• mimocrocodil <4denizzz gmail.com> Aug 15 2011
• David Nadlinger <see klickverbot.at> Aug 15 2011
• Jimmy Cao <jcao219 gmail.com> Aug 15 2011
• "Steven Schveighoffer" <schveiguy yahoo.com> Aug 15 2011
• Mafi <mafi example.org> Aug 15 2011
• Vijay Nayar <madric gmail.com> Aug 15 2011

mimocrocodil <4denizzz gmail.com> writes:

I obtain immutable(byte)* and int where contained length of bytes block from C
library.

Can I convert this into byte[] without explict copying etc.

Something like:

byte* p; // bytes
int size; // size of bytes block

byte[] b;
b.length = size;
b.ptr = p;

// now b contains bytes from library

David Nadlinger <see klickverbot.at> writes:

On 8/15/11 7:16 PM, mimocrocodil wrote:
 Can I convert this into byte[] without explict copying etc.
 […]
 byte* p; // bytes
 int size; // size of bytes block


Simply slice the pointer: p[0 .. size]

David

Jimmy Cao <jcao219 gmail.com> writes:

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On Mon, Aug 15, 2011 at 12:16 PM, mimocrocodil <4denizzz gmail.com> wrote:

 I obtain immutable(byte)* and int where contained length of bytes block
 from C library.

 Can I convert this into byte[] without explict copying etc.

 Something like:

 byte* p; // bytes
 int size; // size of bytes block

 byte[] b;
 b.length = size;
 b.ptr = p;

 // now b contains bytes from library



Yes:

byte[] b = p[0 .. size];

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<div><div class=3D"gmail_quote">On Mon, Aug 15, 2011 at 12:16 PM, mimocroco=
dil <span dir=3D"ltr">&lt;<a href=3D"mailto:4denizzz gmail.com">4denizzz gm=
ail.com</a>&gt;</span> wrote:<br><blockquote class=3D"gmail_quote" style=3D=
"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">

I obtain immutable(byte)* and int where contained length of bytes block fro=
m C library.<br>
<br>
Can I convert this into byte[] without explict copying etc.<br>
<br>
Something like:<br>
<br>
byte* p; // bytes<br>
int size; // size of bytes block<br>
<br>
byte[] b;<br>
b.length =3D size;<br>
b.ptr =3D p;<br>
<br>
// now b contains bytes from library<br></blockquote><div><br></div><div><b=
r></div><div>Yes:</div><div><br></div></div></div><blockquote class=3D"webk=
it-indent-blockquote" style=3D"margin: 0 0 0 40px; border: none; padding: 0=
px;">

<div><div class=3D"gmail_quote"><div>byte[] b =3D p[0 .. size];=A0</div></d=
iv></div></blockquote><div><br></div>

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"Steven Schveighoffer" <schveiguy yahoo.com> writes:

On Mon, 15 Aug 2011 13:16:54 -0400, mimocrocodil <4denizzz gmail.com>  
wrote:

 I obtain immutable(byte)* and int where contained length of bytes block  
 from C library.

 Can I convert this into byte[] without explict copying etc.

 Something like:

 byte* p; // bytes
 int size; // size of bytes block

 byte[] b;
 b.length = size;
 b.ptr = p;

 // now b contains bytes from library


It's even easier:

auto b = p[0..size]; // b is of type byte[]

I.e. you can use a slice operation on a pointer to create a  
correctly-typed slice.

Keep in mind, the size is the number of *elements* for the slice, not the  
number of *bytes*.  In your case it happens to be identical, but for  
larger element types it would be different.

-Steve

Mafi <mafi example.org> writes:

Am 15.08.2011 19:16, schrieb mimocrocodil:
 I obtain immutable(byte)* and int where contained length of bytes block from C
library.

 Can I convert this into byte[] without explict copying etc.

 Something like:

 byte* p; // bytes
 int size; // size of bytes block

 byte[] b;
 b.length = size;
 b.ptr = p;

 // now b contains bytes from library


Pointers support indexing (like C) but additionally also slicing so if 
you have a byte* b and a int/long/size_t (or similiar) len then
b[0.. len]
is a slice starting b conatining len elements of type typeof(*b)[].

Vijay Nayar <madric gmail.com> writes:

On Mon, 15 Aug 2011 17:16:54 +0000, mimocrocodil wrote:

 I obtain immutable(byte)* and int where contained length of bytes block
 from C library.
 
 Can I convert this into byte[] without explict copying etc.
 
 Something like:
 
 byte* p; // bytes
 int size; // size of bytes block
 
 byte[] b;
 b.length = size;
 b.ptr = p;
 
 // now b contains bytes from library


The question presented here really has two parts:
  1) Can you declare a new symbol without duplicating the original data?
  2) Can a mutable symbol reference an immutable pointer?

The short answer to (1) is yes, if you do not need mutability.
The short answer to (2) is no.

The following small program demonstrates this:

import std.stdio;

void main() {
    // Reference data to play with.
    immutable(byte)[] data = [0, 1, 2, 3, 4, 5, 6, 7];

    // These two lines simulate having an immutable byte pointer.
    immutable(byte)* ptr = data.ptr;
    auto ptr_len = data.length;

    // Making an immutable copy is easy, 'auto' works here too.
    immutable(byte)[] immutableCopy = ptr[2 .. 6];

    // Error!  Cannot implicitly convert to immutable.
    //byte[] badMutableCopy = ptr[2 .. 6]; 
    byte[] goodMutableCopy = immutableCopy.dup;

    // Make a simple edit to our mutable copy.
    foreach (ref b; goodMutableCopy) {
        b++;
    }

    // Output is: 2,3 3,4 4,5 5,6
    for (auto i=0; i < immutableCopy.length; i++) {
        writeln(immutableCopy[i], ',', goodMutableCopy[i]);
    }
}