Re: [Bloom] review starts on 13th of May

From: Joaquin M López Muñoz (joaquinlopezmunoz_at_[hidden])
Date: 2025-05-23 10:03:37

El 23/05/2025 a las 3:13, David Bien via Boost escribió:
> I noticed the specialized emplace() right away - it looked weird so I thought about it for a bit:
>
>    template<typename... Args>
>    BOOST_FORCEINLINE void emplace(Args&&... args)
>    {
>      insert(detail::allocator_constructed<allocator_type,value_type>{
>        get_allocator(),std::forward<Args>(args)...}.value());
>    }
>    template<
>      typename U,
>      typename std::enable_if<
>        std::is_same<T,detail::remove_cvref_t<U>>::value>::type* =nullptr
>    >
>    BOOST_FORCEINLINE void emplace(U&& x)
>    {
>      insert(x); /* avoid value_type construction */
>    }
>
> Given
> T t;
> So, the emplace( std::move(t)) will call emplace(U&&), which doesn’t move anything out of the value x, cuz insert(const&).
> emplace(t) will call the emplace(Args&&… args) because emplace(U&&) can’t accept a const T& - and will make a copy of t before calling insert().

No, emplace(t) also selects

 Â  template<
 Â Â Â  typename U,
 Â Â Â  typename std::enable_if<
std::is_same<T,detail::remove_cvref_t<U>>::value>::type* =nullptr
 Â  >
 Â  BOOST_FORCEINLINE void emplace(U&& x)
 Â  {
 Â Â Â  insert(x); /* avoid value_type construction */
 Â  }

So, no copy of t is incurred. U&& is a so-called universal reference 
that matches
everything, including const T&.

> I get why it was done, but I find it to be obfuscating or confusing.
> You could as well just declare the emplace for T as emplace(const U&) (which also looks weird) and a copy wouldn’t be made but it will still accept the call emplace(std::move(t)).
>
> I think ridding emplace entirely is probably the best move - we aren’t emplacing anything in reality.
>
> That’s my two cents, and also not really a review since I didn’t spend enough time for a real review. Hopefully I’m not annoying anyone - lol.

On the contrary, I'm happy the review is producing such interesting
conversations.

Joaquin M Lopez Munoz