Re: why ushort alias casted to int?

2017-12-22 Thread Patrick Schluter via Digitalmars-d-learn 

On Friday, 22 December 2017 at 10:14:48 UTC, crimaniak wrote:

My code:

alias MemSize = ushort;

struct MemRegion
{
MemSize start;
MemSize length;
@property MemSize end() const { return start+length; }
}

Error: cannot implicitly convert expression 
`cast(int)this.start + cast(int)this.length` of type `int` to 
`ushort`


Both operands are the same type, so as I understand casting to 
longest type is not needed at all, and longest type here is 
ushort in any case. What am I doing wrong?


@property MemSize end() const { return cast(int)(start+length); }

The rule of int promotion of smaller types comes from C as they 
said. There are 2 reason to do it that way. int is supposed in C 
to be the natural arithmetic type of the CPU it runs on, i.e. the 
default size the processor has the least difficulties to handle. 
The second reason is that it allows to detect easily without much 
hassle if the result of the operation is in range or not. When 
doing arithmetic with small integer types, it is easy that the 
result overflows. It is not that easy to define portably this 
overflow behaviour. On some cpus it would require extra 
instructions. D has inherited this behaviour so that copied 
arithmetic code coming from C behaves in the same way.


@property MemSize end() const
{
  MemSize result = start+length;
  assert(result <= MemSize.max);
  return cast(int)result;
}

with overflow arithmetic this code is not possible (as is if 
MemSize was uint or ulong).

Re: why ushort alias casted to int?

2017-12-22 Thread Nathan S. via Digitalmars-d-learn 

On Friday, 22 December 2017 at 10:42:28 UTC, crimaniak wrote:
Hm, really. ok, I will use the explicit cast, but I don't like 
it.


It's because the C programming language has similar integer 
promotion rules. That doesn't make it any more convenient if you 
weren't expecting it but that is the reason behind it.

Re: why ushort alias casted to int?

2017-12-22 Thread crimaniak via Digitalmars-d-learn 

On Friday, 22 December 2017 at 10:18:52 UTC, ketmar wrote:

crimaniak wrote:

Both operands are the same type, so as I understand casting to 
longest type is not needed at all, and longest type here is 
ushort in any case. What am I doing wrong?


it is hidden in specs: all types shorter than int are promoted 
to int before doing any math.


Hm, really. ok, I will use the explicit cast, but I don't like it.

Re: why ushort alias casted to int?

2017-12-22 Thread ketmar via Digitalmars-d-learn 

crimaniak wrote:

Both operands are the same type, so as I understand casting to longest 
type is not needed at all, and longest type here is ushort in any case. 
What am I doing wrong?


it is hidden in specs: all types shorter than int are promoted to int 
before doing any math.

why ushort alias casted to int?

2017-12-22 Thread crimaniak via Digitalmars-d-learn 

My code:

alias MemSize = ushort;

struct MemRegion
{
MemSize start;
MemSize length;
@property MemSize end() const { return start+length; }
}

Error: cannot implicitly convert expression `cast(int)this.start 
+ cast(int)this.length` of type `int` to `ushort`


Both operands are the same type, so as I understand casting to 
longest type is not needed at all, and longest type here is 
ushort in any case. What am I doing wrong?