array initialization problem

[Christopher Wright]

Qian Xu wrote:
> Denis Koroskin wrote:
> 
>> ...
>>
>> For example, let's modify CSTR and see what happens:
>> CSTR[0] = 'J'; // now it is "Jello"
>>
>> printing e.str and e2.str gives us the following output:
>> Jello
>> Jello
>>
>> ...
> 
> Hi again,
> 
> but there is one thing, I do not understand.
> CSTR is a constant. But with "CSTR[0] = 'J'", you can modify a const anyway,
> cannot you?

CSTR is a string constant. It's in a data segment of the binary that DMD 
creates. However, on Windows, string constants are in a read-write area 
of memory, so you can change them; but for efficiency, there is only one 
copy of each string constant in the binary.

On Linux, that code would produce a segmentation fault -- there, string 
constants are in a read-only text segment. (I believe I heard that the 
MinGW compiler on Windows makes string constants read-only, so this may 
be compiler specific.)

> BTW: Do you know, why D do not use copy-on-write semantic instead of
> referencing? IMO, copy-on-write is much performanter.

It makes the compiler a fair bit more complicated. It requires syntax to 
create a copy-on-write array versus a by-reference array, or to refer to 
a COW array by reference (so if you modify it, aliases to the same array 
get modified). And copy-on-write does not give you better performance.

Most of all, nobody's made a compelling case to Walter about this. It's 
easy enough to .dup an array if you're about to modify it, though bugs 
from accidentally modifying an array in place are rather hard to track. 
On the other hand, if you have a reasonable const system, these bugs 
turn into compile errors.

> --Qian