On Sat, 28 Dec 2024, 00:37 Magnus Fromreide, <magfr_at_[hidden]> wrote:

> On Sat, Dec 28, 2024 at 12:10:55AM +0000, Jonathan Wakely via
> Std-Proposals wrote:
> > On Fri, 27 Dec 2024, 22:51 Chris Ryan via Std-Proposals, <
> > std-proposals_at_[hidden]> wrote:
> >
> > > Hi Andre,
> > > .resize(0) actually deletes the storage.
> > >
> >
> > No it doesn't. clear() and resize(0) both preserve capacity.
> >
> >
> > Farrakh is wanting it to preserve the storage (keep the .capacity(), not
> > > have to alloc it again)
> > >
> >
> > That's what clear() does.
> >
> > The only reason to add a new function would be to avoid the destructor
> > calls for the elements, which can be accomplished with a custom Allocator
> > that has an empty destroy function.
>
> Wouldn't that lead to the member objects not beeing destructed when the
> vector's lifetime ends?
>

Yes, because I thought that was the point. I'm not saying I support the
idea. I certainly don't want it added as a member function of std::vector.

If you want to manage object lifetime incorrectly, you should do that
yourself without help from the standard.





> To me it sounds like Farrakh wants an extra abstraction on top of vector
> which
> maintains it's own size which is <= vector.size.
>
> This should be possible to do using a decorator like
>
> template<class C>
> class Farrakh {
> private:
> C c;
> typename C::size_type size_ = 0;
> public:
> // lots of members that forward to c but uses size_ rather than
> // c.size(), but in particular these:
>
> void push_back(auto v) {
> if (size_ < c.size())
> c[size_++] = v;
> else
> c.push_back(v);
> }
> void clear() { size_ = 0; }
> };
>
> I am strongly against anything like this beeing added to the standard.
>
> >
> >
> > > Farrakh,
> > > I think that you might not realize the reason .clear() is slower is
> > > because it has to call the in-place destructor for each element (zero
> > > through size(), but not all the way up to .capacity()) You can't just
> > > change the count or you screw up the object lifetime of those objects
> > > that were not destructed. D'tors won't get called. Index positions may
> > > potentially get overwritten later with subsequent push_backs(...) and
> the
> > > in-place c'tors calls. (thus causing leaks)
> > >
> > > https://en.cppreference.com/w/cpp/container/vector/clear
> > >
> > >
> > > On Fri, Dec 27, 2024 at 2:08 PM Andre Kostur via Std-Proposals <
> > > std-proposals_at_[hidden]> wrote:
> > >
> > >> What’s wrong with .resize(0) ?
> > >>
> > >> On Fri, Dec 27, 2024 at 1:57 PM Фаррах Фаттахов via Std-Proposals <
> > >> std-proposals_at_[hidden]> wrote:
> > >>
> > >>> After working with SFML, I so hated std::vector (I was a bad
> programmer
> > >>> back then and didn't know about the reserve() function, which led to
> not
> > >>> understanding why std::vector is so slow, while a simple array
> performs
> > >>> much better). As a result, in my new project i created my own
> version of
> > >>> std::vector - FE2D::dynamic_array. And there, from the beginning i
> created
> > >>> one useful function - reset()
> > >>> ```C++
> > >>>
> > >>> // Reset the Size of Dynamic Array to Zero, but it's don't Touches
> the Real Occupied Memory of Dynamic Array
> > >>> void reset() { mSize = 0; }
> > >>> ```
> > >>>
> > >>> Instead of clear() it's just setting mSize to zero. It's very useful
> in
> > >>> the situations like
> > >>>
> > >>> ```C++
> > >>> void Render() { // It's called every frame
> > >>> // Add a sprites to the FE2D::dynamic_array to draw it later
> > >>> m_Renderer.AddSprite(m_Sprite_0);
> > >>> m_Renderer.AddSprite(m_Sprite_1);
> > >>> m_Renderer.AddSprite(m_Sprite_2);
> > >>> ...
> > >>> // Draw the Sprites and reset the dynamic arrays to prepare it for
> the next frame
> > >>> m_Renderer.RenderSprites();
> > >>> }
> > >>> ```
> > >>> Here reset() works like clear(), but much faster
> > >>>
> > >>> In the std::vector or std::deque is no functions like reset() and i
> want
> > >>> to add it there, especially sometimes i get errors because my
> > >>> FE2D::dynamic_array is not so good like std::vector and i have to
> use it
> > >>> instead of mine, but there is no function i need. I think reset()
> will be
> > >>> nice thing for std::vector.
> > >>> --
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> > >>> Std-Proposals_at_[hidden]
> > >>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
> > >>>
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> > >>
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> > >
>
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