assignment operator c 11

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Why does the = operator work on structs without having been defined?

Let's look at a simple example:

Then abc_copy is an exact copy of abc .. how is it possible without defining the = operator ?

(This took me by surprise when working on some code..)

7 Answers 7

If you do not define these four methods (six in C++11) the compiler will generate them for you:

Assignment Operator

If you want to know why? It is to maintain backward compatibility with C (because C structs are copyable using = and in declaration). But it also makes writing simple classes easier. Some would argue that it adds problems because of the "shallow copy problem". My argument against that is that you should not have a class with owned RAW pointers in it. By using the appropriate smart pointers that problem goes away.

Default Constructor (If no other constructors are defined)

The compiler generated default constructor will call the base classes default constructor and then each members default constructor (in the order they are declared)

Destructor (If no destructor defined)

Calls the destructor of each member in reverse order of declaration. Then calls the destructor of the base class.

Copy Constructor (If no copy constructor is defined)

Calls the base class copy constructor passing the src object. Then calls the copy constructor of each member using the src objects members as the value to be copied.

Calls the base class assignment operator passing the src object. Then calls the assignment operator on each member using the src object as the value to be copied.

Move Constructor (If no move constructor is defined)

Calls the base class move constructor passing the src object. Then calls the move constructor of each member using the src objects members as the value to be moved.

Move Assignment Operator

Calls the base class move assignment operator passing the src object. Then calls the move assignment operator on each member using the src object as the value to be copied.

If you define a class like this:

What the compiler will build is:

In C++, structs are equivalent to classes where members default to public rather than private access.

C++ compilers will also generate the following special members of a class automatically if they are not provided:

That behavior is necessary in order to maintain source compatibility with C.

C does not give you the ability to define/override operators, so structs are normally copied with the = operator.

But it is defined. In the standard. If you supply no operator =, one is supplied to you. And the default operator just copies each of the member variables. And how does it know which way to copy each member? it calls their operator = (which, if not defined, is supplied by default...).

The assignment operator ( operator= ) is one of the implicitly generated functions for a struct or class in C++.

Here is a reference describing the 4 implicitly generated members: http://www.cs.ucf.edu/~leavens/larchc++manual/lcpp_136.html

In short, the implicitly generated member performs a memberwise shallow copy . Here is the long version from the linked page:

The implicitly-generated assignment operator specification, when needed, is the following. The specification says that the result is the object being assigned ( self ), and that the value of the abstract value of self in the post-state self " is the same as the value of the abstract value of the argument from .

The compiler will synthesise some members for you if you don't define them explicitly yourself. The assignment operator is one of them. A copy constructor is another, and you get a destructor too. You also get a default constructor if you don't provide any constructors of your own. Beyond that I'm not sure what else but I believe there may be others (the link in the answer given by 280Z28 suggests otherwise though and I can't remember where I read it now so maybe it's only four).

structs are basically a concatenation of its components in memory (with some possible padding built in for alignment). When you assign one struct the value of another, the values are just coped over.

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cppreference.com

Operator overloading.

Customizes the C++ operators for operands of user-defined types.

[ edit ] Syntax

Overloaded operators are functions with special function names:

[ edit ] Overloaded operators

When an operator appears in an expression , and at least one of its operands has a class type or an enumeration type , then overload resolution is used to determine the user-defined function to be called among all the functions whose signatures match the following:

Note: for overloading co_await , (since C++20) user-defined conversion functions , user-defined literals , allocation and deallocation see their respective articles.

Overloaded operators (but not the built-in operators) can be called using function notation:

[ edit ] Restrictions

[ edit ] Canonical implementations

Besides the restrictions above, the language puts no other constraints on what the overloaded operators do, or on the return type (it does not participate in overload resolution), but in general, overloaded operators are expected to behave as similar as possible to the built-in operators: operator + is expected to add, rather than multiply its arguments, operator = is expected to assign, etc. The related operators are expected to behave similarly ( operator + and operator + = do the same addition-like operation). The return types are limited by the expressions in which the operator is expected to be used: for example, assignment operators return by reference to make it possible to write a = b = c = d , because the built-in operators allow that.

Commonly overloaded operators have the following typical, canonical forms: [1]

[ edit ] Assignment operator

The assignment operator ( operator = ) has special properties: see copy assignment and move assignment for details.

The canonical copy-assignment operator is expected to perform no action on self-assignment , and to return the lhs by reference:

In those situations where copy assignment cannot benefit from resource reuse (it does not manage a heap-allocated array and does not have a (possibly transitive) member that does, such as a member std::vector or std::string ), there is a popular convenient shorthand: the copy-and-swap assignment operator, which takes its parameter by value (thus working as both copy- and move-assignment depending on the value category of the argument), swaps with the parameter, and lets the destructor clean it up.

This form automatically provides strong exception guarantee , but prohibits resource reuse.

[ edit ] Stream extraction and insertion

The overloads of operator>> and operator<< that take a std:: istream & or std:: ostream & as the left hand argument are known as insertion and extraction operators. Since they take the user-defined type as the right argument ( b in a @ b ), they must be implemented as non-members.

These operators are sometimes implemented as friend functions .

[ edit ] Function call operator

When a user-defined class overloads the function call operator, operator ( ) , it becomes a FunctionObject type.

An object of such a type can be used in a function call expression:

Many standard algorithms, from std:: sort to std:: accumulate accept FunctionObject s to customize behavior. There are no particularly notable canonical forms of operator ( ) , but to illustrate the usage:

[ edit ] Increment and decrement

When the postfix increment or decrement operator appears in an expression, the corresponding user-defined function ( operator ++ or operator -- ) is called with an integer argument 0 . Typically, it is implemented as T operator ++ ( int ) or T operator -- ( int ) , where the argument is ignored. The postfix increment and decrement operators are usually implemented in terms of the prefix versions:

Although the canonical implementations of the prefix increment and decrement operators return by reference, as with any operator overload, the return type is user-defined; for example the overloads of these operators for std::atomic return by value.

[ edit ] Binary arithmetic operators

Binary operators are typically implemented as non-members to maintain symmetry (for example, when adding a complex number and an integer, if operator+ is a member function of the complex type, then only complex + integer would compile, and not integer + complex ). Since for every binary arithmetic operator there exists a corresponding compound assignment operator, canonical forms of binary operators are implemented in terms of their compound assignments:

[ edit ] Comparison operators

Standard algorithms such as std:: sort and containers such as std:: set expect operator < to be defined, by default, for the user-provided types, and expect it to implement strict weak ordering (thus satisfying the Compare requirements). An idiomatic way to implement strict weak ordering for a structure is to use lexicographical comparison provided by std::tie :

Typically, once operator < is provided, the other relational operators are implemented in terms of operator < .

Likewise, the inequality operator is typically implemented in terms of operator == :

When three-way comparison (such as std::memcmp or std::string::compare ) is provided, all six two-way comparison operators may be expressed through that:

[ edit ] Array subscript operator

User-defined classes that provide array-like access that allows both reading and writing typically define two overloads for operator [ ] : const and non-const variants:

If the value type is known to be a scalar type, the const variant should return by value.

Where direct access to the elements of the container is not wanted or not possible or distinguishing between lvalue c [ i ] = v ; and rvalue v = c [ i ] ; usage, operator [ ] may return a proxy. See for example std::bitset::operator[] .

Because a subscript operator can only take one subscript until C++23, to provide multidimensional array access semantics, e.g. to implement a 3D array access a [ i ] [ j ] [ k ] = x ; , operator [ ] has to return a reference to a 2D plane, which has to have its own operator [ ] which returns a reference to a 1D row, which has to have operator [ ] which returns a reference to the element. To avoid this complexity, some libraries opt for overloading operator ( ) instead, so that 3D access expressions have the Fortran-like syntax a ( i, j, k ) = x ; .

[ edit ] Bitwise arithmetic operators

User-defined classes and enumerations that implement the requirements of BitmaskType are required to overload the bitwise arithmetic operators operator & , operator | , operator ^ , operator~ , operator & = , operator | = , and operator ^ = , and may optionally overload the shift operators operator << operator >> , operator >>= , and operator <<= . The canonical implementations usually follow the pattern for binary arithmetic operators described above.

[ edit ] Boolean negation operator

[ edit ] rarely overloaded operators.

The following operators are rarely overloaded:

[ edit ] Notes

[ edit ] example, [ edit ] defect reports.

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

[ edit ] See also

[ edit ] External links

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Assignment operators (C# reference)

The assignment operator = assigns the value of its right-hand operand to a variable, a property , or an indexer element given by its left-hand operand. The result of an assignment expression is the value assigned to the left-hand operand. The type of the right-hand operand must be the same as the type of the left-hand operand or implicitly convertible to it.

The assignment operator = is right-associative, that is, an expression of the form

is evaluated as

The following example demonstrates the usage of the assignment operator with a local variable, a property, and an indexer element as its left-hand operand:

The left-hand operand of an assignment receives the value of the right-hand operand. When the operands are of value types , assignment copies the contents of the right-hand operand. When the operands are of reference types , assignment copies the reference to the object.

This is called value assignment : the value is assigned.

ref assignment

Ref assignment = ref makes its left-hand operand an alias to the right-hand operand. The left-hand operand must be a ref local , ref readonly local , or a ref field in a ref struct . Both operands must be of the same type.

The following example demonstrates the usage of the ref assignment operator:

In the preceding example, the ref local arrayElement variable is initialized as an alias to the first array element. Then, it's reassigned to become an alias to the last array element. As it's an alias, when you update its value with an ordinary assignment operator = , the corresponding array element is also updated.

Compound assignment

For a binary operator op , a compound assignment expression of the form

is equivalent to

except that x is only evaluated once.

Compound assignment is supported by arithmetic , Boolean logical , and bitwise logical and shift operators.

Null-coalescing assignment

You can use the null-coalescing assignment operator ??= to assign the value of its right-hand operand to its left-hand operand only if the left-hand operand evaluates to null . For more information, see the ?? and ??= operators article.

Operator overloadability

A user-defined type can't overload the assignment operator. However, a user-defined type can define an implicit conversion to another type. That way, the value of a user-defined type can be assigned to a variable, a property, or an indexer element of another type. For more information, see User-defined conversion operators .

A user-defined type can't explicitly overload a compound assignment operator. However, if a user-defined type overloads a binary operator op , the op= operator, if it exists, is also implicitly overloaded.

C# language specification

For more information, see the Assignment operators section of the C# language specification .

For more information about the ref assignment operator = ref , see the feature proposal note .

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Additional resources

Assignment Operators in C

The following table lists the assignment operators supported by the C language −

Try the following example to understand all the assignment operators available in C −

When you compile and execute the above program, it produces the following result −

Assignment Operators in C

We use this type of operator to transform as well as assign the values to any variable in an operation. In any given assignment operator, the right side is a value, and the left side is a variable. The value present on the right side of the operator must have the same data type as that of the variable present on the left side. In any other case, the compiler raises an error.

In this article, we will take a look into the Assignment Operators in C according to the GATE Syllabus for CSE (Computer Science Engineering) . Read ahead to know more.

Table of Contents

Types of Assignment Operators in C

An assignment operator is basically a binary operator that helps in modifying the variable to its left with the use of the value to its right. We utilize the assignment operators to transform and assign values to any variables.

Here is a list of the assignment operators that you can find in the C language:

Working of Assignment Operators in C

Here is a table that discusses, in brief, all the Assignment operators that the C language supports:

Example of Assignment Operators in C

Let us look at an example to understand how these work in a code:

#include <stdio.h>

int x = 21;

printf(“Line A – = Example of the Value of y = %d\n”, y );

printf(“Line B – -= Example of the Value of y = %d\n”, y );

printf(“Line C – += Example of the Value of c = %d\n”, c );

printf(“Line D – /= Example of the Value of y = %d\n”, y );

printf(“Line E – *= Example of the Value of y = %d\n”, y );

y <<= 2;

printf(“Line F – <<= Example of the Value of y = %d\n”, y );

printf(“Line G – %= Example of the Value of y = %d\n”, y );

y &= 2;

printf(“Line H – &= Example of the Value of y = %d\n”, y );

y >>= 2;

printf(“Line I – >>= Example of the Value of y = %d\n”, y );

printf(“Line J – |= Example of the Value of y = %d\n”, y );

printf(“Line K – ^= Example of the Value of y = %d\n”, y );

The compilation and execution of the program mentioned above will produce a result as follows:

Line A – = Example of the Value of y = 21

Line B – -= Example of the Value of y = 21

Line C – += Example of the Value of y = 42

Line D – /= Example of the Value of y = 21

Line E – *= Example of the Value of y = 441

Line F – <<= Example of the Value of y = 44

Line G – %= Example of the Value of y = 11

Line H – &= Example of the Value of y = 2

Line I – >>= Example of the Value of y = 11

Line J – |= Example of the Value of y = 2

Line K – ^= Example of the Value of y = 0

Here is another example of how the assignment operators work in the C language:

int y = 10;

printf(“z = x + y = %d \n”,z);

printf(“z += x = %d \n”,z);

printf(“z -= x = %d \n”,z);

printf(“z *= x = %d \n”,z);

printf(“z /= x = %d \n”,z);

printf(“z %= x = %d \n”,z);

c &= x ;

printf(“c &= x = %d \n”,z);

printf(“z ^= x = %d \n”,z);

printf(“z |= x = %d \n”,z);

z <<= 2 ;

printf(“z <<= 2 = %d \n”,z);

z >>= 2 ;

printf(“z >>= 2 = %d \n”,z);

The output generated here will be:

Practice Problems on Assignment Operators in C

1. What would be the output obtained from the program given below?

p+=p+=p+=3; it can written as p+=p+=p=p+3; p=2; Or, p+=p+=5; p=5; Or, p+=p=5+5; p=5; Or, p+=10; p=10; Or, p=p+10; p=10; Or, p=20. So, finally p=20.

2. Which of these is an invalid type of assignment operator?

D. None of these

Answer – D. None of these

All of these are valid types of assignment operators.

How does the /= operator work? Is it a combination of two other operators?

Yes, the /+ operator is a combination of the = and / operators. The / operator divides the current value of the available variable first on the left using the available value on the right. It then assigns the obtained result to the available variable on the left side.

What is the most basic operator among all the assignment operators available in the C language?

The = operator is the most basic one used in the C language. We use this operator to assign the value available in the right to the value mentioned on the left side of the operator.

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C++ Tutorial

C++ functions, c++ classes, c++ examples, c++ assignment operators, assignment operators.

Assignment operators are used to assign values to variables.

In the example below, we use the assignment operator ( = ) to assign the value 10 to a variable called x :

The addition assignment operator ( += ) adds a value to a variable:

A list of all assignment operators:

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C++11 Tutorial: Introducing the Move Constructor and the Move Assignment Operator

Copy constructors sounds like a topic for an article from 1989. And yet, the changes in the new C++ standard affect the design of a class' special member functions fundamentally. Find out more about the impact of move semantics on objects' behavior and learn how to implement the move constructor and the move assignment operator in C++11.

C++11 is the informal name for ISO/IEC 14882:2011, the new C++ standard that was published in September 2011. It includes the TR1 libraries and a large number of new core features (a detailed discussion about these new C++11 features is available here ; also see The Biggest Changes in C++11 (and Why You Should Care) ):

Important as these features may be, the defining feature of C++11 is rvalue references .

The Right Time for Rvalue References

Rvalue references are a new category of reference variables that can bind to rvalues . Rvalues are slippery entities, such as temporaries and literal values; up until now, you haven't been able to bind these safely to reference variables.

Technically, an rvalue is an unnamed value that exists only during the evaluation of an expression. For example, the following expression produces an rvalue:

x+(y*z); // A C++ expression that produces a temporary

C++ creates a temporary (an rvalue) that stores the result of y*z , and then adds it to x . Conceptually, this rvalue evaporates by the time you reach the semicolon at the end of the full expression.

A declaration of an rvalue reference looks like this:

std::string&& rrstr; //C++11 rvalue reference variable

The traditional reference variables of C++ e.g.,

std::string& ref;

are now called lvalue references .

Rvalue references occur almost anywhere, even if you don't use them directly in your code. They affect the semantics and lifetime of objects in C++11. To see how exactly, it's time to talk about move semantics .

Get to Know Move Semantics

Hitherto, copying has been the only means for transferring a state from one object to another (an object's state is the collective set of its non-static data members' values). Formally, copying causes a target object t to end up with the same state as the source s , without modifying s . For example, when you copy a string s1 to s2 , the result is two identical strings with the same state as s1 .

And yet, in many real-world scenarios, you don't copy objects but move them. When my landlord cashes my rent check, he moves money from my account into his. Similarly, removing the SIM card from your mobile phone and installing it in another mobile is a move operation, and so are cutting-and-pasting icons on your desktop, or borrowing a book from a library.

Notwithstanding the conceptual difference between copying and moving, there's a practical difference too: Move operations tend to be faster than copying because they transfer an existing resource to a new destination, whereas copying requires the creation of a new resource from scratch. The efficiency of moving can be witnessed among the rest in functions that return objects by value. Consider:

string func()

//do something with s

string mystr=func();

When func() returns, C++ constructs a temporary copy of s on the caller's stack memory. Next, s is destroyed and the temporary is used for copy-constructing mystr . After that, the temporary itself is destroyed. Moving achieves the same effect without so many copies and destructor calls along the way.

Moving a string is almost free; it merely assigns the values of the source's data members to the corresponding data members of the target. In contrast, copying a string requires the allocation of dynamic memory and copying the characters from the source.

Move Special Member Functions

C++11 introduces two new special member functions: the move constructor and the move assignment operator . They are an addition to the fabulous four you know so well:

If a class doesn't have any user-declared special member functions (save a default constructor), C++ declares its remaining five (or six) special member functions implicitly, including a move constructor and a move assignment operator. For example, the following class

doesn't have any user-declared special member functions. Therefore, C++ declares all of its six special member functions implicitly. Under certain conditions , implicitly declared special member functions become implicitly defined as well. The implicitly-defined move special member functions move their sub-objects and data members in a member-wise fashion. Thus, a move constructor invokes its sub-objects' move constructors, recursively. Similarly, a move assignment operator invokes its sub-objects' move assignment operators, recursively.

What happens to a moved-from object? The state of a moved-from object is unspecified. Therefore, always assume that a moved-from object no longer owns any resources, and that its state is similar to that of an empty (as if default-constructed) object. For example, if you move a string s1 to s2 , after the move operation the state of s2 is identical to that of s1 before the move, whereas s1 is now an empty (though valid) string object.

Designing a Move Constructor

A move constructor looks like this:

C::C(C&& other); //C++11 move constructor

It doesn't allocate new resources. Instead, it pilfers other 's resources and then sets other to its default-constructed state.

Let's look at a concrete example. Suppose you're designing a MemoryPage class that represents a memory buffer:

class MemoryPage

size_t size;

char * buf;

explicit MemoryPage(int sz=512):

size(sz), buf(new char [size]) {}

~MemoryPage( delete[] buf;}

//typical C++03 copy ctor and assignment operator

MemoryPage(const MemoryPage&);

MemoryPage& operator=(const MemoryPage&);

A typical move constructor definition would look like this:

MemoryPage(MemoryPage&& other): size(0), buf(nullptr)

// pilfer other's resource

The move constructor is much faster than a copy constructor because it doesn't allocate memory nor does it copy memory buffers.

Designing a Move Assignment Operator

A move assignment operator has the following signature:

C& C::operator=(C&& other);//C++11 move assignment operator

A move assignment operator is similar to a copy constructor except that before pilfering the source object, it releases any resources that its object may own. The move assignment operator performs four logical steps:

Here's a definition of MemoryPage 's move assignment operator:

MemoryPage& MemoryPage::operator=(MemoryPage&& other)

if (this!=&other)

// release the current object's resources

delete[] buf;

return *this;

Overloading Functions

The overload resolution rules of C++11 were modified to support rvalue references. Standard Library functions such as vector::push_back() now define two overloaded versions: one that takes const T& for lvalue arguments as before, and a new one that takes a parameter of type T&& for rvalue arguments. The following program populates a vector with MemoryPage objects using two push_back () calls:

#include <vector>

using namespace std;

vector<MemoryPage> vm;

vm.push_back(MemoryPage(1024));

vm.push_back(MemoryPage(2048));

Both push_back() calls resolve as push_back(T&&) because their arguments are rvalues. push_back(T&&) moves the resources from the argument into vector 's internal MemoryPage objects using MemoryPage 's move constructor. In older versions of C++, the same program would generate copies of the argument since the copy constructor of MemoryPage would be called instead.

As I said earlier, push_back(const T&) is called when the argument is an lvalue:

MemoryPage mp1(1024);//lvalue

vm.push_back(mp); //push_back(const T&)

However, you can enforce the selection of push_back(T&&) even in this case by casting an lvalue to an rvalue reference using static_cast :

//calls push_back(T&&)

vm.push_back(static_cast<MemoryPage&&>(mp));

Alternatively, use the new standard function std::move() for the same purpose:

vm.push_back(std::move(mp));//calls push_back(T&&)

It may seem as if push_back(T&&) is always the best choice because it eliminates unnecessary copies. However, remember that push_back(T&&) empties its argument. If you want the argument to retain its state after a push_back() call, stick to copy semantics. Generally speaking, don't rush to throw away the copy constructor and the copy assignment operator. In some cases, the same class could be used in a context that requires pure copy semantics, whereas in other contexts move semantics would be preferable.

In Conclusion

C++11 is a different and better C++. Its rvalue references and move-oriented Standard Library eliminate many unnecessary copy operations, thereby improving performance significantly, with minimal, if any, code changes. The move constructor and the move assignment operator are the vehicles of move operations. It takes a while to internalize the principles of move semantics – and to design classes accordingly. However, the benefits are substantial. I would dare predicting that other programming languages will soon find ways to usher-in move semantics too.

Danny Kalev is a certified system analyst by the Israeli Chamber of System Analysts and software engineer specializing in C++. Kalev has written several C++ textbooks and contributes C++ content regularly on various software developers' sites. He was a member of the C++ standards committee and has a master's degree in general linguistics.

Additional Links

std::move in Utility in C++ | Move Semantics, Move Constructors and Move Assignment Operators

Last Updated : 11 Feb, 2022

Prerequisites:

References:

In C++ there are two types of references-

Move Constructor And Semantics:

The move constructor was introduced in C++11 . The need or purpose of a move constructor is to steal or move as many resources as it can from the source (original) object , as fast as possible, because the source does not need to have a meaningful value anymore, and/or because it is going to be destroyed in a moment anyway. So that one can avoid unnecessarily creating copies of an object and make efficient use of the resources

While one can steal the resources, but one must leave the source (original) object in a valid state where it can be correctly destroyed.

Move constructors typically “steal” the resource of the source (original) object rather than making several copies of them, and leaves the source object in a “valid but unspecified state”.

The copy constructor uses the lvalue references which are marked with one ampersand (&) while the move constructor uses the rvalue references are marked with two ampersands (&&).

std::move() is a function used to convert an lvalue reference into the rvalue reference. Used to move the resources from a source object i.e. for efficient transfer of resources from one object to another. std::move() is defined in the <utility> header .

template< class T > typename std::remove_reference<T>::type&& move(T&& t) noexcept; (since C++11)(until C++14) template< class T > constexpr std::remove_reference_t<T>&& move(T&& t) noexcept (since C++14)

Example: Below is the C++ program to show what happens without using move semantics i.e. before C++11.

Explanation:

Assuming the program is compiled and executed using a compiler that doesn’t support move semantics. In the main() function,

1. std::vector<std::string> vecString;- An empty vector is created with no elements in it. 2. vecString = createAndInsert();- The createAndInsert() function is called. 3. In createAndInsert() function-

Note: Here, we unnecessarily allocate & deallocate the memory of the temporary string object. which can be optimized (improved) further just by moving the data from the source object.

Example: Below is the C++ program to implement the above concept using move semantics i.e. since C++11 and later.

Here, in order to use the move semantics. The compiler must support the C++11 standards or above. The story of execution for the main() function and createAndInsert() function remains the same till the line vec.push_back( str );

A question may arise why the temporary object is not moved to vector vec using std::move(). The reason behind it is the push_back() method of the vector. Since C++11 the push_back() method has been provided with its new overloaded version.

Syntax:

constexpr void push_back(const T& value); (since C++20) void push_back(T&& value); (since C++11) (until C++20) void push_back(const T& value); (until C++20) constexpr void push_back(T&& value); (since C++20)

A question may arise while returning the vec object to its caller. As it is not required anymore and also a whole temporary object of a vector is going to be created and also local vector vec will be destroyed, then why std::move() is not used to steal the value and return it. Its answer is simple and obvious, there is optimization at the compiler level known as (Named) Return Value Object, more popularly known as RVO .

Some Fallback Of Move Semantics:

Note: The foo() function have all necessary types of arguments.

Below is the C++ program to implement all the above concepts-

Summary:

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Assignment Operators in C++

Introduction to Assignment Operators in C++

Here, let us begin on knowing about assignment operators in C++. As the name already suggests, these operators help in assigning values to variables. We discuss these with respect to operators and operands. These operators help us in allocating a particular value to the operands.

The main simple assignment operator is ‘=’. We have to be sure that both the left and right sides of the operator must have the same data type. We have different levels of operators. Let us learn about each of those in C++. Assignment operators are a part of binary operators. Examples for these are: =, +=, -=, *=, /=, %=. Let us learn about each of these with examples.

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Assignment Operators

There are three levels of operators.

Assignment operators are a part of binary operators.

Examples for these are: =, +=, -=, *=, /=, %=. Let us learn about each of these with examples.

Examples #1

Let us start with the first example with the ‘=’ operator as given below.

So, as an example, we can understand that the assignment operator ‘=’ is just going to assign values based on the data type using the operands and variables. Above, we can see that for the value b which is given data type of int, gives only the value till the decimal point. Only if we give the data type float like the variable d, the complete value gets displayed. So, we can say that data type also plays an important role in setting and displaying the values of different operands based on their values.

Examples #2

In this example let us learn how ‘+=’ this operand works.

As observed above, the example of how we got the value for variable c, is the process of how the operator ‘+=’ works. As per the operand, first, the left side operand is added to the left side operand and then the final value is being assigned to the left side operand. This is how ‘+=’ is used.

Examples #3

Now, let us learn on the operand ‘-=’. This is almost similar to that of ‘+=’. The above operand adds the value, but here it gets subtracted. Let us see an example below.

Here also we have given example for variable ‘c’ on how actually the assignment of value is done. First, the left side operand value subtracts the right-hand value from it. In this process, there might be a negative value obtained.

Examples #4

Here let us have an example for the assignment operator *=, /= and %= together.

Now coming to the interpretation of the above example.

Examples #5

Now let us look at how operators &=, ^= and |= as shown below.

Did we expect this output? Let us now see how it is being calculated. We would start with ‘&=’ operation. Below is the table for the bitwise operator values.

We already know, ‘&’ is a bitwise and operator. So, for the output of variable, ‘a’ our calculation would be as follows. Initial value of ‘a’: 5 – 101 in Binary The right value is 3 – 011 in Binary. Now calculating the ‘&’ value between those two gives 001. This is number one (1) in normal decimals. That is how we get the output.

Examples #6

Here, let us learn about the operators <<= and >>= as given below.

So, let us now see how these operators are actually performing its operations. For the operator, >>=; which is called the right shift operator. First, the left operand values are converted to their binary representation. Here a: 5 – 101 in binary and the right-side value is 1 (001). Now from the binary values shift one digit to right and the empty place at the left is replaced by zero.

101 → _10 1→ 010 1 → 010 → 2 (in Decimal conversion)

For the same let me explain to you with example 5>>=2.

101 → _ _ 1 01 → 001 01→ 001→1 (in Decimal conversion)

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1. C++ Arithmetic Operators

Arithmetic operators are used to perform arithmetic operations on variables and data. For example,

Here, the + operator is used to add two variables a and b . Similarly there are various other arithmetic operators in C++.

Example 1: Arithmetic Operators

Here, the operators + , - and * compute addition, subtraction, and multiplication respectively as we might have expected.

/ Division Operator

Note the operation (a / b) in our program. The / operator is the division operator.

As we can see from the above example, if an integer is divided by another integer, we will get the quotient. However, if either divisor or dividend is a floating-point number, we will get the result in decimals.

% Modulo Operator

The modulo operator % computes the remainder. When a = 9 is divided by b = 4 , the remainder is 1 .

Note: The % operator can only be used with integers.

Increment and Decrement Operators

C++ also provides increment and decrement operators: ++ and -- respectively.

For example,

Here, the code ++num; increases the value of num by 1 .

Example 2: Increment and Decrement Operators

In the above program, we have used the ++ and -- operators as prefixes (++a and --b) . However, we can also use these operators as postfix (a++ and b--) .

To learn more, visit increment and decrement operators .

2. C++ Assignment Operators

In C++, assignment operators are used to assign values to variables. For example,

Here, we have assigned a value of 5 to the variable a .

Example 3: Assignment Operators

3. c++ relational operators.

A relational operator is used to check the relationship between two operands. For example,

Here, > is a relational operator. It checks if a is greater than b or not.

If the relation is true , it returns 1 whereas if the relation is false , it returns 0 .

Example 4: Relational Operators

Note : Relational operators are used in decision-making and loops.

4. C++ Logical Operators

Logical operators are used to check whether an expression is true or false . If the expression is true , it returns 1 whereas if the expression is false , it returns 0 .

In C++, logical operators are commonly used in decision making. To further understand the logical operators, let's see the following examples,

Example 5: Logical Operators

Explanation of logical operator program

5. C++ Bitwise Operators

In C++, bitwise operators are used to perform operations on individual bits. They can only be used alongside char and int data types.

To learn more, visit C++ bitwise operators .

6. Other C++ Operators

Here's a list of some other common operators available in C++. We will learn about them in later tutorials.

Sorry about that.

C++. Move constructor and move operator

Move constructor and move operator. the purpose. examples. the noexcept keyword. an example of vector class (dynamic array).

Before exploring this topic, it is recommended that you familiarize yourself with the following topic:

Class constructor. Features of use constructors in the classes. The default constructor. Parameterized constructor. Examples of calsses that contain constructors

1. What special class functions are provided by the compiler by default? Example

2. move constructor and move operator. the purpose. features of use. general form, 3. figure showing the purpose of a move constructor in a class, 4. ways of calling the move constructor. references of type rvalue- and lvalue-. actions performed in the move constructor, 5. move operator. general form, 6.2.1. the components of the vector class, 6.2.2. internal class fields, 6.2.3. internal private functions free() , copyarray(), 6.2.4. constructor with two parameters vector(double*, int), 6.2.5. destructor ~vector(), 6.2.6. parameterless constructor vector(), 6.2.7. copy constructor vector(const vector&), 6.2.8. copy operator operator=(const vector&), 6.2.9. copy constructor vector(vector&&), 6.2.10. move operator operator=(vector&&), 6.2.11. method set(double*, int) . set a new array, 6.2.12. method print(string) . display an array with the given message, 6.2.13. general structure of the vector class, 6.2.14. method getv() . get a random array, 6.2.15. function main(), 6.2.16. general structure of the program, 6.2.17. run the program, related topics.

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When the class is initially declared, the compiler provides 6 special functions by default that can be overridden:

Any of the above functions can be overridden in a class with your own implementation.

Example . Let an empty class (stub) named MyClass be given

For the class MyClass , the compiler generates 6 special functions by default. The signature of these functions is as follows:

The move constructor and the move operator were added in C++ 11. The main idea behind using these two constructs is to speed up program execution by avoiding copying data during initial initialization and assignment of so-called rvalue references.

It is advisable to declare the move constructor and the move operator in classes containing large data arrays. Nobody bothers to declare a move constructor or a move operator in the class in which there are only a few simple fields of small dimension. However, in this case, the effect of using the move constructor will be negligible (or not at all).

Also, if it becomes necessary to use these structures, it is recommended to add them in pairs (both structures).

If the move constructor is not implemented in the class , then its call is replaced by the copy constructor. If the move operator is not implemented in the class, then its call is replaced by the copy operator.

The general form of declaring a move constructor in a class

The above general form uses the noexcept keyword. This specifier indicates that our function (the move constructor) does not throw (throw) an exception or crash. The compiler recommends using the word noexcept for the move constructor and the move operator. In the move constructor, no memory operations are performed (memory allocation, memory release, writing data to the allocated memory area, etc.), but a simple assignment of the pointer (s) takes place.

Figure 1 shows the case when the class does not implement the move constructor and the copy constructor is called instead. Copying a large data array A with dimension n elements for a class named MyClass is considered. As you can see from the figure, the entire array is copied element by element. If the number of elements in this array is large, this process will take some time.

C++. Calling the copy constructor. Copying the entire array

Figure 1. Actions performed by the copy constructor. Copying the entire array

After adding the move constructor to the MyClass class, the data is no longer copied (Figure 2). This gives a significant gain in the speed of program execution if the number of elements in the obj.A array is significant (for example, n = 1E6).

C++. Move constructor. Assigning (redirecting) a pointer to the source array

Figure 2. Move constructor. Assigning (redirecting) a pointer to the source array. Data is not copied

This topic covers only the general features of rvalue- and lvalue- references. A detailed overview of lvalues and rvalues is a completely different topic that requires a separate thorough study.

If a move constructor is declared in a class, then it is called in cases when the expression that initializes the value of an instance of this class with the = operator receives another instance, which is a so-called rvalue reference.

Below we consider one of the possible situations of calling the move constructor.

In the most general case, any function that returns an instance of the ClassName class looks like this:

The move constructor is called when the SomeFunc() function is called at the time the class instance is declared.

The obj instance is located on the left side of the assignment operator and is an lvalue reference. Such a reference is scoped within curly braces { } and is available after the current expression completes. In other words, you can use the obj instance in the future, for example

here SomeInternalMethod() is some public method from the ClassName class.

In turn, the SumeFunc() function is placed on the right side of the = operator. The result returned by the function is an instance of the ClassName class. In this case, this instance is a temporary rvalue reference. The scope of this temporary object is determined by the current expression. This temporary instance will not be used after the assignment (Figure 3).

C++. The case where the move constructor is called

Figure 3. The case where the move constructor is called. Scopes for lvalue and rvalue references

If a class implements its own move constructor, such initializations will call that constructor instead of the copy constructor.

If you make a correct assignment from an rvalue to an lvalue reference in the move constructor, you can avoid unnecessary copying of data from the memory area pointed to by the rvalue reference to the area pointed to by the lvalue reference. With an increase in the amount of data in the class (for example, large amounts of information), the effect of using the move constructor will increase.

The following actions are performed in the move constructor:

internal pointers should be redirected to external data, which should be copied into the internal fields of the class. In other words, internal pointers receive the values of addresses of memory areas containing data received from outside (see example below).

The purpose of using the move operator is the same as that of the move constructor – to speed up program execution by avoiding direct copying of data when assigning so-called rvalue references, which are used in expressions on the right side of the assignment operator.

If a move operator is declared in a class, then it is called in cases when an instance of a class is obtained in the assignment operator ( = ), which is the result of a return from another function.

here SomeFunc() is some function that returns the instance of the class ClassName .

If the class does not implement the move operator, then this operator is replaced by the copy operator.

The general form of declaring a move operator in a class:

The move operator has more sequence of actions than the copy constructor, namely:

For more details on the implementation of the move operator, see the example below.

6. An example of implementation of the Vector class (dynamic array). Basic set of methods. The move constructor and the move operator in the class

The example demonstrates the declaration and use of the Vector class, which implements a dynamic array of type double* . The class uses a basic set of special class functions and methods for demonstration purposes. These functions ensure the correct functioning of class instances (memory allocation, deallocation, exception handling, etc.). Optionally, you can reprogram this class to a one-dimensional array for the generic type T .

You can also extend the class by adding new methods that operate on an array. For example, you can add methods for reversing an array, concatenating, accessing array elements by index, etc.

Develop a class that is a dynamic array. In the class, form a minimum set of special functions for organizing work with an array.

6.2. Solution

The class contains the following components:

The above list is the basic (minimum) set of functions to ensure the correct functioning of the class. Optionally, this set can be expanded with additional functions.

A dynamic array of elements of type double is declared with double* . The number of elements in the array is count. After entering these variables, the class looks like this

In our case, it is agreed that the check for the presence of an empty array is carried out based on the value of count . If count> 0 , then the array is not empty. In all other cases, the array is considered empty. The control over the filling of the array lies entirely with the count variable.

In different methods of the class, the program code will be repeated. The basic operations commonly used on an array are:

Therefore, it is advisable to implement the corresponding internal private-functions Free() and CopyArray() in the class. After entering the functions, the program code of the class is as follows:

The Free() function frees the memory allocated for the array A . This function will be called from other functions in cases when it is necessary to reallocate memory or free memory.

In the Free() function, the fact of the presence of parts in the array is checked by the count variable ( count==0 ). Therefore, in the case of an empty array, you do not need to assign nullptr to the pointer A every time.

When designing classes, a different number of constructors can be used to initialize internal data. The first is the constructor that initializes the array to cover the largest number of internal fields of the class. In our case, the following constructor with two parameters is introduced into the public section:

The constructor uses the internal CopyArray() function to copy data to the internal array A .

After declaring a parameterized constructor, a destructor must be declared that calls the internal function Free() .

Another constructor that can be used to create an empty array is the parameterless constructor. This constructor delegates its authority to the constructor with two parameters. The constructor is introduced in the public section.

This is the only time the program uses nullptr to assign a value to pointer A . In all other cases, you do not need to set pointer A to nullptr , since control over the presence of an empty array is entirely in the variable count .

Since our class uses dynamic memory allocation for internal data, it is imperative to use the copy constructor to avoid the disadvantages of bitwise copying. More details about this are described here and here .

In our case, the copy constructor code is extremely simple. The CopyArray() function is called to do all the necessary work.

The copy operator must be implemented in cases where the class uses dynamic memory allocation. In our case, the copy operator code contains a call to the CopyArray() function.

The transfer constructor code does not perform any memory operations (memory allocation, deallocation, etc.).

In the move constructor, the most important line is

This action is required, because when assigning pointers

we get a situation that both pointers ( A and obj.A ) point to the same memory area. In the case of freeing memory for pointers, the same memory area will be freed twice, and this will result in an exception being thrown. To avoid this, the number of elements in the temporary object obj.count is set to 0. When calling the function Free() , which frees memory, count is checked for a nonzero value; if count == 0 , then the memory is not freed, and, therefore, unnecessary (unnecessary) memory will not be freed.

Since our class has a dynamic array that can have an arbitrary number of elements, it is recommended to declare a move operator in it.

For demonstration purposes, the class implements the Set() method, which makes a copy of the external array, which is an input parameter, into the internal array.

It would be wrong to assign like

since two arrays (external and internal) will point to the same memory location, which in some situations can lead to subtle errors.

To get the current state of the class, the Print() method is introduced.

After completing clauses 4.2.2 – 4.2.13, the Vector class in its abbreviated form will be as follows.

To demonstrate how to call the move constructor and the move operator outside the Vector class, the GetV() function is introduced, which forms an arbitrary array and returns an instance of the Vector type.

The test of the Vector class is done in the main() function.

In the most general case, the program structure has the form

After combining all the above code snippets, running the program will produce the following result

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COMMENTS

Assignment operators
The direct assignment operator expects a modifiable lvalue as its left operand and an rvalue expression or a braced-init-list (since C++11) as its right operand, and returns an lvalue identifying the left operand after modification. The result is a bit-field if the left operand is a bit-field.
C Assignment Operators
The assignment operators in C can both transform and assign values in a single operation. C provides the following assignment operators: In assignment, the type of the right-hand value is converted to the type of the left-hand value, and the value is stored in the left operand after the assignment has taken place.
c
Later on, section 6.5.16 "Assignment operators", point 3 states: The side effect of updating the stored value of the left operand is sequenced after the value computations of the left and right operands. The evaluations of the operands are unsequenced. ... this is not well defined in C11 although it is in C++11.
c++
The assignment operator ( operator=) is one of the implicitly generated functions for a struct or class in C++. Here is a reference describing the 4 implicitly generated members: http://www.cs.ucf.edu/~leavens/larchc++manual/lcpp_136.html In short, the implicitly generated member performs a memberwise shallow copy.
Assignment Operators in C/C++
Assignment operators are used to assigning value to a variable. The left side operand of the assignment operator is a variable and right side operand of the assignment operator is a value. The value on the right side must be of the same data-type of the variable on the left side otherwise the compiler will raise an error.
operator overloading
The assignment operator ( operator=) has special properties: see copy assignment and move assignment for details. The canonical copy-assignment operator is expected to perform no action on self-assignment, and to return the lhs by reference:
Assignment operators
The assignment operator = is right-associative, that is, an expression of the form C# a = b = c is evaluated as C# a = (b = c) The following example demonstrates the usage of the assignment operator with a local variable, a property, and an indexer element as its left-hand operand: C#
Assignment Operators in C
Line 1 - = Operator Example, Value of c = 21 Line 2 - += Operator Example, Value of c = 42 Line 3 - -= Operator Example, Value of c = 21 Line 4 - *= Operator Example, Value of c = 441 Line 5 - /= Operator Example, Value of c = 21 Line 6 - %= Operator Example, Value of c = 11 Line 7 - >= Operator Example, Value of c = 11 Line 9 - &= Operator …
Assignment Operators in C
An assignment operator is basically a binary operator that helps in modifying the variable to its left with the use of the value to its right. We utilize the assignment operators to transform and assign values to any variables. Here is a list of the assignment operators that you can find in the C language: basic assignment ( = )
C++ Assignment Operators
Assignment operators are used to assign values to variables. In the example below, we use the assignment operator ( =) to assign the value 10 to a variable called x: Example int x = 10; Try it Yourself » The addition assignment operator ( +=) adds a value to a variable: Example int x = 10; x += 5; Try it Yourself »
C++11 Tutorial: Introducing the Move Constructor and the Move
A move assignment operator has the following signature: C& C::operator=(C&& other);//C++11 move assignment operator. A move assignment operator is similar to a copy constructor except that before pilfering the source object, it releases any resources that its object may own. The move assignment operator performs four logical steps:
Operators in C
C Increment and Decrement Operators. C programming has two operators increment ++ and decrement -- to change the value of an operand (constant or variable) by 1. Increment ++ increases the value by 1 whereas decrement -- decreases the value by 1. These two operators are unary operators, meaning they only operate on a single operand.
Assignment operator (C++)
In the C++ programming language, the assignment operator, =, is the operator used for assignment.Like most other operators in C++, it can be overloaded.. The copy assignment operator, often just called the "assignment operator", is a special case of assignment operator where the source (right-hand side) and destination (left-hand side) are of the same class type.
Different List of Assignment Operators in C
Introduction to Assignment Operators in C. Assignment operators are used for assigning value to the variable. Like any other operator, C also supports Assignment Operator which is a binary operator that operates on any two operands. It has 2 values such as the right value and the left value. ... Example #11. Program to use Bitwise inclusive OR ...
Assignment Operators in C Example
The Assignment operators in C are some of the Programming operators that are useful for assigning the values to the declared variables. Equals (=) operator is the most commonly used assignment operator. For example: int i = 10; The below table displays all the assignment operators present in C Programming with an example.
std::move in Utility in C++
The move constructor was introduced in C++11.The need or purpose of a move constructor is to steal or move as many resources as it can from the source (original) object, as fast as possible, because the source does not need to have a meaningful value anymore, and/or because it is going to be destroyed in a moment anyway.So that one can avoid unnecessarily creating copies of an object and make ...
Assignment Operators in C++
We have to be sure that both the left and right sides of the operator must have the same data type. We have different levels of operators. Let us learn about each of those in C++. Assignment operators are a part of binary operators. Examples for these are: =, +=, -=, *=, /=, %=. Let us learn about each of these with examples.
C++ Operators
In programming, an operator is a symbol that operates on a value or a variable. Operators are symbols that perform operations on variables and values. For example, + is an operator used for addition, while - is an operator used for subtraction. Operators in C++ can be classified into 6 types: Arithmetic Operators. Assignment Operators.
C++ 我们需要移动和复制任务吗_C++_C++11_Move Semantics_Assignment Operator
C++ 我们需要移动和复制任务吗,c++,c++11,move-semantics,assignment-operator,C++,C++11,Move Semantics,Assignment Operator
C++. Move constructor and move operator
The move constructor and the move operator were added in C++ 11. The main idea behind using these two constructs is to speed up program execution by avoiding copying data during initial initialization and assignment of so-called rvalue references. ... The obj instance is located on the left side of the assignment operator and is an lvalue ...