STL (Standard Template Library) in C++
Updated on 09th Dec, 23 9.1K Views

In this blog, we will explore all the major components and various types of each component of STL in C++, which includes containers, Iterators, and algorithms. Further, we will be exploring the advantages and disadvantages of STL in C++.

Table of Contents

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What is STL in C++?

STL (Standard Template Library) in C++ is a collection of C++ template classes and functions used for various purposes in the optimization and simplification of complex codes.

What is STL in C++?

The biggest perk of STL is that it offers ready-to-use components and algorithms that enhance the efficiency and productivity development of C++. 

Components of STL in C++

The Standard Template Library is made up of mainly three components that help in implementing generic programming:

  • Containers
  • Iterators
  • Algorithms

Further in this blog, we will discuss these components in detail and cover their types in depth.

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Containers

Containers are the foundation of the Standard Template Library (STL). It is a collection of data structures for storing and managing collections of data efficiently. STL containers are classified into four categories:

Sequence Containers

Sequence containers store elements in a sequential order that enables efficient insertion, deletion, and access to elements based on their position. Here are the main types of Sequence Containers:

  • Vector: It is a dynamically resizable array that provides fast random access to elements.
             vector<object_type> vector_name;
  • List: It is a doubly-linked list that supports efficient insertion and deletion at any position.
             list<object_type> list_name;
  • Deque: It is a double-ended queue that supports efficient insertion and deletion at both ends.
             deque<object_type> deque_name;

Associative Containers

Associative containers store elements based on key-value pairs, this allows efficient access to elements based on their associated keys instead of accessing their position.

  • Map: A collection of key-value pairs where each key is unique and mapped to a single value.
                map<key_object_type, value_object_type> map_name;
  • Set: A collection of unique elements, with no duplicate values allowed.
                  set<object_type> set_name;

Unordered Associative Containers

Unordered associative containers store elements based on key-value pairs. Unlike associative containers, they use hash tables for efficient search operations that give slightly faster performance:

  • Unordered_map: An unordered collection of key-value pairs where each key is unique and mapped to a single value in which the sequence of keys does not matter.
               unordered_map<key_object_type, value_object_type> unordered_map_name;
  • Unordered_set: An unordered collection of unique elements, and in this data type, duplicate values are not allowed.
               unordered_set<object_type> unordered_set_name;

Container Adapters

Container adapters provide specialized views or subsets of existing containers, offering different perspectives on the underlying data. They include:

  • Queue: A FIFO (First In, First Out) queue that supports efficient insertion at the back and deletion at the front.
                      queue<data_type> queue_name;
  • Stack: A LIFO (Last In, First Out) stack that supports efficient insertion and deletion at the top.
               unordered_map<key_object_type, value_object_type> unordered_map_name;
  • Priority Queue: A queue where elements are prioritized based on their associated values.

                           priority_queue<object_type> priority_queue_name;

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Iterators

Iterators are fundamental components of the Standard Template Library (STL) in C++. They serve as a uniform interface for traversing and accessing elements within containers, enabling efficient manipulation of data structures. STL containers provide different types of iterators, each catering to specific access and manipulation capabilities.

Example of a declaration of Iterator:

       container_name<object_type>::iterator iterator_name;
Iterators

Output Iterator

An output iterator facilitates the sequential insertion of elements into a container. It can only be used to append elements to the end of the container, allowing for writing operations. For example, an output iterator can be used to write data to a file or append elements to a linked list.

Input Iterator

An input iterator allows for sequential forward traversal of a container that supports only read operations. It can be used to iterate through elements and access their values, but it cannot modify them. For example, an input iterator can be used to read data from a file or iterate through a vector of integers.

Forward Iterator

A forward iterator combines the capabilities of both input and output iterators. It allows for both sequential traversal and element insertion, but it restricts movement to the forward direction. Forward iterators are the most common type of iterator and are used for most basic data manipulation tasks.

Bidirectional Iterator

A bidirectional iterator extends the capabilities of a forward iterator by enabling backward traversal. It allows for sequential movement in both forward and backward directions, allowing for iteration from both ends of a container. This makes it useful for algorithms that require accessing elements in both directions, such as reversing a list.

Random Access Iterator

A random access iterator offers the highest level of access and manipulation capabilities among all iterator types. It allows for not only sequential traversal but also direct random access to elements using their indices. This enables efficient element insertion, deletion, and modification at any position in the container. Random access iterators are typically used for algorithms that require fast and efficient random access to elements.

Iterator Operations

Iterators provide various operators that enable traversal and element manipulation. These include:

  • Increment (++): Increments the iterator’s position, moving it forward in the container.
  • Decrement (–): Decrements the iterator’s position, moving it backward in the container.
  • Dereference (*): Dereferences the iterator, accessing the element it points to.
  • Equality (==) and Inequality (!=): Checks if two iterators point to the same element.
  • Less-than (<) and Greater-than (>): Compares the positions of two iterators.

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Algorithms

Algorithms are essential components of the Standard Template Library (STL) in C++. They provide a wide range of predefined operations on data structures, enabling efficient manipulation and analysis of data. STL algorithms can be broadly categorized into several types based on their functionality:

Non-manipulative Algorithms

Non-manipulative algorithms perform operations without modifying the original data structures. They are primarily used for searching, counting, and copying elements. Here are some examples:

  • copy: Copies elements from one container to another.
  • find: Searches for a specific element in a container.
  • count: Counts the occurrences of a specific element in a container.

Manipulative Algorithms

Manipulative algorithms modify the original data structures in-place. They are used for tasks such as filling, replacing, and removing elements. Some examples include:

  • fill: Replaces all elements in a container with a specific value.
  • replace: Replaces all occurrences of a specific value in a container with another value.
  • remove: Removes all occurrences of a specific value from a container.

Sorting Algorithms

Sorting algorithms arrange elements in a container in a specific order, typically ascending or descending. They are commonly used for data organization and analysis. Here are some examples:

  • sort: Sorts elements in a container in ascending order.
  • stable_sort: Sorts elements in a container in ascending order while preserving the original order of equal elements.

Set Algorithms

Set algorithms combine or compare elements from two or more containers based on their set operations. They are useful for tasks such as finding unions and intersections of sets. Some examples include:

  • set_union: Combines the unique elements from two containers into a third container.
  • set_intersection: Finds the common elements between two containers and stores them in a third container.

Relational Algorithms

Relational algorithms compare two containers to determine their relationship, such as equality or sortedness. They are used for validating data consistency and verifying algorithm results. For example:

  • equal: Checks whether two containers have the same elements in the same order.
  • is_sorted: Checks whether the elements in a container are sorted in ascending order.

Advantages of the STL in C++

The Standard Template Library (STL) is a powerful and versatile collection of generic components that provide a wide range of data structures, algorithms, and function objects for C++ programming. It offers numerous advantages that make it an essential tool for C++ developers.

  • Reusability: STL components are designed to be highly reusable, allowing developers to use existing code rather than reinventing the wheel. This significantly reduces development time and improves code maintainability.
  • Efficiency: STL algorithms are optimized for performance, ensuring efficient data manipulation and processing. This is particularly beneficial for handling large datasets and computationally intensive tasks.
  • Generic Programming: STL promotes generic programming, enabling developers to write code that works with various data types without sacrificing efficiency. This makes the code more versatile and adaptable to changing requirements.
  • Abstraction: STL components encapsulate complex data manipulation and algorithm implementation, allowing developers to focus on the higher-level logic of their applications. This abstraction simplifies code development and reduces the risk of errors.
  • Productivity: STL provides a comprehensive set of tools for common programming tasks, such as sorting, searching, and container management. This saves developers time and effort, allowing them to focus on more complex and creative aspects of their work.

Disadvantages of the STL in C++

Despite its numerous advantages, the Standard Template Library (STL) in C++ has some limitations and drawbacks that developers should be aware of:

  • Complexity: STL components can be complex and difficult to understand, especially for beginners. The extensive use of templates and iterators can add to the learning curve.
  • Limited Control: STL algorithms and data structures provide a high level of abstraction, which can limit the developer’s fine-grained control over the underlying implementation. This can be an issue in performance-critical scenarios.
  • Memory Management: STL containers manage memory automatically, which can lead to memory bloat if not used carefully. Developers need to be aware of memory allocation and deallocation patterns to optimize memory usage.
  • Error Messages: STL error messages can be cryptic and difficult to understand, especially for complex template-related issues. This can make debugging challenging.
  • Limited Customization: While STL components can be extended, the level of customization may be limited for certain use cases. Developers may need to resort to writing their own code for specific requirements that go beyond the scope of the STL.

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