help cast

[sdvcn]

On Sunday, 18 March 2018 at 20:07:28 UTC, Jonathan M Davis wrote:
> On Sunday, March 18, 2018 14:56:04 Steven Schveighoffer via 
> Digitalmars-d wrote:
>> On 3/18/18 2:24 PM, Jonathan M Davis wrote:
>> > On Sunday, March 18, 2018 13:10:28 Steven Schveighoffer via 
>> > Digitalmars-d
>> >
>> > wrote:
>> >> On 3/18/18 4:34 AM, sdvcn wrote:
>> >>> dchar v11=dchar.max;
>> >>>
>> >>>       auto vp11 = [v11];
>> >>>
>> >>>       auto v2 = cast(ubyte[]) (vp11);   //v2.length=4
>> >>>       auto v22 = cast(ubyte[])( [v11]); //v2.length=1
>> >>
>> >> This seems like a bug to me.
>> >>
>> >> It appears that v22 has truncated v11 to a byte and made 
>> >> only a single byte array out of it.
>> >
>> > Except that that's precisely how you usually get an array 
>> > any integral type smaller than an integer. e.g.
>> >
>> > auto arr = cast(ubyte[])([1, 2, 3, 4]);
>> >
>> > In this case, you could do
>> >
>> > ubyte[] arr = [1, 2, 3, 4];
>> >
>> > instead, but if you're not dealing with an initializaton or 
>> > assignment like this (e.g. you're passing the array to a 
>> > functon), then the cast is the way you do it. Normally, you 
>> > do it with integer literals, and I could see an argument 
>> > that it shouldn't allow it without VRP being used to make it 
>> > work, but it _is_ a cast, and casts are a bit of a blunt 
>> > instrument.
>> >
>> > So, I really don't think that it's a bug.
>>
>> It's quite possible that you aren't understanding what is 
>> happening:
>>
>> ubyte[] arr = cast(ubyte[])[555];
>> writeln(arr); // [43]
>>
>> Why is this not a bug? I didn't cast the 555 to a ubyte, so it 
>> should either complain that it can't do it, or give me an 
>> array of 4 bytes.
>>
>> I guess it could be explained as the same thing as:
>>
>> ubyte[] arr = [cast(ubyte)555];
>>
>> But this is surprisingly weird behavior.
>
> That's exactly what it's doing, and when you have multiple 
> elements in the literal, it quickly gets a lot more pleasant 
> than casting each element individually. e.g.
>
> cast(ubyte[])[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>
> vs
>
> [cast(ubyte)0, cast(ubyte)1, cast(ubyte)2, cast(ubyte)3, 
> cast(ubyte)4,
>  cast(ubyte)5, cast(ubyte)6, cast(ubyte)7, cast(ubyte)8, 
> cast(ubyte)9,
>  cast(ubyte)10]
>
> I use this trick all the time when creating arrays of integral 
> types smaller than int, precisely because casting each element 
> is a royal pain and way harder to read.
>
> - Jonathan M Davis



代码目的是转换任意类行到ubyte[]
The object of the code is to convert any class of rows to ubyte[]

但是非数组类无法正确转化
But non - array classes can't be converted correctly



void pByte(T)(T v)
{
	ubyte[] uBuf;
	static if(isArray!T){
		uBuf = cast(ubyte[])mKey;
	}else{
		uBuf = cast(ubyte[])[v];
	}
	writeln(uBuf);
}

无法得到正确结果
Unable to get the correct result