Issue 2857: {variant,optional,any}::emplace should return the constructed value

Title: {variant,optional,any}::emplace should return the constructed value
Status: c++17
Section: [optional][variant][any]
Submitter: Zhihao Yuan

Created on 2017-01-25.00:00:00 last changed 89 months ago

Messages

Date: 2017-03-03.22:19:58

Proposed resolution:

This wording is relative to N4618.

Update the following signatures in [optional.optional], class template optional synopsis, as indicated:

[…]
// [optional.assign], assignment
[…]
template <class... Args> voidT& emplace(Args&&...);
template <class U, class... Args>
  voidT& emplace(initializer_list<U>, Args&&...);
[…]

Modify [optional.assign] as indicated:
```
template <class... Args> voidT& emplace(Args&&... args);
```
[…]
-27- Postconditions: *this contains a value.
-?- Returns: A reference to the new contained value.
-28- Throws: Any exception thrown by the selected constructor of T.
```
template <class U, class... Args> voidT& emplace(initializer_list il, Args&&... args);
```
[…]
-31- Postconditions: *this contains a value.
-?- Returns: A reference to the new contained value.
-32- Throws: Any exception thrown by the selected constructor of T.

Modify the following signatures in [variant.variant], class template variant synopsis, as indicated:

[…]
// [variant.mod], modifiers
template <class T, class... Args> voidT& emplace(Args&&...);
template <class T, class U, class... Args>
  voidT& emplace(initializer_list<U>, Args&&...);
template <size_t I, class... Args> voidvariant_alternative_t<I, variant<Types...>>& emplace(Args&&...);
template <size_t I, class U, class... Args>
  voidvariant_alternative_t<I, variant<Types...>>& emplace(initializer_list<U>, Args&&...);
[…]

Modify [variant.mod] as indicated:
```
template <class T, class... Args> voidT& emplace(Args&&... args);
```
-1- Effects: Equivalent to return emplace(std::forward<Args>(args)...); where I is the zero-based index of T in Types....
[…]
```
template <class T, class U, class... Args>
 voidT& emplace(initializer_list il, Args&&... args);
```
-3- Effects: Equivalent to return emplace(il, std::forward<Args>(args)...); where I is the zero-based index of T in Types....
[…]
```
template <size_t I, class... Args> voidvariant_alternative_t<I, variant<Types...>>& emplace(Args&&... args);
```
[…]
-7- Postconditions: index() is I.
-?- Returns: A reference to the new contained value.
-8- Throws: Any exception thrown during the initialization of the contained value.
[…]
```
template <size_t I, class U, class... Args>
 voidvariant_alternative_t<I, variant<Types...>>& emplace(initializer_list il, Args&&... args);
```
[…]
-12- Postconditions: index() is I.
-?- Returns: A reference to the new contained value.
-13- Throws: Any exception thrown during the initialization of the contained value.
[…]

Modify the following signatures in [any.class], class any synopsis, as indicated:

[…]
// [any.modifiers], modifiers
template <class ValueType, class... Args>
  voiddecay_t<ValueType>& emplace(Args&& ...);
template <class ValueType, class U, class... Args>
  voiddecay_t<ValueType>& emplace(initializer_list<U>, Args&&...);
[…]

Modify [any.modifiers] as indicated:
```
template <class ValueType, class... Args>
 voiddecay_t<ValueType>& emplace(Args&&... args);
```
[…]
-4- Postconditions: *this contains a value.
-?- Returns: A reference to the new contained value.
-5- Throws: Any exception thrown by the selected constructor of T.
[…]
```
template <class ValueType, class U, class... Args>
 voiddecay_t<ValueType>& emplace(initializer_list il, Args&&... args);
```
[…]
-10- Postconditions: *this contains a value.
-?- Returns: A reference to the new contained value.
-11- Throws: Any exception thrown by the selected constructor of T.
[…]

msg8833 (view)

Date: 2017-03-03.22:19:58

[ Kona 2017-03-02 ]

Accepted as Immediate because P1.

msg8795 (view)

Date: 2017-01-27.00:00:00

[ 2017-01-27 Telecon ]

Priority 1; send to LEWG.

msg8794 (view)

Date: 2017-01-25.00:00:00

When you want to continue operate on the new contained object constructed by emplace, you probably don't want to go through the type-safe machinery again.

History
Date	User	Action	Args
2017-07-30 20:15:43	admin	set	status: wp -> c++17
2017-03-05 23:46:08	admin	set	status: immediate -> wp
2017-03-03 22:19:58	admin	set	messages: + msg9072
2017-03-03 22:19:58	admin	set	status: lewg -> immediate
2017-01-30 15:36:02	admin	set	messages: + msg8833
2017-01-30 15:36:02	admin	set	status: new -> lewg
2017-01-26 19:43:37	admin	set	messages: + msg8795
2017-01-25 00:00:00	admin	create