std.range.interfaces : InputRange moveFront

[Andrei Alexandrescu]

On 12/03/2017 12:42 AM, Johan Engelen wrote:
> On Friday, 1 December 2017 at 18:33:09 UTC, Ali Çehreli wrote:
>> On 12/01/2017 07:21 AM, Steven Schveighoffer wrote:
>> > On 12/1/17 4:29 AM, Johan Engelen wrote:
>>
>> >> (Also, I would expect "popFront" to return the element
>> popped, but it
>> >> doesn't, OK...
>> >
>> > pop removes the front element, but if getting the front
>> element is
>> > expensive (say if it's a map with a complex lambda function),
>> you don't
>> > want to execute that just so you can return it to someone who
>> doesn't
>> > care. This is why front and popFront are separate.
>>
>> Yet, we're told that compilers are pretty good at eliminating that 
>> unused copy especially for function templates where all code is visible.
> 
> I assume that Steven means "copying the front element" when he wrote 
> "getting the front element"? There is no need for a copy, because the 
> element will be removed from the range, so we can move (whose cost only 
> depends on the size of the element, internal pointers being disallowed 
> by the language).
> If it is expensive to actually get _to_ the front/back element (i.e. 
> find its memory location), then having to do the operation twice is a 
> disadvantage.
> 
> Ali: the compiler can only elide copying/moving of an unused return 
> value when inlining the function. (the duty of making the return value 
> move/copy is on the callee, not the caller)
> 
> Note that because front/back() and popFront/Back() are separate, a copy 
> *is* needed when one wants to "pop an element off". Thus 
> moveFront/Back() and popFront/Back() should be used. OK.
> The fact that "pop" does something different from other programming 
> languages is something important to remember when teaching people about 
> D. And I think should be made clear in the documentation; let's add an 
> example of how one is supposed to use all this in an efficient manner?
> 
> Back on topic: let's change the documentation of moveFront such that it 
> is clear that it does _not_ reduce the number of elements in the range?
> 
>> So, even though exception safety is not a common topic of D community, 
>> the real reason for why popFront() does not return the element is for 
>> strong exception safety guarantee.
> 
> Interesting point. Argh why do we allow the user to throw in move?
> 
>> Regardless, separating front() from popFront() is preferable due to 
>> cohesion: fewer responsibilities per function, especially such low 
>> level ones.
> 
> This doesn't make much sense ;-)
> popFrontN has more responsibility, and one gains better performance than 
> simply calling popFront N times. It's similar here.

Thanks Ali for asking me to comment in this thread. The matter of fact 
is moveFront was needed for different purposes.

First off, moving in D cannot throw; all objects are moveable by means 
of bitwise move.

The main reason for moveFront's existence is supporting ranges that have 
front() return an rvalue. For those, there would otherwise be no 
efficient means to move data out of the range to its user.

Now, why does popFront return void instead of the popped element? We 
need front() anyway as a non-destructive way to look at the current 
element of the range, so having popFront return that element is 
redundant. Plus, it's difficult to optimize away particularly in 
separately compiled code.


Andrei