为什么 std::queue 不支持 clear() 函数？

Question

I have requirement: for a function, I get the input as a stream of numbers. I mean, the function keeps on getting called with single number in each call. I am using std::queue for storing the stream of numbers. I need to process a collected set of numbers only when some condition is satisfied. If the condition is not satisfied I need to put all the elements into the queue and then start storing new numbers in there. For emptying the queue, I couldn't find a clear() method. So I am looping like this:

while(!q.empty())
    q.pop();

I got an efficient algorithm for clearing a std::queue at

How do I clear the std::queue efficiently?

My question is: Why doesn't std::queue support a clear() function?

Since std::deque and std::vector both support a clear() method, what is the technical difficulty in supporting it for std::queue ?

Or is my above use case very rare and hence not supported?

Answer 1

Apart from what has been said already, you can clear a queue very easily:

queue<int> q;
...
q = queue<int>(); // Assign an empty queue

or in C++11

q = {};

Answer 2

According to http://www.cplusplus.com/reference/stl/queue/ ,

queues are implemented as containers adaptors, which are classes that use an encapsulated object of a specific container class as its underlying container, providing a specific set of member functions to access it elements.

which means that the queue uses an already existing container, and is just really is an interface to this container as a FIFO queue.

This means queues are not meant to be cleared. If you need to clear a queue, this means you actually need to use an object that is not a queue, and therefore you should instead use the actual underlying container type, being a deque by default.

Answer 3

queue is just an adapter for some underlying container, by default a deque , with restricted function (as you noted here). If you want the full blown function use the underlying deque instead of queue .

Answer 4

Added this to my growing list of 'make STL readable' functions:

template <typename T> 
void Clear(std::queue<T>& Queue) 
{
    Queue = std::queue<T>(); // Assign to empty queue
}

It's just a wrapper around sellibitze's excellent answer, but means I don't have to also add a comment every time I use the technique.