constructor vs assignment in c

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Copy constructors, assignment operators, and exception safe assignment

Tech Differences

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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

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22.3 — Move constructors and move assignment

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Move Constructors and Move Assignment Operators (C++)

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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

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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.

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Difference Between Constructor and Destructor in C++

Constructor :   A constructor is a member function of a class that has the same name as the class name. It helps to initialize the object of a class. It can either accept the arguments or not. It is used to allocate the memory to an object of the class. It is called whenever an instance of the class is created. It can be defined manually with arguments or without arguments. There can be many constructors in a class. It can be overloaded but it can not be inherited or virtual. There is a concept of copy constructor which is used to initialize an object from another object.  Syntax:   

Destructor :   Like a constructor, Destructor is also a member function of a class that has the same name as the class name preceded by a tilde(~) operator. It helps to deallocate the memory of an object. It is called while the object of the class is freed or deleted. In a class, there is always a single destructor without any parameters so it can’t be overloaded. It is always called in the reverse order of the constructor. if a class is inherited by another class and both the classes have a destructor then the destructor of the child class is called first, followed by the destructor of the parent or base class.  Syntax:   

Note: If we do not specify any access modifiers for the members inside the class then by default the access modifier for the members will be Private.  Example/Implementation of Constructor and Destructor:  

Output:   

Difference between Constructor and Destructor in C++ : 

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What is the difference between initialization and assignment?

I was confused by the following statement:

C++ provides another way of initializing member variables that allows us to initialize member variables when they are created rather than afterwards. This is done through use of an initialization list. Using an initialization list is very similar to doing implicit assignments.

PS: C/C++ examples are appreciated.

• initialization
• variable-assignment

• 1 What's wrong with informit.com/articles/article.aspx?p=376876 ? –  Otávio Décio Commented Sep 8, 2011 at 15:02
• 2 A variable that never, during its lifetime, had a garbage value must habe been initialized ; a variable with a garbage value is probably uninitialized (and the garbage value disappears when the variable get assigned a value) –  pmg Commented Sep 8, 2011 at 15:15

6 Answers 6

Oh my. Initialization and assignment. Well, that's confusion for sure!

To initialize is to make ready for use. And when we're talking about a variable, that means giving the variable a first, useful value . And one way to do that is by using an assignment.

So it's pretty subtle: assignment is one way to do initialization.

Assignment works well for initializing e.g. an int , but it doesn't work well for initializing e.g. a std::string . Why? Because the std::string object contains at least one pointer to dynamically allocated memory, and

if the object has not yet been initialized, that pointer needs to be set to point at a properly allocated buffer (block of memory to hold the string contents), but

if the object has already been initialized, then an assignment may have to deallocate the old buffer and allocate a new one.

So the std::string object's assignment operator evidently has to behave in two different ways, depending on whether the object has already been initialized or not!

Of course it doesn't behave in two different ways. Instead, for a std::string object the initialization is taken care of by a constructor . You can say that a constructor's job is to take the area of memory that will represent the object, and change the arbitrary bits there to something suitable for the object type, something that represents a valid object state.

That initialization from raw memory should ideally be done once for each object, before any other operations on the object.

And the C++ rules effectively guarantee that. At least as long as you don't use very low level facilities. One might call that the C++ construction guarantee .

So, this means that when you do

then you're doing simple construction from raw memory, but when you do

then you're first constructing s (with an object state representing an empty string), and then assigning to this already initialized s .

And that, finally, allows me to answer your question. From the point of view of language independent programming the first useful value is presumably the one that's assigned, and so in this view one thinks of the assignment as initialization. Yet, at the C++ technical level initialization has already been done, by a call of std::string 's default constructor, so at this level one thinks of the declaration as initialization, and the assignment as just a later change of value.

So, especially the term "initialization" depends on the context!

Simply apply some common sense to sort out what Someone Else probably means.

Cheers & hth.,

• So to make things clear, when I write : C f(); C obj = f(); , it first calls the default copy constructor, then the assignment operator ? In that case why doesn't that work if I remove the default copy constructor ? –  Dinaiz Commented Nov 8, 2016 at 21:46
• @Dinaiz: Not quite. C obj = f(); is a C++ initialization, a declaration statement, in spite of using the = symbol. Assignment isn't involved here at all. Other than historically, in the origins of the notation. –  Cheers and hth. - Alf Commented Nov 9, 2016 at 6:22
• 2 @Diniz: The effect is as if a perfect copy constructor with no side effects, had been called. However the compiler is allowed to elide that call (optimize it away). In practice there will be no copy constructor call for C obj = f() , but rather f will construct its result directly in the storage provided by the caller, namely obj (this is called return value optimization, or RVO). –  Cheers and hth. - Alf Commented Nov 9, 2016 at 16:30
• 1 Uh, "or move constructor". Sorry. It depends on the class C . ;-) –  Cheers and hth. - Alf Commented Nov 9, 2016 at 16:48
• 2 @Dinaiz: That last explanation of mine, and correction, is ungood and misleading, I'm sorry. It's a complex issue. The short of it: the = notation for initialization is called copy initialization , for historical reasons. Copy initialization can use a copy constructor or a move constructor, which must be accessible. The extra copy or move constructor call can be optimized away, elided , even if the constructor in question has side effects. And it usually is optimized away. –  Cheers and hth. - Alf Commented Nov 10, 2016 at 3:54

In the simplest of terms:

For built in types its relatively straight forward. For user defined types it can get more complex. Have a look at this article.

For instance:

Initialization is creating an instance(of type) with certain value.

Assignment is to give value to an already created instance(of type).

To Answer your edited Question:

What is the difference between Initializing And Assignment inside constructor? & What is the advantage?

There is a difference between Initializing a member using initializer list and assigning it an value inside the constructor body.

When you initialize fields via initializer list the constructors will be called once.

If you use the assignment then the fields will be first initialized with default constructors and then reassigned (via assignment operator) with actual values.

As you see there is an additional overhead of creation & assignment in the latter, which might be considerable for user defined classes.

For an integer data type or POD class members there is no practical overhead.

An Code Example:

In the above example:

This construct is called a Member Initializer List in C++.

It initializes a member param_ to a value param .

When do you HAVE TO use member Initializer list? You will have(rather forced) to use a Member Initializer list if:

Your class has a reference member Your class has a const member or Your class doesn't have a default constructor

• 2 Types don't have values, objects do. –  Mankarse Commented Sep 8, 2011 at 15:10
• Types typically do have a range of values. void is the exception, though. –  MSalters Commented Sep 8, 2011 at 15:49
• The term "initialization" has several meanings. The basic meaning, from which the others are derived, is incompatible with this attempt at differentiating initialization and assignment. And so I think this answer, as it is as of the writing of this comment, can easily mislead the now unfortunately too common unthinking group-oriented associative novices. –  Cheers and hth. - Alf Commented Jul 8, 2016 at 8:58

Initialisation: giving an object an initial value:

Assignment: assigning a new value to an object:

• 3 @Appy No, it is called an initialisation. –  Mankarse Commented Sep 8, 2011 at 15:14

In C the general syntax for initialization is with {} :

The one for S is called "designated initializers" and is only available from C99 onward.

• Fields that are omitted are automatically initialized with the correct 0 for the corresponding type.
• this syntax applies even to basic data types like double r = { 1.0 };
• There is a catchall initializer that sets all fields to 0 , namely { 0 } .
• if the variable is of static linkage all expressions of the initializer must be constant expressions

This {} syntax can not be used directly for assignment, but in C99 you can use compound literals instead like

So by first creating a temporary object on the RHS and assigning this to your object.

• bool a = false; vs. bool a { false }; What to prefer and why? Thx! –  hfrmobile Commented Sep 22, 2021 at 13:39

One thing that nobody has yet mentioned is the difference between initialisation and assignment of class fields in the constructor.

Let us consider the class:

What we have here is a constructor that initialises Thing::num to the value of 5, and assigns 'a' to Thing::c . In this case the difference is minor, but as was said before if you were to substitute int and char in this example for some arbitrary classes, we would be talking about the difference between calling a parameterised constructor versus a default constructor followed by operator= function.

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