Static array initialisation

[Ali Çehreli]

On 4/1/21 2:30 AM, DLearner wrote:

 >     immutable uint  MemSize=100;  // Memory size in bytes.

 >     ubyte[MemSize]  MemPool = 8;  // Initialised to 8 for debugging.

Valid index values there are from 0 to 99, inclusive.

 >     WkPtr = &MemPool[0];
 >
 >     counter = 1;
 >     while (counter <= 102) {
 >
 >        idx = counter - 1;

So, idx will have the incorrect values of 100 and 101.

 >        WkUbyte = *(WkPtr + idx);

That's undefined behavior. We can talk about variable placement on the 
stack, padding, alignment, etc. but we cannot attempt to prove how much 
memory is allocated for ubyte[100] by using undefined behavior like that 
code.

We can have a ubyte[100] as a member of a struct and then look at the 
bytes of an instance of that struct but we can't walk over the bytes of 
the function call stack randomly. I mean, we can, but the observations 
may not make sense.

Ali

P.S. MemPool.sizeof is the amount of memory used for ubyte[100]. Here is 
proof:

void main() {
   alias T = ubyte[100];
   T[2] arr;
   assert(arr.sizeof == 2 * T.sizeof);    // Passes
}

As you can see, there is nothing other than 100 bytes used for 
ubyte[100]. (The results may be different for different types due to 
alignment requirements.)