Aug 13, 2024 · It is an overloaded constructor. It is a bitwise operator. C++ compiler implicitly provides a copy constructor, if no copy constructor is defined in the class. A bitwise copy gets created, if the Assignment operator is not overloaded. Syntax: className(const className &obj) {// body } Syntax: className obj1, obj2; obj2 = obj1; ... the difference between a copy constructor and an assignment constructor is: In case of a copy constructor it creates a new object.(<classname> <o1>=<o2>) In case of an assignment constructor it will not create any object means it apply on already created objects(<o1>=<o2>). ... 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. ... Feb 14, 2022 · Use an assignment operator operator= that returns a reference to the class type and takes one parameter that's passed by const reference—for example ClassName& operator=(const ClassName& x);. Use the copy constructor. If you don't declare a copy constructor, the compiler generates a member-wise copy constructor for you. ... Copy constructor Assignment operator; An example of an overloaded constructor is the copy constructor. A new object is initialized by an existing object of the same type using the copy constructor. An operator that assigns a value to objects or data members is known as an assignment operator. ... Nov 5, 2024 · We could implement the overloaded copy assignment operator similarly to the copy constructor. However, there is an elegant way to use the copy constructor to implement the overloaded copy assignment operator (the copy-and-swap idiom). The idiom operates as follows: Create a copy of the right-hand side object using the copy constructor. ... Dec 2, 2024 · Conclusion. The difference between a copy constructor and an assignment operator is that a copy constructor helps to create a copy of an already existing object without altering the original value of the created object, whereas an assignment operator helps to assign a new value to a data member or an object in the program. ... Jul 22, 2024 · The difference between the copy constructor and the copy assignment operator causes a lot of confusion for new programmers, but it’s really not all that difficult. Summarizing: If a new object has to be created before the copying can occur, the copy constructor is used (note: this includes passing or returning objects by value). ... Copy Constructor Assignment Operator; The Copy constructor is basically an overloaded constructor: Assignment operator is basically an operator. This initializes the new object with an already existing object: This assigns the value of one object to another object both of which are already exists. ... This is a overloaded constructor. This is a overloaded operator: It can be replaced by default constructor plus assignment operator. This can’t be replaced. If a new object needs to be created before copying the value then copy constructor is called. If no new object needs to be created before copying the value then assignment operator is called. ... ">
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Copy Constructor vs Assignment Operator in C++

Copy constructor and Assignment operator are similar as they are both used to initialize one object using another object. But, there are some basic differences between them:

Consider the following C++ program. 

Explanation: Here, t2 = t1;  calls the assignment operator , same as t2.operator=(t1); and   Test t3 = t1;  calls the copy constructor , same as Test t3(t1);

Must Read: When is a Copy Constructor Called in C++?

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Difference Between Copy Constructor and Assignment Operator in C++

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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:

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  • Difference Between Inheritance and Polymorphism
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Copy constructors and copy assignment operators (C++)

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Starting in C++11, two kinds of assignment are supported in the language: copy assignment and move assignment . In this article "assignment" means copy assignment unless explicitly stated otherwise. For information about move assignment, see Move Constructors and Move Assignment Operators (C++) .

Both the assignment operation and the initialization operation cause objects to be copied.

Assignment : When one object's value is assigned to another object, the first object is copied to the second object. So, this code copies the value of b into a :

Initialization : Initialization occurs when you declare a new object, when you pass function arguments by value, or when you return by value from a function.

You can define the semantics of "copy" for objects of class type. For example, consider this code:

The preceding code could mean "copy the contents of FILE1.DAT to FILE2.DAT" or it could mean "ignore FILE2.DAT and make b a second handle to FILE1.DAT." You must attach appropriate copying semantics to each class, as follows:

Use an assignment operator operator= that returns a reference to the class type and takes one parameter that's passed by const reference—for example ClassName& operator=(const ClassName& x); .

Use the copy constructor.

If you don't declare a copy constructor, the compiler generates a member-wise copy constructor for you. Similarly, if you don't declare a copy assignment operator, the compiler generates a member-wise copy assignment operator for you. Declaring a copy constructor doesn't suppress the compiler-generated copy assignment operator, and vice-versa. If you implement either one, we recommend that you implement the other one, too. When you implement both, the meaning of the code is clear.

The copy constructor takes an argument of type ClassName& , where ClassName is the name of the class. For example:

Make the type of the copy constructor's argument const ClassName& whenever possible. This prevents the copy constructor from accidentally changing the copied object. It also lets you copy from const objects.

Compiler generated copy constructors

Compiler-generated copy constructors, like user-defined copy constructors, have a single argument of type "reference to class-name ." An exception is when all base classes and member classes have copy constructors declared as taking a single argument of type const class-name & . In such a case, the compiler-generated copy constructor's argument is also const .

When the argument type to the copy constructor isn't const , initialization by copying a const object generates an error. The reverse isn't true: If the argument is const , you can initialize by copying an object that's not const .

Compiler-generated assignment operators follow the same pattern for const . They take a single argument of type ClassName& unless the assignment operators in all base and member classes take arguments of type const ClassName& . In this case, the generated assignment operator for the class takes a const argument.

When virtual base classes are initialized by copy constructors, whether compiler-generated or user-defined, they're initialized only once: at the point when they are constructed.

The implications are similar to the copy constructor. When the argument type isn't const , assignment from a const object generates an error. The reverse isn't true: If a const value is assigned to a value that's not const , the assignment succeeds.

For more information about overloaded assignment operators, see Assignment .

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21.12 — Overloading the assignment operator

The copy assignment operator (operator=) is used to copy values from one object to another already existing object .

Related content

As of C++11, C++ also supports “Move assignment”. We discuss move assignment in lesson 22.3 -- Move constructors and move assignment .

Copy assignment vs Copy constructor

The purpose of the copy constructor and the copy assignment operator are almost equivalent -- both copy one object to another. However, the copy constructor initializes new objects, whereas the assignment operator replaces the contents of existing objects.

The difference between the copy constructor and the copy assignment operator causes a lot of confusion for new programmers, but it’s really not all that difficult. Summarizing:

  • If a new object has to be created before the copying can occur, the copy constructor is used (note: this includes passing or returning objects by value).
  • If a new object does not have to be created before the copying can occur, the assignment operator is used.

Overloading the assignment operator

Overloading the copy assignment operator (operator=) is fairly straightforward, with one specific caveat that we’ll get to. The copy assignment operator must be overloaded as a member function.

This prints:

This should all be pretty straightforward by now. Our overloaded operator= returns *this, so that we can chain multiple assignments together:

Issues due to self-assignment

Here’s where things start to get a little more interesting. C++ allows self-assignment:

This will call f1.operator=(f1), and under the simplistic implementation above, all of the members will be assigned to themselves. In this particular example, the self-assignment causes each member to be assigned to itself, which has no overall impact, other than wasting time. In most cases, a self-assignment doesn’t need to do anything at all!

However, in cases where an assignment operator needs to dynamically assign memory, self-assignment can actually be dangerous:

First, run the program as it is. You’ll see that the program prints “Alex” as it should.

Now run the following program:

You’ll probably get garbage output. What happened?

Consider what happens in the overloaded operator= when the implicit object AND the passed in parameter (str) are both variable alex. In this case, m_data is the same as str.m_data. The first thing that happens is that the function checks to see if the implicit object already has a string. If so, it needs to delete it, so we don’t end up with a memory leak. In this case, m_data is allocated, so the function deletes m_data. But because str is the same as *this, the string that we wanted to copy has been deleted and m_data (and str.m_data) are dangling.

Later on, we allocate new memory to m_data (and str.m_data). So when we subsequently copy the data from str.m_data into m_data, we’re copying garbage, because str.m_data was never initialized.

Detecting and handling self-assignment

Fortunately, we can detect when self-assignment occurs. Here’s an updated implementation of our overloaded operator= for the MyString class:

By checking if the address of our implicit object is the same as the address of the object being passed in as a parameter, we can have our assignment operator just return immediately without doing any other work.

Because this is just a pointer comparison, it should be fast, and does not require operator== to be overloaded.

When not to handle self-assignment

Typically the self-assignment check is skipped for copy constructors. Because the object being copy constructed is newly created, the only case where the newly created object can be equal to the object being copied is when you try to initialize a newly defined object with itself:

In such cases, your compiler should warn you that c is an uninitialized variable.

Second, the self-assignment check may be omitted in classes that can naturally handle self-assignment. Consider this Fraction class assignment operator that has a self-assignment guard:

If the self-assignment guard did not exist, this function would still operate correctly during a self-assignment (because all of the operations done by the function can handle self-assignment properly).

Because self-assignment is a rare event, some prominent C++ gurus recommend omitting the self-assignment guard even in classes that would benefit from it. We do not recommend this, as we believe it’s a better practice to code defensively and then selectively optimize later.

The copy and swap idiom

A better way to handle self-assignment issues is via what’s called the copy and swap idiom. There’s a great writeup of how this idiom works on Stack Overflow .

The implicit copy assignment operator

Unlike other operators, the compiler will provide an implicit public copy assignment operator for your class if you do not provide a user-defined one. This assignment operator does memberwise assignment (which is essentially the same as the memberwise initialization that default copy constructors do).

Just like other constructors and operators, you can prevent assignments from being made by making your copy assignment operator private or using the delete keyword:

Note that if your class has const members, the compiler will instead define the implicit operator= as deleted. This is because const members can’t be assigned, so the compiler will assume your class should not be assignable.

If you want a class with const members to be assignable (for all members that aren’t const), you will need to explicitly overload operator= and manually assign each non-const member.

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What's the difference between assignment operator and copy constructor in C++?

The Copy constructor and the assignment operators are used to initialize 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 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.

Ankith Reddy

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Difference between Copy constructors and Assignment operator

difference between copy constructor and an overloaded assignment operator

Copy constructor is a special kind of constructor which initializes an object using another object. Compiler always provides a default copy constructor but if needed it can be over-ridden by programmer to meet the requirements.

Assignment operator is basically assigning the new values to already created object. In this case object initialization doesn’t happen as it is already done.

Let’s have a look into a sample program.

Let’s have a look into output of above program.

difference between copy constructor and an overloaded assignment operator

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COMMENTS

  1. Copy Constructor vs Assignment Operator in C++

    Aug 13, 2024 · It is an overloaded constructor. It is a bitwise operator. C++ compiler implicitly provides a copy constructor, if no copy constructor is defined in the class. A bitwise copy gets created, if the Assignment operator is not overloaded. Syntax: className(const className &obj) {// body } Syntax: className obj1, obj2; obj2 = obj1;

  2. What's the difference between assignment operator and copy ...

    the difference between a copy constructor and an assignment constructor is: In case of a copy constructor it creates a new object.(<classname> <o1>=<o2>) In case of an assignment constructor it will not create any object means it apply on already created objects(<o1>=<o2>).

  3. Difference 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.

  4. Copy constructors and copy assignment operators (C++)

    Feb 14, 2022 · Use an assignment operator operator= that returns a reference to the class type and takes one parameter that's passed by const reference—for example ClassName& operator=(const ClassName& x);. Use the copy constructor. If you don't declare a copy constructor, the compiler generates a member-wise copy constructor for you.

  5. The distinction between the C++ copy constructor and ...

    Copy constructor Assignment operator; An example of an overloaded constructor is the copy constructor. A new object is initialized by an existing object of the same type using the copy constructor. An operator that assigns a value to objects or data members is known as an assignment operator.

  6. Copy Constructors and Assignment Operators: Cloning Objects ...

    Nov 5, 2024 · We could implement the overloaded copy assignment operator similarly to the copy constructor. However, there is an elegant way to use the copy constructor to implement the overloaded copy assignment operator (the copy-and-swap idiom). The idiom operates as follows: Create a copy of the right-hand side object using the copy constructor.

  7. Difference Between Copy Constructor and Assignment Operator ...

    Dec 2, 2024 · Conclusion. The difference between a copy constructor and an assignment operator is that a copy constructor helps to create a copy of an already existing object without altering the original value of the created object, whereas an assignment operator helps to assign a new value to a data member or an object in the program.

  8. 21.12 — Overloading the assignment operator – Learn C++

    Jul 22, 2024 · The difference between the copy constructor and the copy assignment operator causes a lot of confusion for new programmers, but it’s really not all that difficult. Summarizing: If a new object has to be created before the copying can occur, the copy constructor is used (note: this includes passing or returning objects by value).

  9. the difference between assignment operator and copy ...">What's the difference between assignment operator and copy ...

    Copy Constructor Assignment Operator; The Copy constructor is basically an overloaded constructor: Assignment operator is basically an operator. This initializes the new object with an already existing object: This assigns the value of one object to another object both of which are already exists.

  10. Difference between Copy constructors and Assignment operator">Difference between Copy constructors and Assignment operator

    This is a overloaded constructor. This is a overloaded operator: It can be replaced by default constructor plus assignment operator. This can’t be replaced. If a new object needs to be created before copying the value then copy constructor is called. If no new object needs to be created before copying the value then assignment operator is called.