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

Can I not do this cast because it's immutable?

On 2015-02-06 at 18:09, Charles wrote:
>          readString(toBytes!char(['t','e','s','t']),0,4).writeln;
>          readString(toBytes!string("test"),0,4).writeln;    // This is line 39

That second line makes no sense (you didn't provide an array of strings).
Why toBytes!string("test") and not toBytes!char("test") or rather:
toBytes!(immutable char)("test") ?

On Fri, 06 Feb 2015 17:09:28 +0000, Charles wrote:

>          readString(toBytes!string("test"),0,4).writeln;

if you'll take a look into druntime sources, you'll find that string is just an alias to `immutable(char)[]`. so you actually doing thing:

  readString(toBytes!(immutable(char)[])("test"),0,4).writeln;

i bet that this is not what you meant. ;-)

On Friday, 6 February 2015 at 17:40:31 UTC, ketmar wrote:
> On Fri, 06 Feb 2015 17:09:28 +0000, Charles wrote:
>
>>          readString(toBytes!string("test"),0,4).writeln;
>
> if you'll take a look into druntime sources, you'll find that string is
> just an alias to `immutable(char)[]`. so you actually doing thing:
>
>   readString(toBytes!(immutable(char)[])("test"),0,4).writeln;
>
> i bet that this is not what you meant. ;-)

Thanks!

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;
     }
?

On Saturday, 7 February 2015 at 12:04:12 UTC, Nicholas Wilson wrote:
> 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;
>      }
> ?

The original code I was using was written in Java, and only had a method for strings. This is closer to what I wanted. My unit tests were just going back and forth with readString function, so I was completely missing this for other types. Nice catch!

There were a couple issues with your code so I've included the corrected version:

    ubyte[] toUbytes(T)(T[] arr)
    {
        if (arr is null)
        {
            return null;
        }

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

        foreach (i, val; arr)
        {
            buffer[] = cast(ubyte[T.sizeof])(&(arr[i]))[0 .. T.sizeof]; // Parenthesis and missing semicolon
            result[i * T.sizeof .. (i * T.sizeof) + T.sizeof] = buffer; // Specify appropriate slice for buffer to be inserted into
        }

        return result;
    }

> The original code I was using was written in Java, and only had a method for strings. This is closer to what I wanted. My unit tests were just going back and forth with readString function, so I was completely missing this for other types. Nice catch!
>
> There were a couple issues with your code so I've included the corrected version:
>

That's what I get for replying at 11pm =p
>     ubyte[] toUbytes(T)(T[] arr)
>     {
>         if (arr is null)
>         {
>             return null;
>         }
>
>         ubyte[T.sizeof] buffer;
>         ubyte[] result = new ubyte[arr.length * T.sizeof];
>
>         foreach (i, val; arr)
>         {
>             buffer[] = cast(ubyte[T.sizeof])(&(arr[i]))[0 .. T.sizeof]; // Parenthesis and missing semicolon
>             result[i * T.sizeof .. (i * T.sizeof) + T.sizeof] = buffer; // Specify appropriate slice for buffer to be inserted into
>         }
>
>         return result;
>     }
 thinking about it again this can be done in a single memcpy

     ubyte[] toUbytes(T)(T[] arr)
     {
         if (arr is null)
         {
             return null;
         }

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

        memcpy(result.ptr, arr.ptr , arr.length * T.sizeof);
        return result;
    }

and an asUbytes can be done as a cast

     ubyte[] toUbytes(T)(T[] arr)
     {
         if (arr is null)
         {
             return null;
         }
         return cast(ubyte[]) arr.ptr [0 .. arr.length * T.sizeof];
     }