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Showing content from https://www.tutorialspoint.com/cpp_standard_library/cpp_vector_front.htm below:

C++ vector::front() Function

The C++ vector::front() function returns a reference to the first element and was available in the <vector> header file. A segmentation fault error will be returned if we attempt to call the front() function on a vector that is empty.

In contrast to begin() function which returns an iterator pointing to the first element of the vector, the front() function returns a reference to the first element in the vector container. The time complexity of the front() function is constant.

Following is the syntax for C++ vector::front() Function −

reference front();const_reference front() const;

It doesn't accept any kind of parameters.

Let's consider the following example, where we are going to use front() function.

#include <iostream>
#include <vector>
using namespace std;

int main(void) {
   vector<int> v = {1, 2, 3, 4, 5};
   cout << "First element of vector = " << v.front() << endl;
   return 0;
}

When we compile and run the above program, this will produce the following result −

First element of vector = 1

Considering the another scenario, where we are going to use the string value and applying the frot() function.

#include<iostream>
#include<vector>
using namespace std;

int main() {  
   vector<string>  myvector{"SHINCHAN","SHEERO","KICK"};  
   cout<<myvector.front();  
   return 0;   
}

On running the above program, it will produce the following result −

SHINCHAN

In the following example, we are going to use push_back() function and then applying the front() function.

#include <iostream>
#include <vector>
using namespace std;

int main(){ 
   vector<int> myvector; 
   myvector.push_back(11); 
   myvector.push_back(33); 
   myvector.push_back(98); 
   cout << myvector.front(); 
   return 0; 
}

When we execute the above program, it will produce the following result −

11

Following is the example, where we are going to do subtraction assignment.

#include <iostream>
#include <vector>

int main (){
   std::vector<int> tutorial;
   tutorial.push_back(33);
   tutorial.push_back(13);
   tutorial.front() -= tutorial.back();
   std::cout << "Element is: " << tutorial.front() << '\n';
   return 0;
} 

On running the above program, it will produce the following result −

Element is: 20

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