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2783.stack::emplace() andqueue::emplace() should returndecltype(auto)

Section: 23.6.3.1[queue.defn], 23.6.6.2[stack.defn]Status:C++20Submitter: Jonathan WakelyOpened: 2016-10-14Last modified: 2021-02-25

Priority:2

View all otherissues in [queue.defn].

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Discussion:

Thestack andqueue adaptors are now defined as:

template <class... Args>reference emplace(Args&&... args) { return c.emplace_back(std::forward<Args>(args)...); }

This breaks any code usingqueue<UserDefinedSequence> orstack<UserDefinedSequence> until the user-defined containers are updated to meet the new C++17 requirements.

If we defined them as returningdecltype(auto) then we don't break any code. When used withstd::vector orstd::deque they will returnreference, as required, but when used with C++14-conforming containers they will returnvoid, as before.

[2016-11-12, Issaquah]

Sat AM: P2

[2017-03-04, Kona]

Status to Tentatively Ready.

Proposed resolution:

This wording is relative to N4606.

1. Change return type ofemplace in class definition in 23.6.3.1[queue.defn]:

   template <class... Args>referencedecltype(auto) emplace(Args&&... args) { return c.emplace_back(std::forward<Args>(args)...); }

2. Change return type ofemplace in class definition in 23.6.6.2[stack.defn]:

   template <class... Args>referencedecltype(auto) emplace(Args&&... args) { return c.emplace_back(std::forward<Args>(args)...); }