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C++ Vector Clear Method

The C++ vector::clear() function is used to remove every element from a vector and is defined in the <vector> library. When the vector is empty, its size is 0. Vectors are useful data structures that function like dynamic one-dimensional arrays.The time complexity of the clear() function is linear.

Calling this function does not guarantee that a reallocation will take place or that the vector capacity will change. It invalidates any iterators, references, or pointers referring to the contained elements. Any past-the-end iterators is also revoked. The clear() function has linear complexity that scales with the size of the container.

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

void clear() noexcept;

It doesn't contains any kind of parameters.

Let's consider the following example, where we are going to use the clear() function and retrieving the output which print total elements in vector before using clear() and after using clear().

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

int main(){
   vector<int> tutorial = {12,23,34,45,56,67};
   unsigned int vectorsize = tutorial.size();
   cout << "Before clear() :" << vectorsize << endl;
   tutorial.clear();
   vectorsize = tutorial.size();
   cout << "After clear(): " << vectorsize << endl;
   cout << endl;
   return 0;
}

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

Before clear() :6
After clear(): 0

In the following example, we are going to use the clear() function and observing that the capacity of the vector remains same after the use of clear().

#include <iostream>
#include <algorithm>
#include <vector>

int main(){
   std::vector<int> tutorial{11,22,33,44,55};
   auto print = [](const int& n) {
      std::cout << " " << n;
   };
   std::cout << "Before clear:";
   std::for_each(tutorial.begin(), tutorial.end(), print);
   std::cout << "\nSize=" << tutorial.size() << ", Capacity=" << tutorial.capacity() << '\n';
   std::cout << "Clear\n";
   tutorial.clear();
   std::cout << "After clear:";
   std::for_each(tutorial.begin(), tutorial.end(), print);
   std::cout << "\nSize=" << tutorial.size() << ", Capacity=" << tutorial.capacity() << '\n';
}

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

Before clear: 11 22 33 44 55
Size=5, Capacity=5
Clear
After clear:
Size=0, Capacity=5

Considering the following example, which throws an error as we declared vector with constant that makes the clear() function to throw error.

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

int main(){
   const vector<char> x = {'W', 'E', 'L', 'C', '0', 'M','E'};
   for (vector<char>::const_iterator it = x.begin(); it != x.end(); it++)
      cout << *it << ' ';
   cout << endl;
   x.clear();
   for (vector<char>::const_iterator it = x.begin(); it != x.end(); it++)
      cout << *it << ' ';
   cout << endl;
   return 0;
}

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

main.cpp:10:12: error: passing 'const std::vector<char>

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