.array changes the order

[xtreak via Digitalmars-d-learn]

On Thursday, 12 May 2016 at 13:43:10 UTC, thedeemon wrote:
> On Thursday, 12 May 2016 at 10:17:19 UTC, xtreak wrote:
>
>> Thanks a lot. Can you kindly elaborate a little more on 
>> File.byLine with an example of the scenario so that I don't 
>> get bitten by it. File.byLine.array works as expected for me. 
>> A little more explanation on the permutations will also be 
>> helpful since joiner.map!array converts the subranges to 
>> arrays, so does joiner work by mutating state if so how does 
>> it do.
>
> With some ranges the value they return by .front is only valid 
> until the next call to popFront(). For example, File.byLine() 
> reuses its buffer so after popFront() is called the buffer 
> contains different data and if you had references to the 
> contents of previously returned value they become invalid. This 
> is why byLineCopy() was added. In the same vein permutations() 
> returns a range that has a mutable array of indices inside, 
> which changes every time popFront() is called, and since every 
> value returned by its .front refers to that indices array, if 
> you collect such permutations they will all use the same array 
> of indices and show the same order of elements, the same 
> permutation.
> Because of this mutable nature of some ranges it's important to 
> understand in which order calls to .front and .popFront() 
> happen. The array() function calls popFront on its input in  a 
> loop, consuming the mutable ranges, while map() does not. So in
>  r.map!f.array
> function f will receive different valid values, before they get 
> invalidated in the loop of array(). But if they contain 
> references to something mutable, it makes sense to make copies 
> before the thing they refer to mutates.

Thanks a lot for the info that really clears a lot of things for 
me :) I see from the source that byLineCopy makes a .dup on each 
line and cached at 
https://github.com/dlang/phobos/blob/master/std/stdio.d#L2029-L2034 with gotFront when front is called a lot of times.  I could see a function called emplaceref being used in array. Any ideas on that please.