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I need to be able to add (append) the contents of one std::queue to another, preferably in much the same way as using std::deque::insert, but using std::vector? I would prefer to continue using std::vectors, rather than a major rewrite to implement std::deques.

As with some of my previous posts in this same project, I have limited mobility in terms of what I can use having to work with some legacy code. And much of that limitation has to do with speed. Nevertheless, members of this forum have come up with some elegant and unique solutions; and I'm hoping to find another.

// This works...
std::deque<std::vector<uint8_t>> aaa;
std::deque<std::vector<uint8_t>> bbb;
aaa.insert(aaa.end(), bbb.begin(), bbb.end());

// This, of course, does not work...
std::queue<std::vector<uint8_t>> ccc;
std::queue<std::vector<uint8_t>> ddd;
ccc.insert(ccc.end(), ddd.begin(), ddd.end());

Obviously will not compile as insert is not supported for ccc type std::queue

A few important notes: There will never be a case where the container being used (queue, deque, etc) will need anything other than FIFO. Also, the processing handles queue volumes in the range of 80,000 to 100,000 elements per second, often very small, occasionally very large.

Weedware
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  • Possible duplicate of [Merge two std::queue](https://stackoverflow.com/questions/5862216/merge-two-stdqueue) – bznein Jul 12 '19 at 15:59
  • I have tried to solve this problem using std::list, but the the performance metrics went down the toilet. I was actually shocked. I'm sure std::list has important and valuable features, but in terms of this application it is doesn't meet the need. – Weedware Jul 12 '19 at 16:09
  • Are you actually after multiple `std::vector`s? From your question it sounds like you are trying to use `std::vector` as the underlying container for `std::queue`, which isn't possible – Caleth Jul 12 '19 at 16:12

2 Answers2

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std::queue is (by default) an adapter on top of std::deque, and one specifically designed to remove operations not associated with FIFO queue structure.

Inserting series of elements, in particular, is not something which is defined for FIFO container, and the whole reason of using std::queue in the first place is to restrict operations logically not part of the FIFO container interface.

Having said that, std::queue exposes the underlying container it uses as a protected member variable C, so one way you can workaround this limitation is by inheriting your custom adapter from std::deque and exposing underlying container.

However, that somewhat defeats the purpose, and may be you'd be better off starting directly with std::deque?

SergeyA
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I made the decision to make the necessary modifications to use std::deque. The problem with std::deque is that it allows removal from the back and pushing onto the front. Our security, best practices guidelines strongly suggest the use of std::queue for exactly that reason. But I made my case successfully (throwing Stack Overflow under the bus lol). Many thanks to all, again!

Weedware
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