• spir (12/12) Feb 25 2011 Hello,
• Jesse Phillips (4/13) Feb 25 2011 You aren't making a pointer to the data, only the array. I probably woul...
• Steven Schveighoffer (6/13) Feb 25 2011 Because indexing a pointer like ptr[n] is the equivalent of *(ptr + n).
• Andrej Mitrovic (2/4) Feb 25 2011 Can't you do the same with -noboundscheck ?
• Steven Schveighoffer (8/12) Feb 25 2011 No, -noboundscheck stops bounds checking everywhere. arr.ptr[n] stops
• spir (10/24) Feb 25 2011 All right! This is what I did not get. Thank you, Steve.

spir <denis.spir gmail.com> writes:

Hello,

I thought it worked, just like implicite deref on (struct, class) member 
access. But I cannot have it work:

     auto a = [1,2,3];
     auto pa = &a;
     writeln((*pa)[2]);      // ok
     writeln(pa[2]);         // segfault

Denis
-- 
_________________
vita es estrany
spir.wikidot.com

Jesse Phillips <jessekphillips+D gmail.com> writes:

spir Wrote:

 Hello,
 
 I thought it worked, just like implicite deref on (struct, class) member 
 access. But I cannot have it work:
 
      auto a = [1,2,3];
      auto pa = &a;
      writeln((*pa)[2]);      // ok
      writeln(pa[2]);         // segfault

You aren't making a pointer to the data, only the array. I probably would have
made the same mistake, but I think you want to use:

auto pa = a.ptr;

Don't know if there is a ptr property for array.

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

On Fri, 25 Feb 2011 10:15:52 -0500, spir <denis.spir gmail.com> wrote:

 Hello,

 I thought it worked, just like implicite deref on (struct, class) member  
 access. But I cannot have it work:

      auto a = [1,2,3];
      auto pa = &a;
      writeln((*pa)[2]);      // ok
      writeln(pa[2]);         // segfault

Because indexing a pointer like ptr[n] is the equivalent of *(ptr + n).

This is how it is in C.

Fun fact, you can avoid array bounds checks (if you know the index is  
valid) by doing arr.ptr[n]

-Steve

Andrej Mitrovic <andrej.mitrovich gmail.com> writes:

On 2/25/11, Steven Schveighoffer <schveiguy yahoo.com> wrote:
 Fun fact, you can avoid array bounds checks (if you know the index is
 valid) by doing arr.ptr[n]

Can't you do the same with -noboundscheck ?

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

On Fri, 25 Feb 2011 10:58:49 -0500, Andrej Mitrovic  
<andrej.mitrovich gmail.com> wrote:

 On 2/25/11, Steven Schveighoffer <schveiguy yahoo.com> wrote:
 Fun fact, you can avoid array bounds checks (if you know the index is
 valid) by doing arr.ptr[n]

 Can't you do the same with -noboundscheck ?

No, -noboundscheck stops bounds checking everywhere.  arr.ptr[n] stops  
bounds checking for that one statement.  It's a lot easier to prove that  
one time that bounds checking is not necessary than it is to prove that no  
bounds checking is necessary anywhere.

Plus, you can't always control the command line.

-Steve

spir <denis.spir gmail.com> writes:

On 02/25/2011 04:43 PM, Steven Schveighoffer wrote:
 On Fri, 25 Feb 2011 10:15:52 -0500, spir <denis.spir gmail.com> wrote:

 Hello,

 I thought it worked, just like implicite deref on (struct, class) member
 access. But I cannot have it work:

 auto a = [1,2,3];
 auto pa = &a;
 writeln((*pa)[2]); // ok
 writeln(pa[2]); // segfault

 Because indexing a pointer like ptr[n] is the equivalent of *(ptr + n).

 This is how it is in C.

 Fun fact, you can avoid array bounds checks (if you know the index is valid) by
 doing arr.ptr[n]

All right! This is what I did not get. Thank you, Steve.
Too bad. Anyway, in the meanwhile I have opted for another approach. (FWIW, in 
Oberon implicite deref works on array indexing just like on struct [record] 
member access.)

Denis
-- 
_________________
vita es estrany
spir.wikidot.com