constructor vs assignment operator

• C++ Classes and Objects
• C++ Polymorphism

C++ Inheritance

• C++ Abstraction
• C++ Encapsulation
• C++ OOPs Interview Questions
• C++ OOPs MCQ

C++ Interview Questions

C++ function overloading.

• C++ Programs
• C++ Preprocessor

C++ Templates

• C++ Programming Language

C++ Overview

• Introduction to C++ Programming Language
• Features of C++
• History of C++
• Interesting Facts about C++
• Setting up C++ Development Environment
• Difference between C and C++
• Writing First C++ Program - Hello World Example
• C++ Basic Syntax
• C++ Comments
• Tokens in C
• C++ Keywords
• Difference between Keyword and Identifier in C

C++ Variables and Constants

• C++ Variables
• Constants in C
• Scope of Variables in C++
• Storage Classes in C++ with Examples
• Static Keyword in C++

C++ Data Types and Literals

• C++ Data Types
• Literals in C
• Derived Data Types in C++
• User Defined Data Types in C++
• Data Type Ranges and their macros in C++
• C++ Type Modifiers
• Type Conversion in C++
• Casting Operators in C++

C++ Operators

• Operators in C++
• C++ Arithmetic Operators
• Unary operators in C
• Bitwise Operators in C
• Assignment Operators in C
• C++ sizeof Operator
• Scope resolution operator in C++

C++ Input/Output

• Basic Input / Output in C++
• cout in C++
• cerr - Standard Error Stream Object in C++
• Manipulators in C++ with Examples

C++ Control Statements

• Decision Making in C (if , if..else, Nested if, if-else-if )
• C++ if Statement
• C++ if else Statement
• C++ if else if Ladder
• Switch Statement in C++
• Jump statements in C++
• for Loop in C++
• Range-based for loop in C++
• C++ While Loop
• C++ Do/While Loop

C++ Functions

• Functions in C++
• return statement in C++ with Examples
• Parameter Passing Techniques in C
• Difference Between Call by Value and Call by Reference in C
• Default Arguments in C++
• Inline Functions in C++
• Lambda expression in C++

C++ Pointers and References

• Pointers and References in C++
• C++ Pointers
• Dangling, Void , Null and Wild Pointers in C
• Applications of Pointers in C
• Understanding nullptr in C++
• References in C++
• Can References Refer to Invalid Location in C++?
• Pointers vs References in C++
• Passing By Pointer vs Passing By Reference in C++
• When do we pass arguments by pointer?
• Variable Length Arrays (VLAs) in C
• Pointer to an Array | Array Pointer
• How to print size of array parameter in C++?
• Pass Array to Functions in C
• What is Array Decay in C++? How can it be prevented?

C++ Strings

• Strings in C++
• std::string class in C++
• Array of Strings in C++ - 5 Different Ways to Create
• String Concatenation in C++
• Tokenizing a string in C++
• Substring in C++

C++ Structures and Unions

• Structures, Unions and Enumerations in C++
• Structures in C++
• C++ - Pointer to Structure
• Self Referential Structures
• Difference Between C Structures and C++ Structures
• Enumeration in C++
• typedef in C++
• Array of Structures vs Array within a Structure in C

C++ Dynamic Memory Management

• Dynamic Memory Allocation in C using malloc(), calloc(), free() and realloc()
• new and delete Operators in C++ For Dynamic Memory
• new vs malloc() and free() vs delete in C++
• What is Memory Leak? How can we avoid?
• Difference between Static and Dynamic Memory Allocation in C

C++ Object-Oriented Programming

• Object Oriented Programming in C++
• Access Modifiers in C++
• Friend Class and Function in C++
• Constructors in C++
• Default Constructors in C++

Copy Constructor in C++

• Destructors in C++
• Private Destructor in C++
• When is a Copy Constructor Called in C++?
• Shallow Copy and Deep Copy in C++
• When Should We Write Our Own Copy Constructor in C++?
• Does C++ compiler create default constructor when we write our own?
• C++ Static Data Members
• Static Member Function in C++
• 'this' pointer in C++
• Scope Resolution Operator vs this pointer in C++
• Local Classes in C++
• Nested Classes in C++
• Enum Classes in C++ and Their Advantage over Enum DataType
• Difference Between Structure and Class in C++
• Why C++ is partially Object Oriented Language?

C++ Encapsulation and Abstraction

• Encapsulation in C++
• Abstraction in C++
• Difference between Abstraction and Encapsulation in C++
• Function Overriding in C++
• Virtual Functions and Runtime Polymorphism in C++
• Difference between Inheritance and Polymorphism
• Function Overloading in C++
• Constructor Overloading in C++
• Functions that cannot be overloaded in C++
• Function overloading and const keyword
• Function Overloading and Return Type in C++
• Function Overloading and float in C++
• C++ Function Overloading and Default Arguments
• Can main() be overloaded in C++?
• Function Overloading vs Function Overriding in C++
• Advantages and Disadvantages of Function Overloading in C++

C++ Operator Overloading

• Operator Overloading in C++
• Types of Operator Overloading in C++
• Functors in C++
• What are the Operators that Can be and Cannot be Overloaded in C++?
• Inheritance in C++
• C++ Inheritance Access
• Multiple Inheritance in C++
• C++ Hierarchical Inheritance
• C++ Multilevel Inheritance
• Constructor in Multiple Inheritance in C++
• Inheritance and Friendship in C++
• Does overloading work with Inheritance?

C++ Virtual Functions

• Virtual Function in C++
• Virtual Functions in Derived Classes in C++
• Default Arguments and Virtual Function in C++
• Can Virtual Functions be Inlined in C++?
• Virtual Destructor
• Advanced C++ | Virtual Constructor
• Advanced C++ | Virtual Copy Constructor
• Pure Virtual Functions and Abstract Classes in C++
• Pure Virtual Destructor in C++
• Can Static Functions Be Virtual in C++?
• RTTI (Run-Time Type Information) in C++
• Can Virtual Functions be Private in C++?

C++ Exception Handling

• Exception Handling in C++
• Exception Handling using classes in C++
• Stack Unwinding in C++
• User-defined Custom Exception with class in C++

C++ Files and Streams

• File Handling through C++ Classes
• I/O Redirection in C++
• Templates in C++ with Examples
• Template Specialization in C++
• Using Keyword in C++ STL

C++ Standard Template Library (STL)

• The C++ Standard Template Library (STL)
• Containers in C++ STL (Standard Template Library)
• Introduction to Iterators in C++
• Algorithm Library | C++ Magicians STL Algorithm

C++ Preprocessors

• C Preprocessors
• C Preprocessor Directives
• #include in C
• Difference between Preprocessor Directives and Function Templates in C++

C++ Namespace

• Namespace in C++ | Set 1 (Introduction)
• namespace in C++ | Set 2 (Extending namespace and Unnamed namespace)
• Namespace in C++ | Set 3 (Accessing, creating header, nesting and aliasing)
• C++ Inline Namespaces and Usage of the "using" Directive Inside Namespaces

Advanced C++

• Multithreading in C++
• Smart Pointers in C++
• auto_ptr vs unique_ptr vs shared_ptr vs weak_ptr in C++
• Type of 'this' Pointer in C++
• "delete this" in C++
• Passing a Function as a Parameter in C++
• Signal Handling in C++
• Generics in C++
• Difference between C++ and Objective C
• Write a C program that won't compile in C++
• Write a program that produces different results in C and C++
• How does 'void*' differ in C and C++?
• Type Difference of Character Literals in C and C++
• Cin-Cout vs Scanf-Printf

C++ vs Java

• Similarities and Difference between Java and C++
• Comparison of Inheritance in C++ and Java
• How Does Default Virtual Behavior Differ in C++ and Java?
• Comparison of Exception Handling in C++ and Java
• Foreach in C++ and Java
• Templates in C++ vs Generics in Java
• Floating Point Operations & Associativity in C, C++ and Java

Competitive Programming in C++

• Competitive Programming - A Complete Guide
• C++ tricks for competitive programming (for C++ 11)
• Writing C/C++ code efficiently in Competitive programming
• Why C++ is best for Competitive Programming?
• Test Case Generation | Set 1 (Random Numbers, Arrays and Matrices)
• Fast I/O for Competitive Programming
• Setting up Sublime Text for C++ Competitive Programming Environment
• How to setup Competitive Programming in Visual Studio Code for C++
• Which C++ libraries are useful for competitive programming?
• Common mistakes to be avoided in Competitive Programming in C++ | Beginners
• C++ Interview Questions and Answers (2024)
• Top C++ STL Interview Questions and Answers
• 30 OOPs Interview Questions and Answers (2024)
• Top C++ Exception Handling Interview Questions and Answers
• C++ Programming Examples

Pre-requisite: Constructor in C++ 

A copy constructor is a member function that initializes an object using another object of the same class. In simple terms, a constructor which creates an object by initializing it with an object of the same class, which has been created previously is known as a copy constructor .  

Copy constructor is used to initialize the members of a newly created object by copying the members of an already existing object.

Copy constructor takes a reference to an object of the same class as an argument.

The process of initializing members of an object through a copy constructor is known as copy initialization.

It is also called member-wise initialization because the copy constructor initializes one object with the existing object, both belonging to the same class on a member by member copy basis.

The copy constructor can be defined explicitly by the programmer. If the programmer does not define the copy constructor, the compiler does it for us.  

Syntax of Copy Constructor

Characteristics of Copy Constructor

1. The copy constructor is used to initialize the members of a newly created object by copying the members of an already existing object.

2. Copy constructor takes a reference to an object of the same class as an argument. If you pass the object by value in the copy constructor, it would result in a recursive call to the copy constructor itself. This happens because passing by value involves making a copy, and making a copy involves calling the copy constructor, leading to an infinite loop. Using a reference avoids this recursion. So we use reference of Objects to avoid infinite calls.

3. The process of initializing members of an object through a copy constructor is known as copy initialization.

4 . It is also called member-wise initialization because the copy constructor initializes one object with the existing object, both belonging to the same class on a member-by-member copy basis.

5. The copy constructor can be defined explicitly by the programmer. If the programmer does not define the copy constructor, the compiler does it for us.

Types of Copy Constructors

1. default copy constructor.

An implicitly defined copy constructor will copy the bases and members of an object in the same order that a constructor would initialize the bases and members of the object.

2. User Defined Copy Constructor 

A user-defined copy constructor is generally needed when an object owns pointers or non-shareable references, such as to a file, in which case a destructor and an assignment operator should also be written

When is the copy constructor called? 

In C++, a Copy Constructor may be called in the following cases: 

• When an object of the class is returned by value. 
• When an object of the class is passed (to a function) by value as an argument. 
• When an object is constructed based on another object of the same class. 
• When the compiler generates a temporary object.

It is, however, not guaranteed that a copy constructor will be called in all these cases, because the C++ Standard allows the compiler to optimize the copy away in certain cases, one example is the return value optimization (sometimes referred to as RVO).

Copy Elision

In copy elision , the compiler prevents the making of extra copies which results in saving space and better the program complexity(both time and space); Hence making the code more optimized.  

Now it is on the compiler to decide what it wants to print, it could either print the above output or it could print case 1 or case 2 below, and this is what Return Value Optimization is. In simple words, RVO is a technique that gives the compiler some additional power to terminate the temporary object created which results in changing the observable behavior/characteristics of the final program.

When is a user-defined copy constructor needed? 

If we don’t define our own copy constructor, the C++ compiler creates a default copy constructor for each class which does a member-wise copy between objects. The compiler-created copy constructor works fine in general. We need to define our own copy constructor only if an object has pointers or any runtime allocation of the resource like a file handle , a network connection, etc.  

The default constructor does only shallow copy.  

Deep copy is possible only with a user-defined copy constructor. In a user-defined copy constructor, we make sure that pointers (or references) of copied objects point to new memory locations.  

Copy constructor vs Assignment Operator 

The main difference between Copy Constructor and Assignment Operator is that the Copy constructor makes a new memory storage every time it is called while the assignment operator does not make new memory storage.

Which of the following two statements calls the copy constructor and which one calls the assignment operator? 

A copy constructor is called when a new object is created from an existing object, as a copy of the existing object. The assignment operator is called when an already initialized object is assigned a new value from another existing object. In the above example (1) calls the copy constructor and (2) calls the assignment operator. See this for more details.

Example – Class Where a Copy Constructor is Required 

Following is a complete C++ program to demonstrate the use of the Copy constructor. In the following String class, we must write a copy constructor. 

What would be the problem if we remove the copy constructor from the above code? 

If we remove the copy constructor from the above program, we don’t get the expected output. The changes made to str2 reflect in str1 as well which is never expected.   

Can we make the copy constructor private?  

Yes, a copy constructor can be made private. When we make a copy constructor private in a class, objects of that class become non-copyable. This is particularly useful when our class has pointers or dynamically allocated resources. In such situations, we can either write our own copy constructor like the above String example or make a private copy constructor so that users get compiler errors rather than surprises at runtime.

Why argument to a copy constructor must be passed as a reference?  

A copy constructor is called when an object is passed by value. Copy constructor itself is a function. So if we pass an argument by value in a copy constructor, a call to the copy constructor would be made to call the copy constructor which becomes a non-terminating chain of calls. Therefore compiler doesn’t allow parameters to be passed by value.

Why argument to a copy constructor should be const?

One reason for passing const reference is, that we should use const in C++ wherever possible so that objects are not accidentally modified. This is one good reason for passing reference as const , but there is more to it than ‘ Why argument to a copy constructor should be const?’

Please Login to comment...

Similar reads.

• cpp-constructor

Improve your Coding Skills with Practice

What kind of Experience do you want to share?

• Graphics and multimedia
• Language Features
• Unix/Linux programming
• Source Code
• Standard Library
• Tips and Tricks
• Tools and Libraries
• Windows API
• Copy constructors, assignment operators,

Copy constructors, assignment operators, and exception safe assignment

Home » Technology » IT » Programming » What is the Difference Between Copy Constructor and Assignment Operator

What is the Difference Between Copy Constructor and Assignment Operator

The main difference between copy constructor and assignment operator is that copy constructor is a type of constructor that helps to create a copy of an already existing object without affecting the values of the original object while assignment operator is an operator that helps to assign a new value to a variable in the program.

A constructor is a special method that helps to initialize an object when creating it. It has the same name as the class name and has no return type. A programmer can write a constructor to give initial values to the instance variables in the class. If there is no constructor in the program, the default constructor will be called. Copy constructor is a type of constructor that helps to create a copy of an existing object. On the other hand, assignment operator helps to assign a new value to a variable.

Key Areas Covered

1. What is Copy Constructor      – Definition, Functionality 2. What is Assignment Operator      – Definition, Functionality 3. What is the Difference Between Copy Constructor and Assignment Operator      – Comparison of Key Differences

Constructor, Copy Constructor, Assignment Operator, Variable

What is Copy Constructor

In programming, sometimes it is necessary to create a separate copy of an object without affecting the original object. Copy constructor is useful in these situations. It allows creating a replication of an existing object of the same class. Refer the below example.

Figure 1: Program with copy constructor

The class Triangle has two instance variables called base and height. In line 8, there is a parameterized constructor. It takes two arguments. These values are assigned to the instance variables base and height. In line 13, there is a copy constructor. It takes an argument of type Triangle. New object’s base value is assigned to the instance variable base. Similarly, the new object’s height value is assigned to the instance variable height. Furthermore, there is a method called calArea to calculate and return area.

In the main method, t1 and t2 are Triangle objects. The object t1 is passed when creating the t2 object. The copy constructor is called to create t2 object. Therefore, the base and the height of the t2 object is the same as the base and height of t1 object. Finally, both objects have the same area.    

What is Assignment Operator

An assignment operator is useful to assign a new value to a variable. The assignment operator is “=”.  When there is a statement as c = a + b; the summation of ‘a’ and ‘b’ is assigned to the variable ‘c’.

Figure 2: Program with assignment operator

The class Number has an instance variable called num. There is a no parameter constructor in line 7. However, there is a parameterized constructor in line 9. It takes an argument and assigns it to the instance variable using the assignment operator. In line 12, there is a method called display to display the number. In the main method, num1 and num2 are two objects of type Number. Printing num1 and num2 gives the references to those objects. The num3 is of type Number. In line 24, num1 is assigned to num3 using the assignment operator. Therefore, num3 is referring to num1 object. So, printing num3 gives the num1 reference.  

The assignment operator and its variations are as follows.

Difference Between Copy Constructor and Assignment Operator

Copy constructor is a special constructor for creating a new object as a copy of an existing object. In contrast, assignment operator is an operator that is used to assign a new value to a variable. These definitions explain the basic difference between copy constructor and assignment operator.

Functionality with Objects

Functionality with objects is also a major difference between copy constructor and assignment operator. Copy constructor initializes the new object with an already existing object while assignment operator assigns the value of one object to another object which is already in existence.

Copy constructor helps to create a copy of an existing object while assignment operator helps to assign a new value to a variable. This is another difference between copy constructor and assignment operator.

The difference between copy constructor and assignment operator is that copy constructor is a type of constructor that helps to create a copy of an already existing object without affecting the values of the original object while assignment operator is an operator that helps to assign a new value to a variable in the program.

1. Thakur, Dinesh. “Copy Constructor in Java Example.” Computer Notes, Available here .

About the Author: Lithmee

Lithmee holds a Bachelor of Science degree in Computer Systems Engineering and is reading for her Master’s degree in Computer Science. She is passionate about sharing her knowldge in the areas of programming, data science, and computer systems.

​You May Also Like These

Leave a reply cancel reply.

Tech Differences

Know the Technical Differences

Difference Between Copy Constructor and Assignment Operator in C++

Let us study the difference between the copy constructor and assignment operator.

Content: Copy Constructor Vs Assignment Operator

Comparison chart.

• Key Differences

Definition of Copy Constructor

A “copy constructor” is a form of an overloaded constructor . A copy constructor is only called or invoked for initialization purpose. A copy constructor initializes the newly created object by another existing object.

When a copy constructor is used to initialize the newly created target object, then both the target object and the source object shares a different memory location. Changes done to the source object do not reflect in the target object. The general form of the copy constructor is

If the programmer does not create a copy constructor in a C++ program, then the compiler implicitly provides a copy constructor. An implicit copy constructor provided by the compiler does the member-wise copy of the source object. But, sometimes the member-wise copy is not sufficient, as the object may contain a pointer variable.

Copying a pointer variable means, we copy the address stored in the pointer variable, but we do not want to copy address stored in the pointer variable, instead, we want to copy what pointer points to. Hence, there is a need of explicit ‘copy constructor’ in the program to solve this kind of problems.

A copy constructor is invoked in three conditions as follow:

• Copy constructor invokes when a new object is initialized with an existing one.
• The object passed to a function as a non-reference parameter.
• The object is returned from the function.

Let us understand copy constructor with an example.

In the code above, I had explicitly declared a constructor “copy( copy &c )”. This copy constructor is being called when object B is initialized using object A. Second time it is called when object C is being initialized using object A.

When object D is initialized using object A the copy constructor is not called because when D is being initialized it is already in the existence, not the newly created one. Hence, here the assignment operator is invoked.

Definition of Assignment Operator

The assignment operator is an assigning operator of C++.  The “=” operator is used to invoke the assignment operator. It copies the data in one object identically to another object. The assignment operator copies one object to another member-wise. If you do not overload the assignment operator, it performs the bitwise copy. Therefore, you need to overload the assignment operator.

In above code when object A is assigned to object B the assignment operator is being invoked as both the objects are already in existence. Similarly, same is the case when object C is initialized with object A.

When the bitwise assignment is performed both the object shares the same memory location and changes in one object reflect in another object.

Key Differences Between Copy Constructor and Assignment Operator

• A copy constructor is an overloaded constructor whereas an assignment operator is a bitwise operator.
• Using copy constructor you can initialize a new object with an already existing object. On the other hand, an assignment operator copies one object to the other object, both of which are already in existence.
• A copy constructor is initialized whenever a new object is initialized with an already existing object, when an object is passed to a function as a non-reference parameter, or when an object is returned from a function. On the other hand, an assignment operator is invoked only when an object is being assigned to another object.
• When an object is being initialized using copy constructor, the initializing object and the initialized object shares the different memory location. On the other hand, when an object is being initialized using an assignment operator then the initialized and initializing objects share the same memory location.
• If you do not explicitly define a copy constructor then the compiler provides one. On the other hand, if you do not overload an assignment operator then a bitwise copy operation is performed.

The Copy constructor is best for copying one object to another when the object contains raw pointers.

Related Differences:

• Difference Between & and &&
• Difference Between Recursion and Iteration
• Difference Between new and malloc( )
• Difference Between Inheritance and Polymorphism
• Difference Between Constructor and Destructor

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

• Selected Reading
• UPSC IAS Exams Notes
• Developer's Best Practices
• Questions and Answers
• Effective Resume Writing
• HR Interview Questions
• Computer Glossary

Copy constructor vs assignment operator in C++

The Copy constructor and the assignment operators are used to initializing one object to another object. The main difference between them is that the copy constructor creates a separate memory block for the new object. But the assignment operator does not make new memory space. It uses the reference variable to point to the previous memory block.

Copy Constructor (Syntax)

Assignment operator (syntax).

Let us see the detailed differences between Copy constructor and Assignment Operator.

Related Articles

• Difference Between Copy Constructor and Assignment Operator in C++
• What's the difference between assignment operator and copy constructor in C++?
• Virtual Copy Constructor in C++
• How to use an assignment operator in C#?
• What is a copy constructor in C#?
• When is copy constructor called in C++?
• When should we write our own assignment operator in C++?
• What is Multiplication Assignment Operator (*=) in JavaScript?
• What is Addition Assignment Operator (+=) in JavaScript?
• Explain the concept of logical and assignment operator in C language
• When should we write our own assignment operator in C++ programming?
• Ternary operator ?: vs if…else in C/C++
• What is Bitwise OR Assignment Operator (|=) in JavaScript?
• What is Bitwise XOR Assignment Operator (^=) in JavaScript?
• Why do we need a copy constructor and when should we use a copy constructor in Java?

Kickstart Your Career

Get certified by completing the course

14.14 — Introduction to the copy constructor

An implicit copy constructor

Prefer the implicit copy constructor

The copy constructor’s parameter must be a reference

When generateFraction returns a Fraction back to main , the copy constructor is implicitly called again. And when f2 is passed to printFraction , the copy constructor is called a third time.

The rule of three is a well known C++ principle that states that if a class requires a user-defined copy constructor, destructor, or copy assignment operator, then it probably requires all three. In C++11, this was expanded to the rule of five , which adds the move constructor and move assignment operator to the list.

Table of Contents

When is a user-defined copy constructor needed, copy constructor vs assignment operator, how to make the copy constructor private, why does an argument to a copy constructor must be passed as a reference, why should the copy constructor argument be const in c++, understanding c++ copy constructor with example code.

When a new object is generated from an existing object as a copy of the existing object, the copy function Object () { [native code] } is named. When a previously initialized object is given a new value from another object, the assignment operator is used. Function Object () { [native code] } is a member function that uses another object of the same class to initialize an object. The following is a general function prototype for a copy function Object () { [native code] }:

Classname(const classname &obj);

#include<iostream>

using namespace std; 

class Point

     int a, b;

     Point(int x1, int y1) { a =x1; b = y1; } 

     // Copy constructor

     Point(const Point &p1) {a = p1.a; b = p1.b; } 

     int getX()            { return a; }

     int getY()            { return b; }

     Point p1(10, 15); // Normal constructor is called here

     Point p2 = p1; // Copy constructor is called here

     // Let us access values assigned by constructors

     cout << "p1.x = " << p1.getX() << ", p1.y = " << p1.getY();

     cout << "\np2.x = " << p2.getX() << ", p2.y = " << p2.getY();

     return 0;

According to C++ copy constructor the C++ compiler produces a default copy constructor Object() { [native code] } for each class if it doesn’t define own copy constructor Object() { [native code] }, which performs a member-wise copy between items. In general, the copy function Object() { [native code] } generated by the compiler works well. It just needs to define its own copy function Object() [native code] if an object has pointers or any runtime resource allocation, such as a file handle or a network connection.

Stand Out From Your Peers this Appraisal Season

When a new object is generated from an existing object as a copy of the existing object, the copy function Object() { [native code] } is named. When an already initialized object is given a new value from another object, the assignment operator is used.

Myclass c1,c2;

Myclass c3;

The above line illustrates the copy constructor

Where Copy Constructor is Needed:

#include<cstring>

class String

     char *s;

     int size;

     String(const char *str = NULL); // constructor

     ~String() { delete [] s; }// destructor

     String(const String&); // copy constructor

     void print() { cout << s << endl; } // Function to print string

     void change(const char *); // Function to change

 String::String(const char *str)

     size = strlen(str);

     s = new char[size+1];

     strcpy(s, str);

 void String::change(const char *str)

     delete [] s;

String::String(const String& old_str)

     size = old_str.size;

     strcpy(s, old_str.s);

     String str1("Welcome");

     String str2 = str1;

     str1.print(); // what is printed ?

     str2.print();

     str2.change("Welcome you all"); 

     str1.print(); // what is printed now ?

What Happens if We Remove the Copy Constructor from the Above Code?

In  c++ copy constructor users don't get the predicted result if we delete the copy function Object() { [native code] } from the above program. Changes to str2 are reflected in str1 as well, which is unexpected.

         char *s;

         int size;

String(const char *str = NULL); // constructor

         ~String() { delete [] s;  }// destructor

         void print() { cout << s << endl; }

         void change(const char *);  // Function to change

String::String(const char *str)

         size = strlen(str);

         s = new char[size+1];

         strcpy(s, str);

         delete [] s;

         String str1("Welcome");

         String str2 = str1; 

str1.print(); // what is printed ?

str2.print(); 

str2.change("Welcome you all"); 

str1.print(); // what is printed now ?

str2.print();

According to the C + + copy constructor it is possible to make a copy function Object() { [native code] }. When a copy function Object() { [native code] } in a class is made private, objects in that class become non-copyable. This is especially useful when the class contains pointers or resources that are dynamically allocated. In such cases, it can either write our own copy function Object() { [native code] }, as in the String example above, or create a private copy function Object() { [native code] }, in which case users will receive compiler errors rather than warnings.

According to C + + copy constructor, when an object is transferred by value, a copy function Object() { [native code] } is named. The copy function Object() { [native code] } is a function in and of itself. If an argument is passed by value in a copy function Object() { [native code] }, a call to copy function Object() { [native code] } is made to call copy function Object() { [native code] }, resulting in a non-terminating sequence of calls. As a result, the compiler won't let you move parameters by value.

Want a Top Software Development Job? Start Here!

According to C++ copy constructor, we pass an object by reference to the copy function Object() { [native code] }, and we usually pass it as a const reference. One justification for passing a const reference is that it can use const wherever possible in C++ to avoid unintentionally changing objects. This is one compelling excuse to transfer references as const, but there are others.

/* Class data members */

Test(Test &t) { /* Copy data members from t*/}

Test()  { /* Initialize data members */ }

     cout << "fun() Called\n";

     Test t;

     return t;

     Test t1;

     Test t2 = fun();

Need to change the copy constructor by the following:

Test(const Test &t) { cout << "Copy Constructor Called\n"; }

The fun() function returns a value. As a result, the compiler generates a temporary entity, which is then copied to t2 using the original program's copy function Object() { [native code] } (The temporary object is passed as an argument to the copy constructor). The compiler error occurs because compiler-created temporary objects cannot be bound to non-const references, despite the fact that the original program attempts to do so. It doesn't make sense to change something that isn't broken.

Advance your career as a MEAN stack developer with the  Full Stack Web Developer - MEAN Stack Master's Program . Enroll now!

According to C++ copy constructor, when an object is made, a function Object() { [native code] } is a special type of member function that is called automatically. A function Object() { [native code] } in C++ has the same name as the class it belongs to, and it does not have a return form.

In this article you learned about stack , implementation, operations with syntax and examples. To get expertise in C++ programming you can join Simplilearn’s C++ certification training course. For learners, Simplilearn has made a special selection of courses available for free. Join Simplilearn’s free courses and revolutionise your profession with certificates, specialisations, and dozens of other courses.

Have any questions for us? Leave them in the comments section of this article, and our experts will get back to you on them, as soon as possible!

Recommended Reads

Free eBook: Salesforce Developer Salary Report

All You Need to Know About Constructor in Java

A One-Stop Solution Guide to Understand Constructors in C#

Skills Acquisition Vs. Talent Acquisition

Copy File in Python

Operators in C#: An Ultimate C# Operations Guide With Examples

Get Affiliated Certifications with Live Class programs

Post graduate program in full stack web development.

• Live sessions on the latest AI trends, such as generative AI, prompt engineering, explainable AI, and more
• Caltech CTME Post Graduate Certificate

Full Stack Web Developer - MEAN Stack

• Comprehensive Blended Learning program
• 8X higher interaction in live online classes conducted by industry experts
• PMP, PMI, PMBOK, CAPM, PgMP, PfMP, ACP, PBA, RMP, SP, and OPM3 are registered marks of the Project Management Institute, Inc.

cppreference.com

Copy constructors.

A copy constructor is a constructor which can be called with an argument of the same class type and copies the content of the argument without mutating the argument.

[ edit ] Syntax

[ edit ] Explanation

The copy constructor is called whenever an object is initialized (by direct-initialization or copy-initialization ) from another object of the same type (unless overload resolution selects a better match or the call is elided ), which includes

• initialization: T a = b ; or T a ( b ) ; , where b is of type T ;
• function argument passing: f ( a ) ; , where a is of type T and f is void f ( T t ) ;
• function return: return a ; inside a function such as T f ( ) , where a is of type T , which has no move constructor .

[ edit ] Implicitly-declared copy constructor

If no user-defined copy constructors are provided for a class type, the compiler will always declare a copy constructor as a non- explicit inline public member of its class. This implicitly-declared copy constructor has the form T :: T ( const T & ) if all of the following are true:

• each direct and virtual base B of T has a copy constructor whose parameters are of type const B & or const volatile B & ;
• each non-static data member M of T of class type or array of class type has a copy constructor whose parameters are of type const M & or const volatile M & .

Otherwise, the implicitly-declared copy constructor is T :: T ( T & ) .

Due to these rules, the implicitly-declared copy constructor cannot bind to a volatile lvalue argument.

A class can have multiple copy constructors, e.g. both T :: T ( const T & ) and T :: T ( T & ) .

The implicitly-declared (or defaulted on its first declaration) copy constructor has an exception specification as described in dynamic exception specification (until C++17) noexcept specification (since C++17) .

[ edit ] Implicitly-defined copy constructor

If the implicitly-declared copy constructor is not deleted, it is defined (that is, a function body is generated and compiled) by the compiler if odr-used or needed for constant evaluation (since C++11) . For union types, the implicitly-defined copy constructor copies the object representation (as by std::memmove ). For non-union class types, the constructor performs full member-wise copy of the object's direct base subobjects and member subobjects, in their initialization order, using direct initialization. For each non-static data member of a reference type, the copy constructor binds the reference to the same object or function to which the source reference is bound.

[ edit ] Deleted copy constructor

The implicitly-declared or explicitly-defaulted (since C++11) copy constructor for class T is undefined (until C++11) defined as deleted (since C++11) if any of the following conditions is satisfied:

• T has a potentially constructed subobject of class type M (or possibly multi-dimensional array thereof) such that
• M has a destructor that is deleted or (since C++11) inaccessible from the copy constructor, or
• the overload resolution as applied to find M 's copy constructor
• does not result in a usable candidate, or
• in the case of the subobject being a variant member , selects a non-trivial function.

[ edit ] Trivial copy constructor

The copy constructor for class T is trivial if all of the following are true:

• it is not user-provided (that is, it is implicitly-defined or defaulted);
• T has no virtual member functions;
• T has no virtual base classes;
• the copy constructor selected for every direct base of T is trivial;
• the copy constructor selected for every non-static class type (or array of class type) member of T is trivial;

A trivial copy constructor for a non-union class effectively copies every scalar subobject (including, recursively, subobject of subobjects and so forth) of the argument and performs no other action. However, padding bytes need not be copied, and even the object representations of the copied subobjects need not be the same as long as their values are identical.

TriviallyCopyable objects can be copied by copying their object representations manually, e.g. with std::memmove . All data types compatible with the C language (POD types) are trivially copyable.

[ edit ] Eligible copy constructor

Triviality of eligible copy constructors determines whether the class is an implicit-lifetime type , and whether the class is a trivially copyable type .

[ edit ] Notes

In many situations, copy constructors are optimized out even if they would produce observable side-effects, see copy elision .

[ edit ] Example

[ edit ] defect reports.

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[ edit ] See also

• converting constructor
• copy assignment
• copy elision
• default constructor
• aggregate initialization
• constant initialization
• copy initialization
• default initialization
• direct initialization
• initializer list
• list initialization
• reference initialization
• value initialization
• zero initialization
• move assignment
• move constructor
• Recent changes
• Offline version
• What links here
• Related changes
• Upload file
• Special pages
• Printable version
• Permanent link
• Page information
• In other languages
• This page was last modified on 4 June 2024, at 23:47.
• Privacy policy
• About cppreference.com
• Disclaimers

This browser is no longer supported.

Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support.

Move Constructors and Move Assignment Operators (C++)

• 9 contributors

This topic describes how to write a move constructor and a move assignment operator for a C++ class. A move constructor enables the resources owned by an rvalue object to be moved into an lvalue without copying. For more information about move semantics, see Rvalue Reference Declarator: && .

This topic builds upon the following C++ class, MemoryBlock , which manages a memory buffer.

The following procedures describe how to write a move constructor and a move assignment operator for the example C++ class.

To create a move constructor for a C++ class

Define an empty constructor method that takes an rvalue reference to the class type as its parameter, as demonstrated in the following example:

In the move constructor, assign the class data members from the source object to the object that is being constructed:

Assign the data members of the source object to default values. This prevents the destructor from freeing resources (such as memory) multiple times:

To create a move assignment operator for a C++ class

Define an empty assignment operator that takes an rvalue reference to the class type as its parameter and returns a reference to the class type, as demonstrated in the following example:

In the move assignment operator, add a conditional statement that performs no operation if you try to assign the object to itself.

In the conditional statement, free any resources (such as memory) from the object that is being assigned to.

The following example frees the _data member from the object that is being assigned to:

Follow steps 2 and 3 in the first procedure to transfer the data members from the source object to the object that is being constructed:

Return a reference to the current object, as shown in the following example:

Example: Complete move constructor and assignment operator

The following example shows the complete move constructor and move assignment operator for the MemoryBlock class:

Example Use move semantics to improve performance

The following example shows how move semantics can improve the performance of your applications. The example adds two elements to a vector object and then inserts a new element between the two existing elements. The vector class uses move semantics to perform the insertion operation efficiently by moving the elements of the vector instead of copying them.

This example produces the following output:

Before Visual Studio 2010, this example produced the following output:

The version of this example that uses move semantics is more efficient than the version that does not use move semantics because it performs fewer copy, memory allocation, and memory deallocation operations.

Robust Programming

To prevent resource leaks, always free resources (such as memory, file handles, and sockets) in the move assignment operator.

To prevent the unrecoverable destruction of resources, properly handle self-assignment in the move assignment operator.

If you provide both a move constructor and a move assignment operator for your class, you can eliminate redundant code by writing the move constructor to call the move assignment operator. The following example shows a revised version of the move constructor that calls the move assignment operator:

The std::move function converts the lvalue other to an rvalue.

Rvalue Reference Declarator: && std::move

Was this page helpful?

Coming soon: Throughout 2024 we will be phasing out GitHub Issues as the feedback mechanism for content and replacing it with a new feedback system. For more information see: https://aka.ms/ContentUserFeedback .

Submit and view feedback for

Additional resources

• Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers
• Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand
• OverflowAI GenAI features for Teams
• OverflowAPI Train & fine-tune LLMs
• Labs The future of collective knowledge sharing
• About the company Visit the blog

Collectives™ on Stack Overflow

Find centralized, trusted content and collaborate around the technologies you use most.

Q&A for work

Connect and share knowledge within a single location that is structured and easy to search.

Get early access and see previews of new features.

assignment operator vs. copy constructor C++

I have the following code to test out my understanding of basic pointers in C++:

This code works exactly as expected as is, it prints out:

However if I remove the copy constructor it prints out something like:

I don't understand why this is the case. My understanding is that the copy constructor is used when the assignment is to an object that doesn't exist. Here intVal1 does exist, so why isn't the assignment operator called?

• memory-management
• copy-constructor

• 2 Sorry, I haven't read the whole thing, so not answering, but commenting. I believe the phenomenon observed has to do with the fact that default (automatically generated) copy constructor doesn't allocate memory, but just copies pointer (and yes, considering I haven't read, I may be way off;)). –  Michael Krelin - hacker Commented Sep 23, 2013 at 21:21
• intVal1 = intVal2; invokes operator = on intVal1 , not the copy constructor. –  Hristo Iliev Commented Sep 23, 2013 at 21:47

3 Answers 3

The copy constructor is not used during assignment. Copy constructor in your case is used when passing arguments to displayInteger function. The second parameter is passed by value, meaning that it is initailized by copy contructor.

Your version of copy constructor performs deep copying of data owned by the class (just like your assignment operator does). So, everything works correctly with your version of copy constructor.

If you remove your own copy constructor, the compiler will generate one for you implicitly. The compiler-generated copy constructor will perform shallow copying of the object. This will violate the "Rule of Three" and destroy the functionality of your class, which is exactly what you observe in your experiment. Basically, the first call to displayInteger damages your intVal1 object and the second call to displayInteger damages your intVal2 object. After that both of your objects are broken, which is why the third displayInteger call displays garbage.

If you change the declaration of displayInteger to

your code will "work" even without an explicit copy constructor. But it is not a good idea to ignore the "Rule of Three" in any case. A class implemented in this way either has to obey the "Rule of Three" or has to be made non-copyable.

• Ok. I think I understand what you're saying. But if I debug my code (without my copy constructor) and break at the line intVal1 = intVal2 , and ask the values of intVal1 and intVal2 I get 0 and 6299648 respectively. So it seems that only intVal2 has been affected by the shallow copying. Why is this? –  Lukas Bystricky Commented Sep 23, 2013 at 21:36
• 1 @H H: It is just a coincidence or an illusion. Both objects got damaged to the same degree. It just that by coincidence the memory that formerly belonged to intVal1 retained its former value. Without copy constructor your program exhibits undefined behavior . Undefined behavior can take any form. It might even look as if everything is fine. –  AnT stands with Russia Commented Sep 23, 2013 at 21:38
• Ok, I can believe that. The only problem I have is that it seems reproducible that intVal = 0 and intVal2 = something else (changes each run). Granted I've only run it 3 times. –  Lukas Bystricky Commented Sep 23, 2013 at 21:41
• Rule of Five: When C++11 is used (as is becoming more and more prevalent nowadays), the rule of three is replaced by the "Rule of Five". That means that in addition to a copy constructor, destructor, (copy) assignment operator, now also a move constructor and move assignment operator should be defined. –  Piotr99 Commented Sep 23, 2013 at 22:20

The problem you're experiencing is caused by the default copy constructor, which copies the pointer but doesn't associate it with newly allocated memory (like your implementation of copy constructor does). When you pass object by value, a copy is created and when the execution goes out of scope, this copy is destructed. delete from the destructor invalidates the value pointer of intVal1 object, making it dangling pointer , dereferencing of which causes undefined behavior .

Debug outputs might be used to understand the behavior of your code:

Now output of this code:

ctor ctor(int) copy ctor 0x9ed4028 0 destructor copy ctor 0x9ed4038 10 destructor assignment copy ctor 0x9ed4048 10 destructor destructor destructor

which shows that copy constructor is used when passing objects by value. However, important to notice here is the destructor called upon the return from your function. And if you remove your implementation of copy constructor, then the output is:

ctor ctor(int) 0x8134008 0 destructor 0x8134018 10 destructor assignment 0x8134008 135479296 destructor destructor destructor

showing that the first copy called delete on the same pointer (pointing to 0x8134008 ) as was used by third copy later, where the memory pointed by this dangling pointer has been used.

Think about this call:

You are creating a copy of intVal1 into the intObj parameter of displayInteger :

That copy will be pointing to the same int that intVal1 is. When displayInteger returns, intObj is destroyed, which will cause the int to be destroyed, and the pointer in intVal1 to be pointing to an invalid object. At that point all bets are off (A.K.A. undefined behavior) if you try to access the value. A similar thing happens for intVal2 .

At a more general level, by removing the copy constructor, you are violating the Rule of Three, which typically leads to these kinds of problems.

Your Answer

Reminder: Answers generated by artificial intelligence tools are not allowed on Stack Overflow. Learn more

Sign up or log in

Post as a guest.

Required, but never shown

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy .

Not the answer you're looking for? Browse other questions tagged c++ pointers memory-management copy-constructor or ask your own question .

• The Overflow Blog
• How to build open source apps in a highly regulated industry
• Community Products Roadmap Update, July 2024
• Featured on Meta
• We spent a sprint addressing your requests — here’s how it went
• Upcoming initiatives on Stack Overflow and across the Stack Exchange network...
• Policy: Generative AI (e.g., ChatGPT) is banned
• The [lib] tag is being burninated
• What makes a homepage useful for logged-in users

Hot Network Questions

• If a lambda is declared as a default argument, is it different for each call site?
• Why should I meet my advisor even if I have nothing to report?
• Two definitions of antisymmetrization of a tensor?
• Can you always extend an isometry of a subset of a Hilbert Space to the whole space?
• Who originated the idea that the purpose of government is to protect its citizens?
• "Teacher's Materials & Answer Key"
• Is anything implied by the author using the (Greek) phrase translated "work with your hands" in 1 Thessalonians 4:11?
• Remove duplicates in file (without sorting!) leaving the _last_ of the occurences
• Why are my star jasmine flowers turning brown
• Staying in USA longer than 3 months
• How to maintain dependencies shared among microservices?
• How to NDSolve a PDE that contains the integral of the solution?
• spath3 rotations shrink paths
• Use of Compile[] to optimize the code to run faster
• Books using the axiomatic method
• Exercise 5.B.7(b) in "Linear Algebra Done Right" 4th edition by Sheldon Axler
• What is the value of air anisotropy?
• What's the point of Dream Chaser?
• Did Tolkien give his son explicit permission to publish all that unfinished material?
• How did Rashi reach the number of eight people carrying the grapes?
• Correlation for Small Dataset?
• E flat tuning - adjust bass?
• Does the decision of North Korea sending engineering troops to the occupied territory in Ukraine leave them open to further UN sanctions?
• common.apex.runtime.impl.ExecutionException: Unrecognized base64 character: [