Standard Template Library Programmer's Guide

[2] This restriction is the only reason for queue to exist at all. Any container that is both a front insertion sequence and a back insertion sequence can be used as a queue; deque , for example, has member functions front , back , push_front , push_back , pop_front , and pop_back The only reason to use the container adaptor queue instead of the container deque is to make it clear that you are performing only queue operations, and no other operations.

[3] One might wonder why pop() returns void , instead of value_type . That is, why must one use front() and pop() to examine and remove the element at the front of the queue , instead of combining the two in a single member function? In fact, there is a good reason for this design. If pop() returned the front element, it would have to return by value rather than by reference: return by reference would create a dangling pointer. Return by value, however, is inefficient: it involves at least one redundant copy constructor call. Since it is impossible for pop() to return a value in such a way as to be both efficient and correct, it is more sensible for it to return no value at all and to require clients to use front() to inspect the value at the front of the queue .

stack , priority_queue , deque , Container, Sequence

Categories: containers, adaptors

Component type: type

A priority_queue is an adaptor that provides a restricted subset of Container functionality: it provides insertion of elements, and inspection and removal of the top element. It is guaranteed that the top element is the largest element in the priority_queue , where the function object Compare is used for comparisons. [1] Priority_queue does not allow iteration through its elements. [2]

Priority_queue is a container adaptor, meaning that it is implemented on top of some underlying container type. By default that underlying type is vector , but a different type may be selected explicitly.

int main() {

priority_queue Q;

Q.push(1);

Q.push(4);

Q.push(2);

Q.push(8);

Q.push(5);

Q.push(7);

assert(Q.size() == 6);

assert(Q.top() == 8);

Q.pop();

assert(Q.top() == 7);

Q.pop();

assert(Q.top() == 5);

Q.pop();

assert(Q.top() == 4);

Q.pop();

assert(Q.top() == 2);

Q.pop();

assert(Q.top() == 1);

Q.pop();

assert(Q.empty());

}

Defined in the standard header queue, and in the nonstandard backward-compatibility header stack.h.

Assignable, Default Constructible

• T is a model of Assignable.

• Sequence is a model of Sequence.

• Sequence is a model of Random Access Container.

• Sequence::value_type is the same type as T .

• Compare is a model of Binary Predicate.

• Compare induces a strict weak ordering, as defined in the LessThan Comparable requirements, on its argument type.

• T is convertible to Compare 's argument type.

None.

These members are not defined in the Assignable and Default Constructible requirements, but are specific to priority_queue .

[1] Priority queues are a standard concept, and can be implemented in many different ways; this implementation uses heaps. Priority queues are discussed in all algorithm books; see, for example, section 5.2.3 of Knuth. (D. E. Knuth, The Art of Computer Programming. Volume 3: Sorting and Searching . Addison-Wesley, 1975.)