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Section: 24.6.6.1 [queue.defn], 24.6.8.2 [stack.defn] Status: C++20 Submitter: Jonathan Wakely Opened: 2016-10-14 Last modified: 2021-02-25
Priority: 2
View all issues with C++20 status.
Discussion:
The stack and queue adaptors are now defined as:
template <class... Args>
reference emplace(Args&&... args) { return c.emplace_back(std::forward<Args>(args)...); }
This breaks any code using queue<UserDefinedSequence> or stack<UserDefinedSequence> until the user-defined containers are updated to meet the new C++17 requirements.
If we defined them as returning decltype(auto) then we don't break any code. When used with std::vector or std::deque they will return reference, as required, but when used with C++14-conforming containers they will return void, 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.
Change return type of emplace in class definition in 24.6.6.1 [queue.defn]:
template <class... Args>referencedecltype(auto) emplace(Args&&... args) { return c.emplace_back(std::forward<Args>(args)...); }
Change return type of emplace in class definition in 24.6.8.2 [stack.defn]:
template <class... Args>referencedecltype(auto) emplace(Args&&... args) { return c.emplace_back(std::forward<Args>(args)...); }