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From: Andrey Tarasevich <andreytarasevich@hotmail.com>
Newsgroups: comp.lang.c++
Subject: Re: std::map::emplace useful in any way?
Date: Sun, 30 Jun 2024 13:32:00 -0700
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On 05/25/24 8:57 AM, Marcel Mueller wrote:
> The emplace functions generally allows to construct container objects
> directly at the target location. But std::map expects a value type of
> std::pair<const Key, T>. Now the implementation of emplace needs to
> instantiate the value type just to get the key to find the insert
> location. This cannot happen at the target location, isn't it?

Yes, it sorta-kinda can happen "at target location", If I correctly 
understand what you mean by "target location".

In mainstream implementations `emplace` does not really construct a some 
local `value_type t` object. Instead, it immediately allocates and 
constructs out of the supplied arguments a _node_ object. This node 
object is what the `std::map` is literally built of and, of course, that 
node object contains a `value_type` object inside.

If the key is not present in the map, the node object is immediately 
_attached_ directly into to the map. Otherwise, it is destroyed and 
deallocated. In other words, that node object is the "target location".

So, yes, there is an "optimistic" construction of a `value_type` object 
(which might prove to be a waste), but there's no subsequent 
copying/moving from that `value_type` object to the "target location". 
The `value_type` object is created as part of the "target location" from 
the very beginning.

This approach is what permits usage of `emplace` for insertion of 
non-copyable/non-movable objects into the map through 
`std::piecewise_construct` (which is likely what Stefan refers to in his 
comment).

-- 
Best regards,
Andrey