C++ 'Using' vs 'Typedef'

C++ has two keywords that can be used to define new types: typedef and using. Both of these keywords allow you to create a new type name that can be used to declare variables, but they work in slightly different ways.

typedef is an older keyword that has been part of the C++ language since the beginning. It is used to create a new type name that is an alias for an existing type. For example, you could use typedef to create a new type named MyInt that is an alias for the built-in int type:

Syntax

After this definition, you can use MyInt to declare variables just like you would use int:

Snippet

Using is a newer keyword that was introduced in C++11. It works in a similar way to typedef, but it has a more flexible syntax and can be used in more places in your code. For example, you can use using to define a new type name in the same way as typedef:

Snippet

This creates a new type named MyInt, which is an alias for the built-in int type. You can then use MyInt to declare variables just like you would use int.

In general, using is considered to be a more modern and flexible way to define new type names in C++. typedef is still supported for backward compatibility, but the newest code should use using instead.

Here is an example of using the using keyword to define a new type named MyInt that is an alias for the built-in 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:

x = 5

Note that using is not limited to defining type aliases for built-in types like int. You can use it to define type aliases for any type, including user-defined types, template types, and more.

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 code, we create a 'vector' object that can hold a sequence of integers. We then add some elements to the vector using the 'push_back' method and print its size and capacity. Next, we remove the last element from the vector using the 'pop_back' method and print its size and capacity again

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, we use the 'typedef' keyword to create an alias called 'IntVector' for a 'vector' of integers. We then use this alias to create a 'vector' object and add some elements to it.

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