Are members of a C struct initialized to 0 by default

Question

I have this struct   struct Snapshot       double x       int y       I want x and y to be 0  Will they be 0 by default or do I have to do   Snapshot s    0 0     What are the other ways to zero out the structure

User · Answer

Move pod members to a base class to shorten your initializer list   struct foo pod       int x      int y      int z      struct foo   foo pod       std  string name      foo std  string name            foo pod             name name                  int main         foo f  bar        printf   d  d  d  s n   f x  f y  f z  f name c str

User · Answer

Since this is a POD  essentially a C struct  there is little harm in initialising it the C way   Snapshot s  memset  amp s  0  sizeof  s      or similarly  Snapshot  sp   new Snapshot  memset sp  0  sizeof   sp      I wouldn t go so far as to use calloc   in a C   program though

User · Answer

In general  no  However  a struct declared as file-scope or static in a function  will  be initialized to 0  just like all other variables of those scopes    int x     0 int y   42     42 struct   int a  b    foo     0  0  void foo       struct   int a  b    bar     undefined   static struct   int c  d    quux     0  0

User · Answer

I believe the correct answer is that their values are undefined   Often  they are initialized to 0 when running debug versions of the code   This is usually not the case when running release versions

User · Answer

With POD you can also write  Snapshot s         You shouldn t use memset in C    memset has the drawback that if there is a non-POD in the struct it will destroy it   or like this   struct init     template  lt typename T gt    operator T              return new T            Snapshot  s   init

User · Answer

No  they are not 0 by default   The simplest way to ensure that all values or defaulted to 0 is to define a constructor  Snapshot     x 0   y 0        This ensures that all uses of Snapshot will have initialized values

User · Answer

In C    use no-argument constructors  In C you can t have constructors  so use either memset or - the interesting solution - designated initializers   struct Snapshot s      x   0 0   y   0 0

User · Answer

They are not null if you don t initialize the struct    Snapshot s     receives no initialization Snapshot s          value initializes all members   The second will make all members zero  the first leaves them at unspecified values  Note that it is recursive   struct Parent   Snapshot s     Parent p     receives no initialization Parent p          value initializes all members   The second will make p s  x y  zero  You cannot use these aggregate initializer lists if you ve got constructors in your struct  If that is the case  you will have to add proper initalization to those constructors  struct Snapshot       int x      double y      Snapshot   x 0  y 0             other ctors   functions         Will initialize both x and y to 0  Note that you can use x    y   to initialize them disregarding of their type  That s then value initialization  and usually yields a proper initial value  0 for int  0 0 for double  calling the default constructor for user defined types that have user declared constructors        This is important especially if your struct is a template

[c++] Are members of a C++ struct initialized to 0 by default?

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