Container Adapter

Container adapters in the STL are essentially classes that provide a specific interface to an underlying container class, allowing it to be used in a manner different from its original design. These adapters wrap around existing containers and offer a restricted set of operations to achieve specific goals or constraints.

The STL provides three main types of container adapters: stack, queue, and priority_queue.

Anatomy of std::queue

The std::queue container adapter is part of the C++ standard library and is defined in the <queue> header. It is implemented as an adapter over other STL containers, such as std::deque, std::list, or std::vector. The adapter nature means that std::queue provides a higher-level interface built on top of an underlying container, allowing developers to focus on the logical operations of a queue without worrying about the low-level details of memory management.

The primary goal is to simplify the implementation of a queue by offering a high-level interface while abstracting the underlying container.

Characteristics:

• FIFO Behavior: Enforces the first-in, first-out order for elements.
• Adapter Nature: Built on top of an existing container, abstracting implementation details.

Creating a std::queue

To create a std::queue, you declare an instance and optionally specify the underlying container type:

#include <queue>

std::queue<int> myQueue; // Default underlying container is std::deque<int>
std::queue<int, std::vector<int>> myVectorQueue; // Using std::vector<int> as the underlying container

Basic Operations

Enqueueing Elements:

Adding elements to the back of the queue is achieved using the push member function:

myQueue.push(42);

Dequeueing Elements:

Removing elements from the front of the queue is done with the pop member function:

myQueue.pop();

Functions:

1. Element Access

front():

• Returns a reference to the front element of the queue.
• Access the front element without removing it, you can use the front member function.

Example:

std::queue<int> myQueue:
int frontElement = myqueue.front();

2. Capacity

empty():

• Checks if the queue is empty.

Example:

std::queue<int> myQueue;
bool isEmpty = myQueue.empty();

size():

• Returns the number of elements in the queue.

Example:

std::queue<int>myQueue;
size_t sizeOfQueue = myQueue.size();

3. Modifiers Functions:

push(cont T& value):

• Adds an element to the back of the queue.

Example:

std::queue<int> myQueue;
myQueue.push(42);

pop():

• Removes the front element from the queue.

Example:

std::queue<int> myQueue;
myQueue.pop();