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There is an issue that has bothered me for a long time. In order to extract the top element from std::priority_queue, we have to write something like this:
auto x = queue.top(); // Copy constructor or copy assignment.
queue.pop();
Unfortunately, we cannot move a top value from the queue because top() is of const_reference type (https://eel.is/c++draft/priority.queue#priqueue.members) . This can cause a performance hit if the value type is expensive to copy (large strings, etc.).
The idea is to provide something like a pop(T&) method overload:
// Implementation example. noexcept-based computations and choices are omitted.
template <typename U>
void pop(U& to) {
to = std::move(container.front());
pop();
}
// Usage:
T x; // or std::optional<T>
queue.pop(x); // move if T::operator=(T&&) is noexcept
Alternative interface:
T extract_top() {
T val = std::move(container.front());
pop();
return val;
}
I like it even more than the first one.
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