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

C++ Vector Size

The main difference between the size() and capacity() is that the size is exaclty the number of elements in a given vector whereas the capacity is maximum number of elements a vector can have without the reallocation of the memory. When this limit is reached then the capacity is expanded automatically. The time complexity of the size() is constant.

The main distinction between size() and capacity() is that the size() returns the exact number of elements in a given vector, capacity() returns the maximum number of elements a vector may contain without reallocating memory. When this limit is achieved, the capacity is automatically increased.

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

size_type size() const noexcept;

It doesn't accept any kind of parameter.

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

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

int main(){
   vector<int> tutorial{11,22,33,44};
   cout << tutorial.size();
   return 0;
}

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

4

Considering the another scenario, where we are going to take an vector with no elements and applying size() function.

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

int main() {
   vector<int> myvector;
   cout << myvector.size();
}

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

0

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

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

int main() {
   vector<int> nums;
   nums.push_back(11);
   nums.push_back(22);
   nums.push_back(33);
   nums.push_back(44);
   cout << nums.size();
}

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

4

Following is the another example, where we are going to take string values and applying the size() function.

#include<iostream>  
#include<vector>  
using namespace std;
  
int main(){  
   vector<string> myvector{"Welcome To TP","ABCD"};  
   int n=myvector.size();  
   cout<<"Size :"<<n;  
   return 0;  
}  

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

Size :2

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