• tcak (20/20) Feb 05 2015 I am on 64-bit Linux.
• tcak (9/29) Feb 05 2015 By the way,
• Baz (25/63) Feb 05 2015 I've tried this;
• =?UTF-8?B?QWxpIMOHZWhyZWxp?= (8/10) Feb 05 2015 In pointer arithmetic, the increment value means "that many *objects*

"tcak" <tcak gmail.com> writes:

I am on 64-bit Linux.

I defined a struct that it 8 bytes in total.

align(1) struct MessageBase{
align(1):
	ushort qc;
	ushort wc;
	ushort id;
	ushort contentLength;
	void[0] content;
}


I defined a function in this struct that tries to set a pointer 
to "contentLength" field.

writeln( "Without: ", (&this + id.offsetof) );
writeln( "With   : ", (cast(size_t)&this + id.offsetof) );

Results:
Without: 7FFFF74F5030
With   : 140737342558228

0x7FFFF74F5030 => 140737342558256


As it is seen, there is 28 bytes of difference between them. What 
is this behaviour exactly?

"tcak" <tcak gmail.com> writes:

On Friday, 6 February 2015 at 03:59:51 UTC, tcak wrote:
 I am on 64-bit Linux.

 I defined a struct that it 8 bytes in total.

 align(1) struct MessageBase{
 align(1):
 	ushort qc;
 	ushort wc;
 	ushort id;
 	ushort contentLength;
 	void[0] content;
 }


 I defined a function in this struct that tries to set a pointer 
 to "contentLength" field.

 writeln( "Without: ", (&this + id.offsetof) );
 writeln( "With   : ", (cast(size_t)&this + id.offsetof) );

 Results:
 Without: 7FFFF74F5030
 With   : 140737342558228

 0x7FFFF74F5030 => 140737342558256


 As it is seen, there is 28 bytes of difference between them. 
 What is this behaviour exactly?

By the way,

writeln("Base Normal: ", &this);
writeln("Base Cast  : ", cast(size_t)&this);

Result:
Base Normal: 7FFFF74F5010
Base Cast  : 140737342558224

0x7FFFF74F5010 => 140737342558224

These are same. So, the issue is about addition process.

"Baz" <bb.temp gmx.com> writes:

On Friday, 6 February 2015 at 04:10:08 UTC, tcak wrote:
 On Friday, 6 February 2015 at 03:59:51 UTC, tcak wrote:
 I am on 64-bit Linux.

 I defined a struct that it 8 bytes in total.

 align(1) struct MessageBase{
 align(1):
 	ushort qc;
 	ushort wc;
 	ushort id;
 	ushort contentLength;
 	void[0] content;
 }


 I defined a function in this struct that tries to set a 
 pointer to "contentLength" field.

 writeln( "Without: ", (&this + id.offsetof) );
 writeln( "With   : ", (cast(size_t)&this + id.offsetof) );

 Results:
 Without: 7FFFF74F5030
 With   : 140737342558228

 0x7FFFF74F5030 => 140737342558256


 As it is seen, there is 28 bytes of difference between them. 
 What is this behaviour exactly?

 By the way,

 writeln("Base Normal: ", &this);
 writeln("Base Cast  : ", cast(size_t)&this);

 Result:
 Base Normal: 7FFFF74F5010
 Base Cast  : 140737342558224

 0x7FFFF74F5010 => 140737342558224

 These are same. So, the issue is about addition process.

I've tried this;

import std.stdio;

struct MessageBase{
align(1):
	ushort qc;
	ushort wc;
	ushort id;
	ushort contentLength;
	void[0] content;

     void a(){
         writefln("%.8X",(cast(void*)&this + id.offsetof));
         writefln("%.8X",(cast(size_t)&this + id.offsetof));
     }
}


void main()
{
     MessageBase mb;
     mb.a;
}

and got the same value for both. Your problem is:

typeof(&this).stringof is "MessageBase*" so with ptr arithmetic 
when you add 1 you actually shift by 1 * MessageBase.sizeof.

typeof(cast(void*)&this) is "void*" so with ptr arithmetic you 
shift by a certain number of bytes.

=?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:

On 02/05/2015 07:59 PM, tcak wrote:

 writeln( "Without: ", (&this + id.offsetof) );

In pointer arithmetic, the increment value means "that many *objects* 
away", not "than many bytes away".

Since id.offsetof is 4, you are calculating 4 MessageBase objects away 
(4*8==32 bytes away).

 writeln( "With   : ", (cast(size_t)&this + id.offsetof) );

That is not pointer arithmetic, so the increment is just 4.

32 - 4 == 28.

Ali