digitalmars.D.learn - casting and arrays..

Chris Warwick (9/9) Mar 10 2007 std.file.read(filename);

Jarrett Billingsley (10/17) Mar 10 2007 Yes.

Chris Warwick (3/14) Mar 10 2007 Cool.. very nice feature.. :-)
Chris Warwick (8/13) Mar 10 2007 Ok say you have that void[] array returned from std.read.file(). Whats t...

Kirk McDonald (9/27) Mar 10 2007 Assuming that array.length == YourStruct.sizeof, you can say

Chris Warwick (6/26) Mar 10 2007 Well i cant realy do that cause im reading a binary file in, and then i ...

gareis (13/42) Mar 10 2007 Then you need to do pointer arithmetic, it seems. Assuming your struct

Daniel Keep (12/57) Mar 10 2007 Personally, I like std.stream.MemoryStream for things like this. That

Frits van Bommel (14/17) Mar 11 2007 I never saw anyone claim it was _safe_, just that it was _possible_ ;).....

Reiner Pope (11/23) Mar 10 2007 Let D do the memcpy?

Jarrett Billingsley (5/7) Mar 10 2007 I wouldn't really recommend std.file.read for anything more than reading...

Chris Warwick (4/12) Mar 11 2007 Note to self ... RTFM

Jarrett Billingsley (5/6) Mar 11 2007 Hehe :) Though when I first started using D, I thought std.file would b...

"Chris Warwick" <sp m.me.not> writes:

 std.file.read(filename);

returns type void[]

And it it seems i can cast that to whatever type i like, int[], bool[], 
ect.. anyway it compiles... but.. when casting between array types like this 
are runtime checks made to make sure the block being cast has the correct 
granularity.. if the return was void[7] will i get an runtime error casting 
it to int? Does a cast like that even work?

cheers,

cw

Mar 10 2007

"Jarrett Billingsley" <kb3ctd2 yahoo.com> writes:

"Chris Warwick" <sp m.me.not> wrote in message 
news:esvf5h$2317$1 digitalmars.com...
 std.file.read(filename);

 returns type void[]

 And it it seems i can cast that to whatever type i like, int[], bool[], 
 ect.. anyway it compiles... but.. when casting between array types like 
 this are runtime checks made to make sure the block being cast has the 
 correct granularity..

Yes.

 if the return was void[7] will i get an runtime error casting it to int?

Yes.

void main()
{
    ubyte[] a = [cast(ubyte)1, 2, 3, 4, 5, 6, 7];
    int[] x = cast(int[])a;
}

That cast will give an "array cast misalignment" at run-time.

Mar 10 2007

"Chris Warwick" <sp m.me.not> writes:

"Jarrett Billingsley" <kb3ctd2 yahoo.com> wrote in message 
news:esvhdv$261a$1 digitalmars.com...
 "Chris Warwick" <sp m.me.not> wrote in message 
 news:esvf5h$2317$1 digitalmars.com...
 std.file.read(filename);

 returns type void[]

 And it it seems i can cast that to whatever type i like, int[], bool[], 
 ect.. anyway it compiles... but.. when casting between array types like 
 this are runtime checks made to make sure the block being cast has the 
 correct granularity..

 Yes.

Cool.. very nice feature.. :-)

Mar 10 2007

"Chris Warwick" <sp m.me.not> writes:

"Jarrett Billingsley" <kb3ctd2 yahoo.com> wrote in message 
news:esvhdv$261a$1 digitalmars.com...
 "Chris Warwick" <sp m.me.not> wrote in message 
 news:esvf5h$2317$1 digitalmars.com...
 std.file.read(filename);

 returns type void[]


Ok say you have that void[] array returned from std.read.file(). Whats the 
best way to copy a struct out from the void array? Only way i can think is 
memcpy, or some heavy duty casting and pointers. Is there a cleaner - 
pretier way with D?

thanks,

cw

Mar 10 2007

Kirk McDonald <kirklin.mcdonald gmail.com> writes:

Chris Warwick wrote:
 "Jarrett Billingsley" <kb3ctd2 yahoo.com> wrote in message 
 news:esvhdv$261a$1 digitalmars.com...
 "Chris Warwick" <sp m.me.not> wrote in message 
 news:esvf5h$2317$1 digitalmars.com...
 std.file.read(filename);

 returns type void[]


 
 Ok say you have that void[] array returned from std.read.file(). Whats the 
 best way to copy a struct out from the void array? Only way i can think is 
 memcpy, or some heavy duty casting and pointers. Is there a cleaner - 
 pretier way with D?
 
 thanks,
 
 cw 
 
 

Assuming that array.length == YourStruct.sizeof, you can say
     cast(YourStruct*)array.ptr
to treat the array as the struct in-place.

-- 
Kirk McDonald
http://kirkmcdonald.blogspot.com
Pyd: Connecting D and Python
http://pyd.dsource.org

Mar 10 2007

"Chris Warwick" <sp m.me.not> writes:

"Kirk McDonald" <kirklin.mcdonald gmail.com> wrote in message 
news:esvk4l$28mi$1 digitalmars.com...
 Chris Warwick wrote:
 "Jarrett Billingsley" <kb3ctd2 yahoo.com> wrote in message 
 news:esvhdv$261a$1 digitalmars.com...
 "Chris Warwick" <sp m.me.not> wrote in message 
 news:esvf5h$2317$1 digitalmars.com...
 std.file.read(filename);

 returns type void[]


 Ok say you have that void[] array returned from std.read.file(). Whats 
 the best way to copy a struct out from the void array? Only way i can 
 think is memcpy, or some heavy duty casting and pointers. Is there a 
 cleaner - pretier way with D?

 thanks,

 cw

 Assuming that array.length == YourStruct.sizeof, you can say
     cast(YourStruct*)array.ptr
 to treat the array as the struct in-place.

Well i cant realy do that cause im reading a binary file in, and then i want 
to parse it, extract structs and various types here and there.

cheers,

cw

Mar 10 2007

gareis <dhasenan gmail.com> writes:

Chris Warwick wrote:
 "Kirk McDonald" <kirklin.mcdonald gmail.com> wrote in message 
 news:esvk4l$28mi$1 digitalmars.com...
 Chris Warwick wrote:
 "Jarrett Billingsley" <kb3ctd2 yahoo.com> wrote in message 
 news:esvhdv$261a$1 digitalmars.com...
 "Chris Warwick" <sp m.me.not> wrote in message 
 news:esvf5h$2317$1 digitalmars.com...
 std.file.read(filename);

 returns type void[]


 Ok say you have that void[] array returned from std.read.file(). Whats 
 the best way to copy a struct out from the void array? Only way i can 
 think is memcpy, or some heavy duty casting and pointers. Is there a 
 cleaner - pretier way with D?

 thanks,

 cw

 Assuming that array.length == YourStruct.sizeof, you can say
     cast(YourStruct*)array.ptr
 to treat the array as the struct in-place.

 
 Well i cant realy do that cause im reading a binary file in, and then i want 
 to parse it, extract structs and various types here and there.

Then you need to do pointer arithmetic, it seems. Assuming your struct 
has a deserializer method that takes a void*:
---
for (int i = 0; i < array.length; i++) {
     if (someStructStartsHere()) {
         someStruct s;
         s.deserialize(array.ptr + i);
         do_stuff_with_someStruct (s);
     }
}
---

That should probably work.

 cheers,
 
 cw

Mar 10 2007

Daniel Keep <daniel.keep.lists gmail.com> writes:

gareis wrote:
 Chris Warwick wrote:
 "Kirk McDonald" <kirklin.mcdonald gmail.com> wrote in message
 news:esvk4l$28mi$1 digitalmars.com...
 Chris Warwick wrote:
 "Jarrett Billingsley" <kb3ctd2 yahoo.com> wrote in message
 news:esvhdv$261a$1 digitalmars.com...
 "Chris Warwick" <sp m.me.not> wrote in message
 news:esvf5h$2317$1 digitalmars.com...
 std.file.read(filename);

 returns type void[]


 Ok say you have that void[] array returned from std.read.file().
 Whats the best way to copy a struct out from the void array? Only
 way i can think is memcpy, or some heavy duty casting and pointers.
 Is there a cleaner - pretier way with D?

 thanks,

 cw

 Assuming that array.length == YourStruct.sizeof, you can say
     cast(YourStruct*)array.ptr
 to treat the array as the struct in-place.

 Well i cant realy do that cause im reading a binary file in, and then
 i want to parse it, extract structs and various types here and there.

 
 Then you need to do pointer arithmetic, it seems. Assuming your struct
 has a deserializer method that takes a void*:
 ---
 for (int i = 0; i < array.length; i++) {
     if (someStructStartsHere()) {
         someStruct s;
         s.deserialize(array.ptr + i);
         do_stuff_with_someStruct (s);
     }
 }
 ---
 
 That should probably work.
 
 cheers,

 cw


Personally, I like std.stream.MemoryStream for things like this.  That
way, I don't have to stuff around with pointer arithmetic, and
potentially make a mistake.

Incidentally, I never knew that you could safely cast arrays of one type
to another!  Yet another thing I didn't know about D :P

	-- Daniel

-- 
Unlike Knuth, I have neither proven or tried the above; it may not even
make sense.

v2sw5+8Yhw5ln4+5pr6OFPma8u6+7Lw4Tm6+7l6+7D
i28a2Xs3MSr2e4/6+7t4TNSMb6HTOp5en5g6RAHCP  http://hackerkey.com/

Mar 10 2007

Frits van Bommel <fvbommel REMwOVExCAPSs.nl> writes:

Daniel Keep wrote:
 
 Incidentally, I never knew that you could safely cast arrays of one type
 to another!  Yet another thing I didn't know about D :P

I never saw anyone claim it was _safe_, just that it was _possible_ ;)...

Casting arrays is about as safe as pointer casting (which is actually 
pretty much what you're doing), which is to say it isn't safe unless 
you're very careful ;).
For example, if the type you're casting to an array of contains any 
pointers or class references, you've usually got a problem unless you're 
sure those are valid (reading pointers from anything that wasn't always 
in memory is a bad idea, for example; so no reading pointers from file 
or socket connections).
The only thing that's safer about array casting is that the length of 
the data is taken into account, so you won't as easily be accessing 
invalid memory. Which I'd say is more of a property of arrays vs 
pointers, instead of array casting vs pointer casting.

Mar 11 2007

Reiner Pope <some address.com> writes:

gareis wrote:
 Then you need to do pointer arithmetic, it seems. Assuming your struct 
 has a deserializer method that takes a void*:
 ---
 for (int i = 0; i < array.length; i++) {
     if (someStructStartsHere()) {
         someStruct s;
         s.deserialize(array.ptr + i);
         do_stuff_with_someStruct (s);
     }
 }
 ---
 

Let D do the memcpy?

foreach (i, elem; array)
{
     if (someStructStartsHere()) {
         someStruct s = * (cast(someStruct*) &array[i]);
     }
}

That should copy the struct out of the array.

Cheers,

Reiner

Mar 10 2007

"Jarrett Billingsley" <kb3ctd2 yahoo.com> writes:

"Chris Warwick" <sp m.me.not> wrote in message 
news:esvnvu$2ku8$1 digitalmars.com...
 Well i cant realy do that cause im reading a binary file in, and then i 
 want to parse it, extract structs and various types here and there.

I wouldn't really recommend std.file.read for anything more than reading in 
the simplest of files.  Use std.stream.File or BufferedFile for reading in 
complex files.  You can then read in a struct with .readExact.

Mar 10 2007

"Chris Warwick" <sp m.me.not> writes:

"Jarrett Billingsley" <kb3ctd2 yahoo.com> wrote in message 
news:et00eg$66h$1 digitalmars.com...
 "Chris Warwick" <sp m.me.not> wrote in message 
 news:esvnvu$2ku8$1 digitalmars.com...
 Well i cant realy do that cause im reading a binary file in, and then i 
 want to parse it, extract structs and various types here and there.

 I wouldn't really recommend std.file.read for anything more than reading 
 in the simplest of files.  Use std.stream.File or BufferedFile for reading 
 in complex files.  You can then read in a struct with .readExact.

Note to self ... RTFM

;-)

Mar 11 2007

"Jarrett Billingsley" <kb3ctd2 yahoo.com> writes:

"Chris Warwick" <sp m.me.not> wrote in message 
news:et0s2n$1no1$1 digitalmars.com...
 Note to self ... RTFM

Hehe :)  Though when I first started using D, I thought std.file would be 
the best place for file IO too!  Maybe I don't think like a C++ programmer, 
where file IO should be done with streams..

Mar 11 2007

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digitalmars.D.learn - casting and arrays..