Re: implicite deref on array element access? (indexing)
On Friday, February 25, 2011 07:15:52 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 > > Denis The _only_ time that dereferencing is done automatically in D is with the dot operator. - Jonathan M Davis
Re: implicite deref on array element access? (indexing)
On 02/25/2011 04:43 PM, Steven Schveighoffer wrote: On Fri, 25 Feb 2011 10:15:52 -0500, 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 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
Re: implicite deref on array element access? (indexing)
On Fri, 25 Feb 2011 10:58:49 -0500, Andrej Mitrovic wrote: On 2/25/11, Steven Schveighoffer 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
Re: implicite deref on array element access? (indexing)
On 2/25/11, Steven Schveighoffer 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 ?
Re: implicite deref on array element access? (indexing)
On Fri, 25 Feb 2011 10:15:52 -0500, 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 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
Re: implicite deref on array element access? (indexing)
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.
Re: implicite deref on array element access? (indexing)
P.S. I got bitten by this when I was interfacing with C. Only in my case I used multidimensional arrays. I just assumed they were the same thing as in C, but I was wrong.
Re: implicite deref on array element access? (indexing)
Actually that doesn't explain anything! What I mean is, arrays in D are not the same as arrays in C. auto a = [1, 2, 3]; auto pa = &a; writeln(&a[0]); writeln(&a[1]); writeln(&a[2]); writeln(&pa[0]); writeln(&pa[1]); writeln(&pa[2]); 9A2E40 9A2E44 9A2E48 12FE34 12FE3C 12FE44 Afaik D arrays have a length and then a pointer to the contents of the array (someone correct me on this if I'm wrong?).
Re: implicite deref on array element access? (indexing)
This should explain everything: auto a = [1,2,3]; auto pa = &a; writeln(&pa); writeln(&pa+1); Do the math!
implicite deref on array element access? (indexing)
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
