Convenient C struct initialisation

Question

I m trying to find a convenient way to initialise  pod  C   structs  Now  consider the following struct   struct FooBar     int foo    float bar        just to make all examples work in C and C    typedef struct FooBar FooBar    If I want to conveniently initialise this in C      I could simply write      A    FooBar fb      foo   12   bar   3 4       illegal C    legal C   Note that I want to explicitly avoid the following notation  because it strikes me as being made to break my neck if I change anything in the struct in the future      B    FooBar fb     12  3 4       legal C    legal C  bad style    To achieve the same  or at least similar  in C   as in the    A    example  I would have to implement an idiotic constructor   FooBar  FooBar int foo  float bar    foo foo   bar bar        - gt     C    FooBar fb 12  3 4     Which is good for boiling water  but not suitable for lazy people  laziness is a good thing  right    Also  it is pretty much as bad as the    B    example  as it does not explicitly state which value goes to which member   So  my question is basically how I can achieve something similar to    A    or better in C    Alternatively  I would be okay with an explanation why I should not want to do this  i e  why my mental paradigm is bad    EDIT  By convenient  I mean also maintainable and non-redundant

User · Answer

You could use a lambda   const FooBar fb     amp         FooBar fb      fb foo   12      fb bar   3 4      return fb         More information on this idiom can be found on Herb Sutter s blog

User · Answer

Extract the contants into functions that describe them  basic refactoring    FooBar fb     foo    bar        I know that style is very close to the one you didn t want to use  but it enables easier replacement of the constant values and also explain them  thus not needing to edit comments   if they ever change that is   Another thing you could do  since you are lazy  is to make the constructor inline  so you don t have to type as much  removing  Foobar    and time spent switching between h and cpp file    struct FooBar     FooBar int f  float b    foo f   bar b       int foo    float bar

User · Answer

I know this question is old  but there is a way to solve this until C  20 finally brings this feature from C to C    What you can do to solve this is use preprocessor macros with static asserts to check your initialization is valid   I know macros are generally bad  but here I don t see another way   See example code below    define INVALID STRUCT ERROR  Instantiation of struct failed  Type  order or number of attributes is wrong     define CREATE STRUCT 1 type  identifier  m 1  p 1      p 1     static assert offsetof type  m 1     0  INVALID STRUCT ERROR      define CREATE STRUCT 2 type  identifier  m 1  p 1  m 2  p 2      p 1  p 2     static assert offsetof type  m 1     0  INVALID STRUCT ERROR    static assert offsetof type  m 2   gt   sizeof identifier m 1   INVALID STRUCT ERROR      define CREATE STRUCT 3 type  identifier  m 1  p 1  m 2  p 2  m 3  p 3      p 1  p 2  p 3     static assert offsetof type  m 1     0  INVALID STRUCT ERROR    static assert offsetof type  m 2   gt   sizeof identifier m 1   INVALID STRUCT ERROR    static assert offsetof type  m 3   gt    offsetof type  m 2    sizeof identifier m 2    INVALID STRUCT ERROR      define CREATE STRUCT 4 type  identifier  m 1  p 1  m 2  p 2  m 3  p 3  m 4  p 4      p 1  p 2  p 3  p 4     static assert offsetof type  m 1     0  INVALID STRUCT ERROR    static assert offsetof type  m 2   gt   sizeof identifier m 1   INVALID STRUCT ERROR    static assert offsetof type  m 3   gt    offsetof type  m 2    sizeof identifier m 2    INVALID STRUCT ERROR    static assert offsetof type  m 4   gt    offsetof type  m 3    sizeof identifier m 3    INVALID STRUCT ERROR        Create more macros for structs with more attributes      Then when you have a struct with const attributes  you can do this   struct MyStruct       const int attr1      const float attr2      const double attr3      const MyStruct test   CREATE STRUCT 3 MyStruct  test  attr1  1  attr2  2 f  attr3  3      It s a bit inconvenient  because you need macros for every possible number of attributes and you need to repeat the type and name of your instance in the macro call  Also you cannot use the macro in a return statement  because the asserts come after the initialization   But it does solve your problem  When you change the struct  the call will fail at compile-time   If you use C  17  you can even make these macros more strict by forcing the same types  e g     define CREATE STRUCT 3 type  identifier  m 1  p 1  m 2  p 2  m 3  p 3      p 1  p 2  p 3     static assert offsetof type  m 1     0  INVALID STRUCT ERROR    static assert offsetof type  m 2   gt   sizeof identifier m 1   INVALID STRUCT ERROR    static assert offsetof type  m 3   gt    offsetof type  m 2    sizeof identifier m 2    INVALID STRUCT ERROR    static assert typeid p 1     typeid identifier m 1   INVALID STRUCT ERROR    static assert typeid p 2     typeid identifier m 2   INVALID STRUCT ERROR    static assert typeid p 3     typeid identifier m 3   INVALID STRUCT ERROR

User · Answer

Option D   FooBar FooBarMake int foo  float bar   Legal C  legal C    Easily optimizable for PODs  Of course there are no named arguments  but this is like all C    If you want named arguments  Objective C should be better choice   Option E   FooBar fb  memset  amp fb  0  sizeof FooBar    fb foo   4  fb bar   15 5f    Legal C  legal C    Named arguments

User · Answer

For versions of C   prior to C  20  which introduces the named initialization  making your option A valid in C     consider the following  int main         struct TFoo   int val         struct TBar   float val          struct FooBar           TFoo foo          TBar bar              FooBar mystruct     TFoo 12   TBar 3 4          std  cout  lt  lt   quot foo    quot   lt  lt  mystruct foo val  lt  lt   quot  bar    quot   lt  lt  mystruct bar val  lt  lt  std  endl      Note that if you try to initialize the struct with FooBar mystruct     TFoo 12   TFoo 3 4     you will get a compilation error  The downside is that you have to create one additional struct for each variable inside your main struct  and also you have to use the inner value with mystruct foo val  But on the other hand  it s clean  simple  pure and standard

User · Answer

Since style A is not allowed in C   and you don t want style B then how about using style BX   FooBar fb        foo    12     bar    3 4             At least help at some extent

User · Answer

Many compilers  C   frontends  including GCC and clang  understand C initializer syntax  If you can  simply use that method

User · Answer

The way    B    is fine in C   also the C  0x is going to extend the syntax so it is useful for C   containers too  I do not understand why you call it bad style   If you want to indicate parameters with names then you can use boost parameter library  but it may confuse someone unfamiliar with it    Reordering struct members is like reordering function parameters  such refactoring may cause problems if you don t do it very carefully

User · Answer

For me the laziest way to allow inline inizialization is use this macro    define METHOD MEMBER TYPE  NAME  CLASS    CLASS  amp set     NAME const TYPE  amp  val    NAME    val  return  this      TYPE NAME   struct foo       METHOD MEMBER string  attr1  foo      METHOD MEMBER int  attr2  foo      METHOD MEMBER double  attr3  foo         inline usage foo test   foo   set attr1  hi   set attr2 22  set attr3 3 14     That macro create attribute and self reference method

User · Answer

Your question is somewhat difficult because even the function   static FooBar MakeFooBar int foo  float bar     may be called as   FooBar fb   MakeFooBar 3 4  5     because of the promotion and conversions rules for built-in numeric types   C has never been really strongly typed   In C    what you want is achievable  though with the help of templates and static assertions   template  lt typename Integer  typename Real gt  FooBar MakeFooBar Integer foo  Real bar      static assert std  is same lt Integer  int gt   value   foo should be of type int      static assert std  is same lt Real  float gt   value   bar should be of type float      return   foo  bar        In C  you may name the parameters  but you ll never get further   On the other hand  if all you want is named parameters  then you write a lot of cumbersome code   struct FooBarMaker     FooBarMaker int f    f f       FooBar Bar float b  const   return FooBar  f  b       int  f      static FooBarMaker Foo int f    return FooBarMaker f         Usage FooBar fb   Foo 5  Bar 3 4     And you can pepper in type promotion protection if you like

User · Answer

Designated initializes will be supported in c  2a  but you don t have to wait  because they are officialy supported by GCC  Clang and MSVC   include  lt iostream gt   include  lt filesystem gt   struct hello world       const char  hello      const char  world      int main           hello world hw              hello    quot hello   quot            world    quot world  quot                  std  cout  lt  lt  hw hello  lt  lt  hw world  lt  lt  std  endl      return 0     GCC Demo MSVC Demo Update 20201 As  Code Doggo noted  anyone who is using Visual Studio 2019 will need to set  std c  latest   for the  quot C   Language Standard quot  field contained under Configuration Properties - gt  C C   - gt  Language

User · Answer

What about this syntax   typedef struct       int a      short b    ABCD   ABCD abc     abc a   5  abc b   7      Just tested on a Microsoft Visual C   2015 and on g   6 0 2  Working OK  You can make a specific macro also if you want to avoid duplicating variable name

User · Answer

Yet another way in C   is  struct Point   private    int x   int y   public      Point amp  setX int xIn    x   Xin  return  this       Point amp  setY int yIn    y   Yin  return  this       Point pt  pt setX 20  setY 20

[c++] Convenient C++ struct initialisation

Examples related to c++

Examples related to struct

Examples related to initialization