C++ offers two keywords for defining new types: typedef and using. Both of these keywords enable the creation of custom type names for variable declarations, albeit with subtle distinctions in their functionality.

The keyword typedef is a longstanding component of the C++ language, present since its inception. Its purpose is to establish a new type name that functions as a synonym for an existing type. For instance, typedef can be employed to define a new type called MyInt, which serves as an alias for the standard int type:

Syntax

typedef int MyInt;

Following this explanation, you can employ MyInt for declaring variables in the same manner as you would utilize int:

Snippet

MyInt x = 5;

Introduced in C++11, the using keyword is a modern addition that functions similarly to typedef. However, it offers a more adaptable syntax and is applicable in a wider range of contexts within your code. For instance, you can utilize using to establish a new type name just like typedef:

Snippet

using MyInt = int;

This establishes a fresh type called MyInt, serving as a synonym for the existing int type. Subsequently, you can employ MyInt to define variables in a similar manner to how you would utilize int.

In most cases, employing using is viewed as a more contemporary and adaptable method for declaring new type names in C++. The typedef functionality is maintained for compatibility with older code, yet it is recommended to utilize using for newer implementations.

Here is an illustration of employing the using keyword to declare a new type called MyInt, which serves as a synonym for the predefined int type:

C++ code (Example-1)

#include <iostream>

// Define a new type named MyInt that is an alias for int

using MyInt = int;

// The main driver code functionality ends from here

int main()

{

    // Declare a variable of type MyInt

    MyInt x = 5;

    // Print the value of the variable

    std::cout << "x = " << x << std::endl;

    return 0;

// The main driver code functionality ends from here

}

This code will print the following output:

Keep in mind that the utilization of ```

Size: 10

Capacity: 10

Size: 9

Capacity: 10

Size: 0

Capacity: 10

### C++ code (Example-2)

include <iostream>

include <vector>

using namespace std;

int main

{

// Create a vector of integers with an initial capacity of 5 elements

vector<int> numbers(5);

// Add some elements to the vector

numbers.push_back(10);

numbers.push_back(20);

numbers.push_back(30);

numbers.push_back(40);

numbers.push_back(50);

// Print the size and capacity of the vector

cout << "Size: " << numbers.size << endl;

cout << "Capacity: " << numbers.capacity << endl;

// Remove the last element from the vector

numbers.pop_back;

// Print the size and capacity of the vector after removing an element

cout << "Size: " << numbers.size << endl;

cout << "Capacity: " << numbers.capacity << endl;

// Clear all elements from the vector

numbers.clear;

// Print the size and capacity of the vector after clearing

cout << "Size: " << numbers.size << endl;

cout << "Capacity: " << numbers.capacity << endl;

return 0;

}

Output:

Size: 10

Capacity: 10

Size: 9

Capacity: 10

Size: 0

Capacity: 10

In this example, we instantiate a 'vector' instance to store a series of integer values. Subsequently, we append several elements to the vector utilizing the 'push_back' function and display its current size and total capacity. Following that, we eliminate the final element from the vector by employing the 'pop_back' function and exhibit the updated size and capacity of the vector.

### C++ code (typedef example)

include <iostream>

include <vector>

using namespace std;

// Create an alias for a vector of integers

typedef vector<int> IntVector;

int main

{

// Create a vector of integers using the alias

IntVector numbers;

// Add some elements to the vector

numbers.push_back(10);

numbers.push_back(20);

numbers.push_back(30);

// Print the elements of the vector

for (IntVector::iterator it = numbers.begin; it != numbers.end; ++it)

{

cout << *it << " ";

}

return 0;

}

Output:

10 20 30

In this code snippet, the 'typedef' keyword is employed to establish an alias named 'IntVector' for a 'vector' containing integers. Subsequently, this alias is utilized to instantiate a 'vector' object and append elements to it.