The most powerful data storage arrays in Modern C++ are Containers and they are very useful to iterate and search members with their amazing methods and properties. A container is a holder object that stores data elements (a collection of data objects). std::vector, std::array, std::list are these kinds of containers. There is another useful container std::queue that allows you to add members to the back of the member list efficiently. In this post, we explain what std::queue is and give some examples. Before that let’s remind ourselves what containers are in C++ programming and what are their types.
Table of Contents
What is a container in modern C++?
Containers are modern data storage arrays in modern C++ and they are very useful to iterate and search data with their powerful methods and properties.
A container is a holder object that stores data elements (a collection of data objects). They are implemented as a class template to define objects that can be used with modern rules of C++ (The rule of 6), they allow great flexibility in the different data types supported as elements, they can be used with int, float, double, etc. or with struct types, they can be used with other modern types of C++, lambdas and templates. Thus, the developer can create different data sets in memory, these can be static or dynamic, they are safe and optimized well.
What are the basic container types modern C++?
The C++ Standards library defines 4 container types.
- Sequence Containers ( vector, array, deque, list, forward_list )
- Associative Containers ( map, multimap, set, multiset )
- Unordered Associative Containers ( unordered_map, unordered_multimap, unordered_set, unordered_multiset )
- Container Adapters ( stack, queue, priority_queue )
If you want to learn more about these each type, here are some basic details,
What is the queue (std::queue) in modern C++?
In programming, a queue is an abstract data type in which members are stored in a container sequence and new members can be added to the end of the sequence and the first member can be extracted.
In C++, the Queue (std::queue) is a container adapter that defined in <queue> header and can be used in FIFO (First In First Out) data operations where members are inserted into one end of the container and extracted from the other.
The Queue is a class that uses an encapsulated object of a specific container class as its underlying container that members are pushed into the back of the specific data container and members are popped from its front of specific data container.
Is there a simple example about the Queue (std::queue) in modern C++?
Here is an example how you can define and add new members to the back of std::queue.
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std::queue<std::string> myq; myq.push("First"); myq.push("Second"); |
Here is another example how you can delete members from front and back;
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std::queue<std::string> myq = { "one", "two", "three", "four", "five" }; myq.pop(); // deletes "one" |
Is there a simple example of how to use the queue (std::queue) in C++?
Here is a full example about std::queue,
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#include <iostream> #include <queue> #include <string> int main() { std::queue<std::string> myq; myq.push("First"); // First element myq.push("Second"); myq.push("Third"); myq.push("Fourth"); std::cout << "Size: of my queue:" << myq.size() << std::endl; myq.pop(); // delete the "First" std::cout << "Listing my queue: "; for (; !myq.empty(); myq.pop()) std::cout << myq.front() << ", "; std::cout << std::endl; std::cout << "Size: of my queue:" << myq.size() << std::endl; system("pause"); return 0; } |
and the output will be as below:
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Size: of my queue:4 Listing my queue: Second, Third, Fourth, Size: of my queue:0 |
Note that, if we want to compare std::queue and std::deque, the queue (std:queue) container adaptors follow the FIFO (First In First Out) rule, where the element that is inserted first is the one that gets removed first. However, the deque (std:deque) does not follow FIFO rule and you can add new members to the front or back of the container.
There is also concurrent queue in modern C++. For more details about Concurrent Queue feature in C++, please see this paper; p0260r7
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