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

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

Section: 22.5 [optional], 22.6 [variant], 22.7 [any] Status: C++17 Submitter: Zhihao Yuan Opened: 2017-01-25 Last modified: 2020-09-06

Priority: 1

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View all issues with C++17 status.

Discussion:

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.

[2017-01-27 Telecon]

Priority 1; send to LEWG.

[Kona 2017-03-02]

Accepted as Immediate because P1.

Proposed resolution:

This wording is relative to N4618.

Update the following signatures in 22.5.3 [optional.optional], class template optional synopsis, as indicated:

[…]
// 22.5.3.4 [optional.assign], assignment
[…]
template <class... Args> voidT& emplace(Args&&...);
template <class U, class... Args>
  voidT& emplace(initializer_list<U>, Args&&...);
[…]

Modify 22.5.3.4 [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 22.6.3 [variant.variant], class template variant synopsis, as indicated:

[…]
// 22.6.3.5 [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 22.6.3.5 [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 22.7.4 [any.class], class any synopsis, as indicated:

[…]
// 22.7.4.4 [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 22.7.4.4 [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.
[…]

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

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