Defining an Adaptor

Each adaptor defines two constructors: the default constructor that creates an empty object, and a constructor that takes a container and initializes the adaptor by copying the given container. For example, assuming that deq is a deque<int>, we can use deq to initialize a new stack as follows:

stack<int> stk(deq);  // copies elements from deq into stk

By default both stack and queue are implemented in terms of deque, and a priority_queue is implemented on a vector. We can override the default container type by naming a sequential container as a second type argument when we create the adaptor:

// empty stack implemented on top of vector
stack<string, vector<string>> str_stk;
// str_stk2 is implemented on top of vector and initially holds a copy of svec
stack<string, vector<string>> str_stk2(svec);

There are constraints on which containers can be used for a given adaptor. All of the adaptors require the ability to add and remove elements. As a result, they cannot be built on an array. Similarly, we cannot use forward_list, because all of the adaptors require operations that add, remove, or access the last element in the container. A stack requires only push_back, pop_back, and back operations, so we can use any of the remaining container types for a stack. The queue adaptor requires back, push_back, front, and push_front, so it can be built on a list or deque but not on a vector. A priority_queue requires random access in addition to the front, push_back, and pop_back operations; it can be built on a vector or a deque but not on a list.