care  dont care  dont care        struct foo bar   FOO DONT CARE  bar id   42  bar current route   new route  update  amp bar     This code has virtually no mental overhead of understanding the indirection  and it is very clear which fields in bar you set explicitly while  safely  ignoring those you do not set

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How about something like   struct foo bar  update init id 42  init dont care  amp bar       with   struct foo  init dont care struct foo  bar      bar- gt id   dont care    bar- gt route   dont care    bar- gt backup route   dont care    bar- gt current route   dont care    return bar      and   struct foo  init id int id  struct foo  bar      bar- gt id   id    return bar      and correspondingly   struct foo  init route int route  struct foo  bar   struct foo  init backup route int backup route  struct foo  bar   struct foo  init current route int current route  struct foo  bar     In C    a similar pattern has a name which I don t remember just now   EDIT  It s called the Named Parameter Idiom

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You could address the problem with an X-Macro  You would change your struct definition into    define LIST OF foo MEMBERS       X int id        X int route        X int backup route        X int current route     define X type name  type name  struct foo       LIST OF foo MEMBERS      undef X   And then you would be able to easily define a flexible function that sets all fields to dont care    define X type name  in- gt name   dont care      void setFooToDontCare struct foo  in        LIST OF foo MEMBERS     undef X   Following the discussion here  one could also define a default value in this way    define X name  dont care  const struct foo foo DONT CARE     LIST OF STRUCT MEMBERS foo     undef X   Which translates into   const struct foo foo DONT CARE    dont care  dont care  dont care  dont care      And use it as in hlovdal answer  with the advantage that here maintenance is easier  i e  changing the number of struct members will automatically update foo DONT CARE  Note that the last  spurious  comma is acceptable   I first learned the concept of X-Macros when I had to address this problem   It is extremely flexible to new fields being added to the struct  If you have different data types  you could define different dont care values depending on the data type  from here  you could take inspiration from the function used to print the values in the second example   If you are ok with an all int struct  then you could omit the data type from LIST OF foo MEMBERS and simply change the X function of the struct definition into  define X name  int name

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Since it looks like that you only need this structure for the update   function  don t use a structure for this at all  it will only obfuscate your intention behind that construct  You should maybe rethink why you are changing and updating those fields and define separate functions or macros for this  little  changes   e g     define set current route id  route  update id  dont care  dont care  route   define set route id  route  update id  dont care  route  dont care   define set backup route id  route  update id  route  dont care  dont care    Or even better write a function for every change case  As you already noticed you don t change every property at the same time  so make it possible to change only one property at a time  This doesn t only improve the readability  but also helps you handling the different cases  e g  you don t have to check for all the  dont care  because you know that only the current route is changed

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The most elegant way would be to update the struct fields directly  without having to use the update   function - but maybe there are good reasons for using it that don t come across in the question   struct foo  bar   get foo ptr    foo ref id   42  foo ref current route   new route    Or you can  like Pukku suggested  create separate access functions for each field of the struct   Otherwise the best solution I can think of is treating a value of  0  in a struct field as a  do not update  flag - so you just create a funciton to return a zeroed out struct  and then use this to update   struct foo empty foo void        struct foo bar      bzero  amp bar  sizeof  struct bar        return bar         struct foo bar   empty foo    bar id 42  bar current route   new route  update  amp bar     However  this might not be very feasible  if 0 is a valid value for fields in the struct

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You can change your secret special value to 0  and exploit C s default structure-member semantics  struct foo bar      id   42   current route   new route    update  amp bar     will then pass 0 as members of bar unspecified in the initializer   Or you can create a macro that will do the default initialization for you    define FOO INIT         id   -1   current route   -1   quux   -1       VA ARGS      struct foo bar   FOO INIT   id   42   current route   new route    update  amp bar

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Perhaps consider using a preprocessor macro definition instead    define UPDATE ID instance  id       instance - gt id   id       define UPDATE ROUTE instance  route       instance - gt route    route       define UPDATE BACKUP ROUTE instance  route       instance - gt backup route    route       define UPDATE CURRENT ROUTE instance  route       instance - gt current route    route        If your instance of  struct foo  is global  then you don t need the parameter for that of course   But I m assuming you probably have more than one instance   Using the           block is a GNU-ism that that applies to GCC   it is a nice  safe  way to keep lines together as a block   If you later need to add more to the macros  such as range validation checking  you won t have to worry about breaking things like if else statements and so forth   This is what I would do  based upon the requirements you indicated   Situations like this are one of the reasons that I started using python a lot  handling default parameters and such becomes a lot simpler than it ever is with C    I guess that s a python plug  sorry  -

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I m rusty with structs  so I m probably missing a few keywords here  But why not start with a global structure with the defaults initialized  copy it to your local variable  then modify it   An initializer like   void init struct  structType   s        memcopy s  amp defaultValues sizeof structType        Then when you want to use it   structType foo  init struct   amp foo       get defaults foo fieldICareAbout   1     modify fields update   amp foo       pass to function

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One pattern gobject uses is a variadic function  and enumerated values for each property  The interface looks something like   update  ID  1          BACKUP ROUTE  4          -1      -1 terminates the parameter list      Writing a varargs function is easy -- see http   www eskimo com  scs cclass int sx11b html  Just match up key -  value pairs and set the appropriate structure attributes

[c] Default values in a C Struct

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