Container Adaptors

Notes from C++ Primer

• stack and queue: based on deque
• priority_queue:    based on vector

Standard library provides three kinds of sequential adaptors: queue, priority_queue and stack. When we use adaptor, we must include the relevant head files:

#include <stack>
#include <queue>	// both queue and priority_queue adaptors

We can use two ways to initialize the adaptor: default constructor or a constructor with one container parameter:

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

Also we can override the underlying container type by passing a sequential container type:

// empty stack implemented on top of vector
stack<string, vector<string> > str_stk;

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

There're constraints on which containers can be used for a given adaptor.

• stack: any sequential container: vector, list, deque
• queue: the underlying container must support push_front, so only based on: list
• priority_queue: the underlying container should support random access, so based on: vecotr or deque

Stack Adaptor Operation

• s.empty()        if the stack is empty, return true
• s.size()            return the size of elements in the stack
• s.pop()            delete the top element of stack, NOT return its value
• s.top()             return the value of top element in stack, not deleting
• s.push(item)    push new element in stack

Codes below show the five operations:

// number of elements we'll put in our stack
const stack<int>::size_type stk_size = 10;
stack<int> intStack;	// empty stack

// fill up the stack
int ix = 0;
while(intStack.size() != stk_size)
	// use postfix increment; want to push old value onto intStack
	intStack.push_back(ix++);		// intStack holds 0...9 inclusive
	
int error_cnt = 0;
while(intStack.empty() == false)
{
	int value = intStack.top();
	
	// read the top element of the stack
	if(value != --ix)
	{
		cerr << "oops! expected " << ix
			 << " received " << value << endl;
			 ++error_cnt;
	}
	intStack.pop();		// pop the top element, and repeat
}

cout << "Our program ran with "
	 << error_cnt << " errors!" << endl;