Issue with template function

[Nicholas Wilson via Digitalmars-d-learn]

On Friday, 6 February 2015 at 17:09:29 UTC, Charles wrote:
> I'm trying to create a template function that can take in any 
> type of array and convert it to a ubyte array. I'm not 
> concerned with endianness at the moment, but I ran into a 
> roadblock when trying to do this with strings. It already works 
> with ints, chars, etc.
>
> Here's the relevant test code:
>
>     module byteReader;
>
>     public import std.system : Endian;
>
>     ubyte[] toBytes(T)(T[] arr)
>     {
>         if (arr == null)
>         {
>             return null;
>         }
>
>         ubyte[] result = new ubyte[arr.length];
>
>         foreach (key, val; arr)
>         {
>             result[key] = cast(ubyte) val;    // This is line 16
>         }
>
>         return result;
>     }
>
>     string readString(ubyte[] buffer, uint offset, uint length)
>     {
>         assert( buffer.length >= offset + length );
>
>         char[] chars = new char[length];
>         foreach(key, val; buffer[offset .. offset + length])
>         {
>             chars[key] = cast(char) val;
>         }
>
>         return cast(string)chars;
>
>     }
>
>     void main() {
>         import std.stdio;
>         readString(toBytes!char(['t','e','s','t']),0,4).writeln;
>         readString(toBytes!string("test"),0,4).writeln;    // 
> This is line 39
>     }
>
> Here's the output:
>     byteReader.d(16): Error: cannot cast val of type string to 
> type ubyte
>     byteReader.d(39): Error: template instance 
> byteReader.toBytes!string error instantiating

Are you wanting to to convert each element in arr to a byte thus 
truncating and losing data (when T.sizeof != 1)?
as in
     toBytes([1,2,3, 42, 500 /*this will be truncated to 244 
*/]);// T  == int here
or are you wanting to convert each element to a ubyte array and 
then concatenate it to the result.
as is
      ubyte[] toBytes(T)(T[] arr)
      {
          ubyte[T.sizeof] buf;
          if (arr is null)
          {
              return null;
          }

          ubyte[] result = new ubyte[arr.length * T.sizeof];

          foreach (i, val; arr)
          {
              buf[] = cast(ubyte[T.sizeof])&(arr[i])[0 .. T.sizeof]
              result ~= buf;
          }

          return result;
      }
?