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From: Thiago Adams <thiago.adams@gmail.com>
Newsgroups: comp.lang.c
Subject: how cast works?
Date: Wed, 7 Aug 2024 08:28:09 -0300
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How cast works?
Does it changes the memory?
For instance, from "unsigned int" to "signed char".
Is it just like discarding bytes or something else?

For instance, any 4 bytes type, cast to 2 bytes type is just the lower 2 
bytes?


[A][B][C][D]
->
[A][B]

I also would like to understand better signed and unsigned.
There is no such think as "signed" or "unsigned" register, right?
How about floating point?


The motivation problem.
I have a union, with unsigned int, unsigned char etc.(all types)
I need to execute a cast in runtime (like script).
The problem is that this causes an explosion of combinations that I am 
trying to avoid.

---------------------------------------------------------

enum type {
     TYPE_UNSIGNED_CHAR,
     TYPE_UNSIGNED_INT,
};
struct number {
     enum type type;
     union U {
         unsigned int unsigned_int_value;
         unsigned char unsigned_char_value;
     } data;
};

struct number cast_to(enum type t, const struct number* n) {
     struct number r = {0};
     r.type = t;
     r.data = n->data;
     //maybe fill with zeros the extra bytes?
     return r;
}

// All combinations...
struct number cast_to2(enum type t, const struct number* n)
{
   struct number r = {0};
   r.type = n->type;

   switch (t)
   {
     case TYPE_UNSIGNED_CHAR:
        switch (n->type)
        {
          case TYPE_UNSIGNED_CHAR:
            r.data.unsigned_char_value = n->data.unsigned_char_value;
            break;
          case TYPE_UNSIGNED_INT:
            r.data.unsigned_char_value = n->data.unsigned_int_value;
            break;
        }
        break;

     case TYPE_UNSIGNED_INT:
       switch (t)
       {
          case TYPE_UNSIGNED_CHAR:
            r.data.unsigned_int_value = n->data.unsigned_char_value;
            break;

         case TYPE_UNSIGNED_INT:
           r.data.unsigned_int_value = n->data.unsigned_int_value;
         break;
    }
    break;
  }
  return r;
}