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Move constructor and move assignment operator in c++

In the C++ programming language, the move constructor and move assignment operator are new and essential class programming features first introduced in C++11. The move constructor and move assignment operator in C++ are used to copy, move, or transfer program resource data information from one source class object to another target destination class object, instead of creating a costly C++ copy.

The move constructor and move assignment operator in C++ are particularly useful when a user-defined class in C++ manages resources such as dynamic memory, files, sockets, or other system resources.

Why do we need move semantics features in C++?

In a C++ move semantics program, we imagine a class data type structure layout that allocates the memory required for the class object in a dynamic order.

class TestClass

{

int* datatype;

public:

TestClass(int element)

{

datatype = new int(element);

}

};

Here we have.

TestClass element(77);

TestClass newelement = element;

In this example, a normal copy operation may require allocating new memory and copying the data.

With C++ move semantics, if the element in this program is a temporary class object whose data value resource is no longer required, we can easily transfer its data value resource to the newelement variable data type instead of creating a duplicate object.

Copy vs. Move Concept in C++ Programs.

C++ Copy Concept.

Class object P ──data──> allot memory P

Class object Q ──data──> allot memory Q

(This copies the class object data value)

C++ Move Concept.

Class object P ──data──> allot memory P

↑

Class object Q ──────────┘

Class object P → nullptr

In this example, ownership of the class object resource is transferred from P to the data value element of Q.

Move Constructor Concept in C++.

A move constructor creates a new object by transferring resource data from an existing temporary/rvalue object.

Syntax of Move Constructor.

ClassName(ClassName&& other);

Here the && ampersand operator indicates an rvalue reference.

Example of Move Constructor.

#include <iostream>

using namespace std;

move class

{

int* datatype;

Public:

move(int element)

{

datatype = new int(element);

}

//Here we use move constructor to class object

Move(Move&& other)

{

datatype = other.datatype;

other.datatype = nullptr;

}

void display()

{

cout << *datatype << endl;

}

~Move()

{

delete datatype;

}

};

int main()

{

move element(77);

Move newelement = std::move(element);

newelement.display();

return 0;

}

The output is.

77

What happens in the move constructor here?

Move newelement = std::move(element);

• Here, std::move(element) treats the element as an rvalue object.
• The move constructor method is called.
• The object pointer is moved or transferred from the element value to the newelement value.
• Finally, element.data is set to the nullptr value.
• No new integer data type value memory is allocated for the newelement class object.

Move Assignment Operator in C++.

The move assignment operator in a C++ program easily transfers a variable object resource from a user-defined class to another existing class object in the program.

Syntax of the Move Assignment Operator.

ClassName& operator=(ClassName&& other);

Example of the Move Assignment Operator.

#include <iostream>

using namespace std;

move class

{

int* datatype;

Public:

move(int element)

{

datatype = new int(element);

}

// Here we use move constructor

Move(Move&& another)

{

datatype = another.datatype;

another.datatype = nullptr;

}

// Here we move assignment operator

Move& operator=(Move&& another)

{

if (this != &another)

{

delete datatype;

datatype = another.datatype;

another.datatype = nullptr;

}

return *this;

}

void preview()

{

if(datatype)

cout << *datatype<< endl;

}

~Move()

{

delete datatype;

}

};

int main()

{

Move element(7);

Move newelement(17);

newelement = std::move(element);

newelement.preview();

return 0;

}

The output is.

7

Here in the Move Assignment Operator example.

• In this example, newelement = std::move(element);
• uses the move assignment operator because newelement already exists.

Main Difference between Move Constructor vs Move Assignment Operator

Simple example of a move constructor.

Move constructor in C++.

Move element(7);

Move newelement(std::move(element));

The newelement class object is being created in it.

Move assignment statement.

Move element(7);

Move newelement(17);

newelement = std::move(element);

Here, newelement is already defined or exists.

Copy vs. Move concept in C++ programming.

These are some important special member functions to better understand the copy and move concepts in general in the C++ programming language.

• Copy constructor method defined in a class
• Use of the copy assignment operator in a class
• Use of the move constructor method
• Use of the move assignment operator

Copy vs. Move concept example.

P q = p; // This is a copy constructor method defined

q = p; // This is a copy assignment method definition

P r = std::move(p); // This is a move constructor method definition

r = std::move(q); // This is a move assignment method definition

Why do move semantics improve program performance in a C++ program?

In a C++ program, assume that a class object has a large block of dynamically allocated class variable data type memory.

Copying a class object may require the following.

Assign new memory to the class object

↓

Copy all data to the destination location with proper values

↓

Keep both resources proper

The move class method allows for easy transfer of ownership of an object.

Transfer Pointer/Ownership with Move

↓

There is no expensive data copying involved in this process.

Because of this, when existing class objects are large or require expensive resources to copy, move semantics can significantly improve program performance.

Essentials of the move constructor and move assignment operator in C++.

• Move semantics were first introduced in C++11.

• The move constructor uses the && symbolic address operator.
• The move assignment operator also uses the && symbolic address operator.
• These transfer ownership/resources instead of copying them, as is common in C++.
• The std::move() function method is commonly used to move a class object in C++ programs.
• After a class object is moved, it remains a valid class object in the program. But its value is generally not something you should trust unless your class indicates it.
• A class object moved into a user-defined class must still be destructible in secure order.

An easy way to remember the move constructor and move assignment operator in C++.

• Move constructor → In C++, the user assigns resources to a new object.
• Move assignment → In C++, the user assigns resources to an existing class object.

Move constructor method in C++.

P → [Class object resource]

↓ Transfer

Q → [Class object resource]

Move assignment method in C++.

P → [Class Object Resource]

↓ Transfer

Q (already existing) → [Class Object Resource]

Move constructor and move assignment operator in a nutshell.

A move constructor and move assignment operator in a user-defined class program help a C++ program efficiently transfer class object resources from one source object location to another target destination location. This avoids unnecessary copying of variable class object resources in the existing class program.