Hello

I'm trying to compile the following program:

module main;

int main(string[] argv){
	short asd = 1;
	short qwe = asd + asd;
	return 0;
}

And the compiler gives this error:

C:\David>dmd simple
simple.d(5): Error: cannot implicitly convert expression (cast(int)asd + cast(in
t)asd) of type int to short

Why is there a cast if I'm adding to shorts?

Regards

On 06/23/2014 10:03 PM, David Zaragoza wrote:> Hello
>
> I'm trying to compile the following program:
>
> module main;
>
> int main(string[] argv){
>      short asd = 1;
>      short qwe = asd + asd;
>      return 0;
> }
>
> And the compiler gives this error:
>
> C:\David>dmd simple
> simple.d(5): Error: cannot implicitly convert expression (cast(int)asd +
> cast(in
> t)asd) of type int to short
>
> Why is there a cast if I'm adding to shorts?
>
> Regards

This is a common gotcha of system languages like C, C++, and D. Operations like + are never executed in types like 'short'.[1] Since the result of that + is int, the compiler does not allow assigning the value back to a short, which can lose data.

However, in this case it is clear that the value of asd is known to be 1 at compile time and that there will not be data loss. The D compilers apply what is known as 'value range propagation'[2], which should take care of your issue. I guess the use in your program is too complicated for the current compiler.

Ali

[1] See both "Integer Promotions" and "Usual Arithmetic Conversions" here:

  http://dlang.org/type.html

[2] "Value Range Propagation"

  http://www.drdobbs.com/tools/value-range-propagation/229300211

Interesting info, I've never seen this behavior in C since the conversion is implicit as [2] notes. A simple cast resolves the problem in D:

module main;

int main(string[] argv){
	short asd = 1;
	short qwe = cast(short)(asd + asd);
	return 0;
}

Thanks for your answer :)