constructor vs assignment operator

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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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Copy Constructor in C++

A copy constructor is a type of constructor 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 .  

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.

Syntax of Copy Constructor in C++

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

Here, the const qualifier is optional but is added so that we do not modify the obj by mistake.

Syntax of Copy Constructor

Examples of Copy Constructor in C++

Example 1: user defined copy constructor.

If the programmer does not define the copy constructor, the compiler does it for us.

Example 2: 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.

Need of User Defined Copy Constructor

If we don’t define our own copy constructor, the C++ compiler creates a default copy constructor for each class which works fine in general. However, 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 because the default constructor does only shallow copy.

Shallow Copy means that only the pointers will be copied not the actual resources that the pointers are pointing to. This can lead to dangling pointers if the original object is deleted.

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 copy of the dynamic resource allocated manually in the copy constructor using new operators.

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. 

Note: Such classes also need the overloaded assignment operator. See this article for more info – C++ Assignment Operator Overloading

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 c hanges made to str2 reflect in str1 as well which is never expected. Also, if the str1 is destroyed, the str2’s data member s will be pointing to the deallocated memory.

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).

Refer to this article for more details – When is a Copy Constructor Called in C++?

Copy Elision

In copy elision, the compiler prevents the making of extra copies by making the use to techniques such as NRVO and RVO which results in saving space and better the program complexity (both time and space); Hence making the code more optimized.

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.

Frequently Asked Questions in C++ Copy Constructors

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?  

If you pass the object by value in the copy constructor, it will 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 recursion. Using a reference avoids this recursion. So, we use reference of objects to avoid infinite calls.

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?’

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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.

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Copy Constructor in C++ vs. Assignment Operator in C++: What's the Difference?

Key Differences

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22.3 — Move constructors and move assignment

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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
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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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Additional resources

NumPy for MATLAB users #

Introduction #.

MATLAB® and NumPy have a lot in common, but NumPy was created to work with Python, not to be a MATLAB clone. This guide will help MATLAB users get started with NumPy.

Some key differences #

Rough equivalents #

The table below gives rough equivalents for some common MATLAB expressions. These are similar expressions, not equivalents. For details, see the documentation .

In the table below, it is assumed that you have executed the following commands in Python:

Also assume below that if the Notes talk about “matrix” that the arguments are two-dimensional entities.

General purpose equivalents #

Linear algebra equivalents #

Submatrix : Assignment to a submatrix can be done with lists of indices using the ix_ command. E.g., for 2D array a , one might do: ind=[1, 3]; a[np.ix_(ind, ind)] += 100 .

HELP : There is no direct equivalent of MATLAB’s which command, but the commands help will usually list the filename where the function is located. Python also has an inspect module (do import inspect ) which provides a getfile that often works.

INDEXING : MATLAB uses one based indexing, so the initial element of a sequence has index 1. Python uses zero based indexing, so the initial element of a sequence has index 0. Confusion and flamewars arise because each has advantages and disadvantages. One based indexing is consistent with common human language usage, where the “first” element of a sequence has index 1. Zero based indexing simplifies indexing . See also a text by prof.dr. Edsger W. Dijkstra .

RANGES : In MATLAB, 0:5 can be used as both a range literal and a ‘slice’ index (inside parentheses); however, in Python, constructs like 0:5 can only be used as a slice index (inside square brackets). Thus the somewhat quirky r_ object was created to allow NumPy to have a similarly terse range construction mechanism. Note that r_ is not called like a function or a constructor, but rather indexed using square brackets, which allows the use of Python’s slice syntax in the arguments.

LOGICOPS : & or | in NumPy is bitwise AND/OR, while in MATLAB & and | are logical AND/OR. The two can appear to work the same, but there are important differences. If you would have used MATLAB’s & or | operators, you should use the NumPy ufuncs logical_and / logical_or . The notable differences between MATLAB’s and NumPy’s & and | operators are:

Non-logical {0,1} inputs: NumPy’s output is the bitwise AND of the inputs. MATLAB treats any non-zero value as 1 and returns the logical AND. For example (3 & 4) in NumPy is 0 , while in MATLAB both 3 and 4 are considered logical true and (3 & 4) returns 1 .

Precedence: NumPy’s & operator is higher precedence than logical operators like < and > ; MATLAB’s is the reverse.

If you know you have boolean arguments, you can get away with using NumPy’s bitwise operators, but be careful with parentheses, like this: z = (x > 1) & (x < 2) . The absence of NumPy operator forms of logical_and and logical_or is an unfortunate consequence of Python’s design.

RESHAPE and LINEAR INDEXING : MATLAB always allows multi-dimensional arrays to be accessed using scalar or linear indices, NumPy does not. Linear indices are common in MATLAB programs, e.g. find() on a matrix returns them, whereas NumPy’s find behaves differently. When converting MATLAB code it might be necessary to first reshape a matrix to a linear sequence, perform some indexing operations and then reshape back. As reshape (usually) produces views onto the same storage, it should be possible to do this fairly efficiently. Note that the scan order used by reshape in NumPy defaults to the ‘C’ order, whereas MATLAB uses the Fortran order. If you are simply converting to a linear sequence and back this doesn’t matter. But if you are converting reshapes from MATLAB code which relies on the scan order, then this MATLAB code: z = reshape(x,3,4); should become z = x.reshape(3,4,order='F').copy() in NumPy.

‘array’ or ‘matrix’? Which should I use? #

Historically, NumPy has provided a special matrix type, np.matrix , which is a subclass of ndarray which makes binary operations linear algebra operations. You may see it used in some existing code instead of np.array . So, which one to use?

Short answer #

Use arrays .

They support multidimensional array algebra that is supported in MATLAB

They are the standard vector/matrix/tensor type of NumPy. Many NumPy functions return arrays, not matrices.

There is a clear distinction between element-wise operations and linear algebra operations.

You can have standard vectors or row/column vectors if you like.

Until Python 3.5 the only disadvantage of using the array type was that you had to use dot instead of * to multiply (reduce) two tensors (scalar product, matrix vector multiplication etc.). Since Python 3.5 you can use the matrix multiplication @ operator.

Given the above, we intend to deprecate matrix eventually.

Long answer #

NumPy contains both an array class and a matrix class. The array class is intended to be a general-purpose n-dimensional array for many kinds of numerical computing, while matrix is intended to facilitate linear algebra computations specifically. In practice there are only a handful of key differences between the two.

Operators * and @ , functions dot() , and multiply() :

For array , ``*`` means element-wise multiplication , while ``@`` means matrix multiplication ; they have associated functions multiply() and dot() . (Before Python 3.5, @ did not exist and one had to use dot() for matrix multiplication).

For matrix , ``*`` means matrix multiplication , and for element-wise multiplication one has to use the multiply() function.

Handling of vectors (one-dimensional arrays)

For array , the vector shapes 1xN, Nx1, and N are all different things . Operations like A[:,1] return a one-dimensional array of shape N, not a two-dimensional array of shape Nx1. Transpose on a one-dimensional array does nothing.

For matrix , one-dimensional arrays are always upconverted to 1xN or Nx1 matrices (row or column vectors). A[:,1] returns a two-dimensional matrix of shape Nx1.

Handling of higher-dimensional arrays (ndim > 2)

array objects can have number of dimensions > 2 ;

matrix objects always have exactly two dimensions .

Convenience attributes

array has a .T attribute , which returns the transpose of the data.

matrix also has .H, .I, and .A attributes , which return the conjugate transpose, inverse, and asarray() of the matrix, respectively.

Convenience constructor

The array constructor takes (nested) Python sequences as initializers . As in, array([[1,2,3],[4,5,6]]) .

The matrix constructor additionally takes a convenient string initializer . As in matrix("[1 2 3; 4 5 6]") .

There are pros and cons to using both:

:) Element-wise multiplication is easy: A*B .

:( You have to remember that matrix multiplication has its own operator, @ .

:) You can treat one-dimensional arrays as either row or column vectors. A @ v treats v as a column vector, while v @ A treats v as a row vector. This can save you having to type a lot of transposes.

:) array is the “default” NumPy type, so it gets the most testing, and is the type most likely to be returned by 3rd party code that uses NumPy.

:) Is quite at home handling data of any number of dimensions.

:) Closer in semantics to tensor algebra, if you are familiar with that.

:) All operations ( * , / , + , - etc.) are element-wise.

:( Sparse matrices from scipy.sparse do not interact as well with arrays.

:\\ Behavior is more like that of MATLAB matrices.

<:( Maximum of two-dimensional. To hold three-dimensional data you need array or perhaps a Python list of matrix .

<:( Minimum of two-dimensional. You cannot have vectors. They must be cast as single-column or single-row matrices.

<:( Since array is the default in NumPy, some functions may return an array even if you give them a matrix as an argument. This shouldn’t happen with NumPy functions (if it does it’s a bug), but 3rd party code based on NumPy may not honor type preservation like NumPy does.

:) A*B is matrix multiplication, so it looks just like you write it in linear algebra (For Python >= 3.5 plain arrays have the same convenience with the @ operator).

<:( Element-wise multiplication requires calling a function, multiply(A,B) .

<:( The use of operator overloading is a bit illogical: * does not work element-wise but / does.

Interaction with scipy.sparse is a bit cleaner.

The array is thus much more advisable to use. Indeed, we intend to deprecate matrix eventually.

Customizing your environment #

In MATLAB the main tool available to you for customizing the environment is to modify the search path with the locations of your favorite functions. You can put such customizations into a startup script that MATLAB will run on startup.

NumPy, or rather Python, has similar facilities.

To modify your Python search path to include the locations of your own modules, define the PYTHONPATH environment variable.

To have a particular script file executed when the interactive Python interpreter is started, define the PYTHONSTARTUP environment variable to contain the name of your startup script.

Unlike MATLAB, where anything on your path can be called immediately, with Python you need to first do an ‘import’ statement to make functions in a particular file accessible.

For example you might make a startup script that looks like this (Note: this is just an example, not a statement of “best practices”):

To use the deprecated matrix and other matlib functions:

Another somewhat outdated MATLAB/NumPy cross-reference can be found at http://mathesaurus.sf.net/

An extensive list of tools for scientific work with Python can be found in the topical software page .

See List of Python software: scripting for a list of software that use Python as a scripting language

MATLAB® and SimuLink® are registered trademarks of The MathWorks, Inc.

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Why does assignment operator call constructor?

I am just playing around to understand smart pointers and trying to make mine but I come across a situation that I do not fully understand. Here is the code:

The code compiles and on the line of holder = b; the constructor of Holder class is called. I thought compiler would give an error. It is not the assingment operator but why is it calling constructor?

• constructor
• variable-assignment
• smart-pointers

• Holder has two constructors, which one is called? –  suszterpatt Commented Dec 8, 2010 at 0:16

4 Answers 4

holder = b attempts to assign from b to Holder . b is of type A* , and holder is of type Holder<A> .

The Holder template defines assignment from another instance of the same Holder type, so the compiler looks for a conversion from A* to Holder<A> . It finds the constructor, and uses that.

Constructors which may take exactly one argument may be used for implicit conversions, unless you tag them with the explicit keyword.

Both the constructor and assignment operator are called. You can check this by printing something in operator = .

This happens because operator = is defined to take a const Holder & , but b is of type A * . So first the Holder(T *) constructor is called to create a temporary object, then this object is assigned to holder through operator = .

If you define an operator =(const T *) , only the assignment operator will be called.

I do not see a version of the assignment operator that takes a right hand side of A*

You have a constructor taking a T* . Your assigment has a rhs pointer ,so it construct a temp-obj with that pointer as argument and assigns this to holder.

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