• Chris (17/17) May 23 2014 The following:
• Meta (4/21) May 23 2014 It is design I believe, but it should be possible for the
• Chris (2/29) May 23 2014 That's what I thought too.
• John Colvin (3/20) May 23 2014 The language guarantees catching the out-of-bounds at runtime.
• bearophile (27/44) May 23 2014 Spotting the second bug is beyond the very limited analysis

"Chris" <wendlec tcd.ie> writes:

The following:

import std.stdio;

void main() {
   int[5] arg;
   arg[10] = 3;              // Compiler says (of course): Error: 
array index 10 is out of bounds arg[0 .. 5]
}

import std.stdio;

void main() {
   int[5] arg;
   foreach (i; 0..10) {
     arg[i] = i;
   }
}

Compiler says nothing, but we get a runtime error 'Range 
violation'.

Bug or design?

"Meta" <jared771 gmail.com> writes:

On Friday, 23 May 2014 at 15:14:47 UTC, Chris wrote:
 The following:

 import std.stdio;

 void main() {
   int[5] arg;
   arg[10] = 3;              // Compiler says (of course): 
 Error: array index 10 is out of bounds arg[0 .. 5]
 }

 import std.stdio;

 void main() {
   int[5] arg;
   foreach (i; 0..10) {
     arg[i] = i;
   }
 }

 Compiler says nothing, but we get a runtime error 'Range 
 violation'.

 Bug or design?

It is design I believe, but it should be possible for the 
compiler to unroll this loop at compile time and detect the out 
of bounds access.

"Chris" <wendlec tcd.ie> writes:

On Friday, 23 May 2014 at 15:25:37 UTC, Meta wrote:
 On Friday, 23 May 2014 at 15:14:47 UTC, Chris wrote:
 The following:

 import std.stdio;

 void main() {
  int[5] arg;
  arg[10] = 3;              // Compiler says (of course): 
 Error: array index 10 is out of bounds arg[0 .. 5]
 }

 import std.stdio;

 void main() {
  int[5] arg;
  foreach (i; 0..10) {
    arg[i] = i;
  }
 }

 Compiler says nothing, but we get a runtime error 'Range 
 violation'.

 Bug or design?

 It is design I believe, but it should be possible for the 
 compiler to unroll this loop at compile time and detect the out 
 of bounds access.

That's what I thought too.

"John Colvin" <john.loughran.colvin gmail.com> writes:

On Friday, 23 May 2014 at 15:14:47 UTC, Chris wrote:
 The following:

 import std.stdio;

 void main() {
   int[5] arg;
   arg[10] = 3;              // Compiler says (of course): 
 Error: array index 10 is out of bounds arg[0 .. 5]
 }

 import std.stdio;

 void main() {
   int[5] arg;
   foreach (i; 0..10) {
     arg[i] = i;
   }
 }

 Compiler says nothing, but we get a runtime error 'Range 
 violation'.

 Bug or design?

The language guarantees catching the out-of-bounds at runtime. 
The compile-time check is merely a courtesy.

"bearophile" <bearophileHUGS lycos.com> writes:

Chris:

 The following:

 import std.stdio;

 void main() {
   int[5] arg;
   arg[10] = 3;              // Compiler says (of course): 
 Error: array index 10 is out of bounds arg[0 .. 5]
 }

 import std.stdio;

 void main() {
   int[5] arg;
   foreach (i; 0..10) {
     arg[i] = i;
   }
 }

 Compiler says nothing, but we get a runtime error 'Range 
 violation'.

 Bug or design?

Spotting the second bug is beyond the very limited analysis 
capabilities of the D compiler. But a slightly different but 
still rather common kind of code (where the index is not mutable) 
could be spotted as buggy by the compiler:


void main() {
     int[5] data;
     foreach (const i; 0 .. 10)
         data[i] = 0;
     foreach (immutable i; 0 .. 10)
         data[i] = 0;
     int[10] big;
     foreach (const i, x; big)
         data[i] = x;
}

(In D it's very good to tag as const/immutable the indexes of 
arrays, as work around a design bug of D foreach).

But the compiler must recognize this as correct code:


void main() {
     int[5] data;
     foreach (const i; 0 .. 10)
         if (i < 5)
             data[i] = 0;
}


So can we add this logic in the compiler?

Bye,
bearophile