How to Use Deleted Functions to Prevent Object Copying in C++?

In C++, the class always has a copy constructor and assignment operator, whether it is default or user-defined which allows the program to create copies of the objects of that class. But sometimes, we need to create a class whose object should not be copied. In this article, we will learn how to use the deleted function concept to prevent object copying in C++.

Prevent Object Copying using Deleted Functions

We can prevent the copying of an object belonging to a particular class by deleting the constructor and copy assignment operator using the delete keyword. To delete a function, we can use the following syntax:

function_declaration {} = delete;

C++ Program to Prevent Object Copying Using Deleted Functions

// C++ program demonstrating a class that cannot be copied
#include <iostream>

using namespace std;

class NonCopyables {
public:
    NonCopyables() = default;
    // Delete copy constructor
    NonCopyables(const NonCopyables&) = delete;
    // Delete assignment operator
    NonCopyables& operator=(const NonCopyables&) = delete;

    // Member function to display a message
    void showMessage() const
    {
        cout << "This object cannot be copied." << endl;
    }
};

int main()
{
    // Create an instance of the NonCopyables class
    NonCopyables obj1;
    // Call the showMessage method to display a message
    NonCopyables obj2 = obj1;
    return 0;
}

Output

main.cpp: In function ‘int main()’:
main.cpp:26:25: error: use of deleted function ‘NonCopyables::NonCopyables(const NonCopyables&)’
   26 |     NonCopyables obj2 = obj1;
      |                         ^~~~
main.cpp:10:5: note: declared here
   10 |     NonCopyables(const NonCopyables&) = delete;
      |     ^~~~~~~~~~~~

Explanation

We define a class NonCopyables with the default constructor and delete the copy constructor and assignment operator using the delete keyword. In the main() function, we attempt to create a copy of obj1 which results in a compilation error due to a deleted copy constructor. Then we tried to create a copy of the obj1 but we got the error instead.