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### 2006. `emplace` broken for associative containers

Section: 24.2.8 [unord.req] Status: NAD Submitter: Pablo Halpern Opened: 2010-10-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The current definition of `emplace(args)` for associative containers as described in Table 99 is:

> Requires: `T` shall be constructible from `args`.

> Effects: Inserts a `T` object `t` constructed with `std::forward<Args>(args)...` if and only if there is no element in the container with key equivalent to the key of `t`. The `bool` component of the returned `pair` is `true` if and only if the insertion takes place, and the iterator component of the `pair` points to the element with key equivalent to the key of `t`.

There is similar language in Table 100 for unordered associative containers.

The first issue is editorial: `T` should be `value_type` throughout both tables.

The major issue is that, if the container is `map`, `multimap`, `unordered_map`, or `unordered_multimap`, then the only way to construct an object of `value_type` is to supply exactly two arguments for `Key` and `Value`, a `pair<Key,Value>`, or a `piecewise_construct_t` followed by two `tuple`s. The original `emplace()` proposal would have allowed you to specify a `Key` value followed by any number of constructor arguments for `Value`. When we removed the variadic constructor to `pair`, this ability went away. I don't think that was deliberate.

Fixing this is non-trivial, I think. I think that `emplace()` for `map` and `multimap` need several overloads: one for each overloaded constructor in `pair<Key,Value>`, and one for the `emplace(Key, valueargs...)` case. And it probably needs some SFINAE meta-programming to ensure that the last case doesn't override any of the other ones. Alternatively, one could say that there are exactly two cases: `emplace(args)` where `pair<Key,Value>` is constructible from `args`, and `emplace(args)` where `Key` is constructible form the first `arg` and `Value` is constructible from the rest.

Alternatively, the status quo is to use `piecewise_construct_t` if you want to construct an object.

[ 2010 Batavia: ]

N3178 was looked at in session and moved to NAD.

Proposed resolution: