An In-Depth Guide to Vectors in C++ STL

Vectors allow you to store data elements dynamically so you can resize as needed. They provide a flexible, powerful way to handle arrays in C++. This guide will serve as your complete reference to leveraging vectors effectively.

Overview

This article aims to explain everything you need to know about vectors in C++ STL. We will start by understanding what vectors are and what benefits they provide over traditional arrays.

We will then explore the various functions you can use to initialize, access, iterate and manipulate vectors. Usage examples are provided to demonstrate how vectors can be leveraged to store custom data types and objects.

We also analyze the performance and complexity tradeoffs of common vector operations. Finally, we conclude by summarizing the key differences between vectors and plain C++ arrays.

So whether you‘re a beginner looking for an introduction or an experienced programmer needing a quick refresher, this vector guide has you covered!

What Exactly is a Vector in C++?

A vector in C++ is a template class from the Standard Template Library that implements a dynamic array. "Dynamic" means vectors can grow or shrink in size automatically as you insert or delete elements.

Some key properties that make vectors flexible and powerful:

• Contiguous storage: Elements stored side-by-side for faster access through direct indexing like arrays.
• Dynamic expansion: More memory allocated as you add elements and capacity exceeds initial allocation.
• Abstraction: No need to directly deal with lower level memory management.
• Flexibility: Vectors can adapt to store any data type.

Let‘s analyze the memory representation of a vector to understand how it differs from traditional arrays:

As you insert items, a vector can dynamically allocate more contiguous memory to accommodate new elements. This is handled automatically without your intervention. Arrays have fixed allocations, so you‘d need to manually create a new larger array and copy data over.

Use cases best suited for vectors:

• Storing data elements of same type together
• Managing collections where size changes frequently
• Require speedy insert/delete from end of sequence
• Require ease of use over manual memory control

So basically, vectors strike the right balance between the ease-of-use of dynamic data structures and speed of static arrays!

Common Vector Operations and Functions

Now that you understand what vectors are and what makes them so useful, let‘s go through some of the most popular functions for initializing, modifying and accessing vector elements:

Initialize a Vector

// Empty vector
vector<int> vec1;  

// Vector with 5 integer elements  
vector<int> vec2(5);  

// Initialize values
vector<int> vec3 {1, 2, 3, 4, 5};

You can also use initializer list constructors:

// Initializer list
vector<int> vec {  
  vec.push_back(1),
  vec.push_back(2),
  vec.push_back(3)
};

Inserting and Removing Elements

// Insert at the beginning
vec.insert(vec.begin(), 3);

// Erase second element
vec.erase(vec.begin()+1);  

// Append element 
vec.push_back(5);

// Delete last element  
vec.pop_back();

Insert and erase operations take linear O(n) time. But push/pop from the end take O(1) time.

Access and Modify Elements

// Modify element at index 3
vec[3] = 10;  

// Front and back element
int first = vec.front(); 
int last = vec.back();

Accessing any element takes constant O(1) time.

Iterating over Elements

for (auto it = vec.begin(); it != vec.end(); ++it) {
  // *it gives element
  cout << *it << "\n";
}

// Reverse iteration
for(auto rit = vec.rbegin(); rit!=vec.rend(); ++rit){    
  cout << *rit << "\n";
}  

Iterators provide an easy way to traverse vectors.

Vector Member Functions

And many more! We‘ve covered the most common and useful vector operations here.

Vector Usage Examples

Vectors are extremely versatile. Let‘s look at some examples of how they can be used to store and process data in C++ programs.

Storing User Defined Objects

We can create vectors to store custom structs and class objects:

struct Employee {
  int id;
  string name; 

  // Constructor 
  Employee(int id, string name) {
    this->id = id;
    this->name = name;
  }
};  

// Vector of employees
vector<Employee> employees;   

Employee e1(101, "John");
employees.push_back(e1); 

// Add more employee objects

Passing Vectors to Functions

Vectors can be normally passed as arguments just like primitive types:

// Function to display vector 
void display(vector<int>& vec) {   
   //...
}

void main() {

  vector<int> myVec{ 1, 2, 3};

  display(myVec); 
}

We pass vector by reference to avoid copying.

Sorting Elements

bool compare(int a, int b) {
  return a > b;  
}

// Sort vector descending
std::sort(vec.begin(), vec.end(), compare);  

The std::sort() algorithm works seamlessly with vectors.

And many more usage examples can be built using vectors!

Vector Arithmetic Operations

We can even overload operators to perform arithmetic vector operations:

// Overload + operator 
vector<int> operator+(const vector<int>& lhs, const vector<int>& rhs) {

  vector<int> result;
  // ...
  return result; 
}

// Usage:
vector<int> v1{1, 2, 3}, v2{4, 5}; 
vector<int> sum = v1 + v2; // {5, 7, 3}

Comparing Performance vs Arrays

Though vectors have many advantages, plain arrays can offer better performance in some cases. Here is a summary of the differences:

Vectors enable easier coding, readability, code reuse and maintainability. But for absolute speed critical applications without dynamic size needs, arrays may edge out vectors.

Wrapping Up

Vectors provide a flexible interface for you to work with dynamic arrays easily in C++ without handling allocations. They strike an optimal balance between performance and ease of use.

Their dynamic expansion, coupled with helpful member functions to access and manipulate data makes vector an extremely versatile container for a variety of applications. I hope this guide gives you a firm grasp of the vector class so you can leverage its power effectively!