Struct tm doesn't initialize to "0"? WTF?

Objective-C Programming (2nd Edition) 11. Structs
1

OK, kept getting “crazy talk” from my struct tm’s. Finally figured out they aren’t empty when they are initialized…inside the “main” function - had to move them out. Why does it matter where they are initialized??

2

Instances of structs in Objective-C are not always guaranteed to be initialized automatically. You just have to remember this and proceed with caution.

For example:

struct String
{
   unsigned int length;
   char * value;
};

s is not initialized:

void bar ()
{
   struct String s;

   unsigned int len = s.length;        // len == ?
   char * value = s.value;             // value == ?
}

s is initialized because it is in global scope:

struct String s;

void bar ()
{
   unsigned int len = s.length;        // len == 0
   char * value = s.value;             // value == 0
}

s is initialized because it is static:

void bar ()
{
   static struct String s;

   unsigned int len = s.length;        // len == 0
   char * value = s.value;             // value == 0
}

s is initialized explicitly:

void bar ()
{
    struct String s = {6, "jibber"};

    unsigned int len = s.length;        // len == 6
    char * value = s.value;             // value == "jibber"

}

s is initialized explicitly:

void bar ()
{
   struct String s;
   memset (&s, 0, sizeof (s));
   unsigned int len = s.length;        // len == 0
   char * value = s.value;             // value == 0

}