CODEWITHSUNDEEP
c++stliteratorqueue
user15884941
user15884941

Reputation: 1

What is the C++ iterator for STL Queue?

I know we can not iterate easily in STL Queue, but I want to do something like this:

void myFun(Node* root) {
    queue<pair<int, Node*>> myQueue;
    myQueue.push(make_pair(0, root));
    auto it = myQueue.front();
}

This works, however, what should I use instead of the auto keyword for the queues?

We use something like this for maps:

map<int, Node*>::iterator it = myMap.begin();

But

queue<pair<int, Node*>>::iterator it = myQueue.front();

This does not work and throws error:

‘iterator’ is not a member of ‘std::queue<std::pair<int, Node*> >’
queue<pair<int, Node*>>::iterator it = myQueue.front();

What's the correct syntax?

Upvotes: 0

Views: 443

Answers (3)

Pete Becker
Pete Becker

Reputation: 76305

std::queue does not provide access to its internals. It's interface is that of a queue: you push elements into it, and you pop elements out of it. That's all. If you need a container that lets you look at its contents, `std::queued isn't the appropriate choice.

Upvotes: 0

Marshall Clow
Marshall Clow

Reputation: 16670

You should use pair<int, Node*> &, (possibly const) since that's what front() returns. None of the containers (or container adaptors) have a front() method that returns an iterator.

What are you trying to accomplish?

Upvotes: 0

cigien
cigien

Reputation: 60238

The front member of std::queue doesn't return an iterator at all, it returns a reference to the first element in the queue. So auto would just be the type inside the queue

std::pair<int, Node*> it = myQueue.front();

This makes a copy of the element, but you can also take a reference, or a const reference, to that element.

As there are no queue member functions that return an iterator, you'll have to look at all the elements with a sequence of front and pop calls.

Upvotes: 2

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