assignment operator overloading

21.12 — Overloading the assignment operator

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

Related content

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

Copy assignment vs Copy constructor

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

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

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

Overloading the assignment operator

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

This prints:

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

Issues due to self-assignment

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

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

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

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

Now run the following program:

You’ll probably get garbage output. What happened?

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

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

Detecting and handling self-assignment

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

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

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

When not to handle self-assignment

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

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

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

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

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

The copy and swap idiom

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

The implicit copy assignment operator

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

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

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

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

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Assignment Operators Overloading in C++

You can overload the assignment operator (=) just as you can other operators and it can be used to create an object just like the copy constructor.

Following example explains how an assignment operator can be overloaded.

When the above code is compiled and executed, it produces the following result −

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How to Implement Assignment Operator Overloading in C++

• How to Implement Assignment Operator …

This article will explain several methods of how to implement assignment operator overloading in C++.

Use copy-assignment operator to Implement Overloaded Assignment Operator in C++

C++ provides the feature to overload operators, a common way to call custom functions when a built-in operator is called on specific classes. These functions should have a special name starting with operator followed by the specific operator symbol itself. E.g., a custom assignment operator can be implemented with the function named operator= . Assignment operator generally should return a reference to its left-hand operand. Note that if the user does not explicitly define the copy assignment operator, the compiler generates one automatically. The generated version is quite capable when the class does not contain any data members manually allocated on the heap memory. It can even handle the array members by assigning each element to the corresponding object members. Although, it has shortcomings when dealing with dynamic memory data members, as shown in the following example code.

The above code defines only copy-constructor explicitly, which results in incorrect behavior when P1 object contents are assigned to the P3 object. Note that the second call to the P1.renamePerson function should not have modified the P3 object’s data members, but it did. The solution to this is to define an overloaded assignment operator i.e., copy-assignment operator. The next code snippet implements the version of the Person class that can copy assign the two objects of the same class correctly. Notice, though, the if statement in the copy-assignment function guarantees that the operator works correctly even when the object is assigned to itself.

Founder of DelftStack.com. Jinku has worked in the robotics and automotive industries for over 8 years. He sharpened his coding skills when he needed to do the automatic testing, data collection from remote servers and report creation from the endurance test. He is from an electrical/electronics engineering background but has expanded his interest to embedded electronics, embedded programming and front-/back-end programming.

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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 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

• The operators :: (scope resolution), . (member access), .* (member access through pointer to member), and ?: (ternary conditional) cannot be overloaded
• New operators such as ** , <> , or &| cannot be created
• The overloads of operators && , || , and , (comma) lose their special properties: short-circuit evaluation and sequencing .

[ edit ] Canonical implementations

The language puts no restrictions 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. To summarize, the canonical "universal assignment operator" implementation is

When there are resources that can be reused in assignment, for example, if the class owns a heap-allocated array, then copy-assignment between arrays of the same size can avoid allocation and deallocation:

[ 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. Many standard algorithms, from std:: sort to std:: accumulate accept objects of such types 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 and decrement appear 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 ) , where the argument is ignored. The postfix increment and decrement operator is usually implemented in terms of the prefix version:

Although canonical form of preincrement/predecrement returns a 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 ] Relational 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. Typically, operator < is provided and the other relational operators are implemented in terms of operator < .

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

[ edit ] Array subscript operator

User-defined classes that provide array-like access typically define two overloads for operator [ ] : one which can be called in rvalue context such as x = a [ i ] ; , and one which can be used in lvalue context, such as a [ i ] = x ; .

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 ] Example

[ edit ] see also.

• Operator precedence
• Alternative operator syntax

[ edit ] References

• ↑ Operator Overloading on StackOverflow C++ FAQ

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In C++, we can define how operators behave for user-defined types like class and structures For example,

The + operator, when used with values of type int , returns their sum. However, when used with objects of a user-defined type, it is an error.

In this case, we can define the behavior of the + operator to work with objects as well.

This concept of defining operators to work with objects and structure variables is known as operator overloading .

• Syntax for C++ Operator Overloading

The syntax for overloading an operator is similar to that of function with the addition of the operator keyword followed by the operator symbol.

• returnType - the return type of the function
• operator - a special keyword
• symbol - the operator we want to overload ( + , < , - , ++ , etc.)
• arguments - the arguments passed to the function
• Overloading the Binary + Operator

Following is a program to demonstrate the overloading of the + operator for the class Complex .

Here, we first created a friend function with a return type Complex .

The operator keyword followed by + indicates that we are overloading the + operator.

The function takes two arguments:

• Complex& indicates that we are passing objects by reference and obj1 and obj2 are references to Complex objects. This is an efficient approach because it avoids unnecessary copying, especially for large objects. To learn more, visit C++ References .
• const indicates that referenced objects are constant, meaning we cannot modify obj1 and obj2 within the function.

Inside the function, we created another Complex object, temp to store the result of addition.

We then add the real parts of two objects and store it into the real attribute of the temp object.

Similarly, we add the imaginary parts of the two objects and store them into the img attribute of the temp object.

At last, we return the temp object from the function.

We can also overload the operators using a member function instead of a friend function. For example,

In this case, the operator is invoked by the first operand. Meaning, the line

translates to

Here, the number of arguments to the operator function is reduced by one because the first argument is used to invoke the function.

The problem with this approach is, not all the time the first operand is an object of a user-defined type. For example:

That's why it is recommended to overload operator functions as a non-member function generally, defined as a friend function.

• Overloading ++ as a Prefix Operator

Following is a program to demonstrate the overloading of the ++ operator for the class Count .

Here, when we use ++count1; , the void operator ++ () is called. This increases the value attribute for the object count1 by 1.

Note : When we overload operators, we can use it to work in any way we like. For example, we could have used ++ to increase value by 100.

However, this makes our code confusing and difficult to understand. It's our job as a programmer to use operator overloading properly and in a consistent and intuitive way.

To overload the ++ operator as a Postfix Operator, we declare a member function operator++() with one argument having type int .

Here, when we use count1++; , the void operator ++ (int) is called. This increments the value attribute for the object count1 by 1.

Note : The argument int is just to indicate that the operator is used as a postfix operator.

• Things to Remember in C++ Operator Overloading

1. By default, operators = and & are already overloaded in C++. For example,

we can directly use the = operator to copy objects of the same class. Here, we do not need to create an operator function.

2. We cannot change the precedence and associativity of operators using operator overloading.

3. We cannot overload following operators in C++:

• :: (scope resolution)
• . (member selection)
• .* (member selection through pointer to function)
• ?: (ternary operator)
• sizeof operator
• typeid Operator

4. We cannot overload operators for fundamental data types like int , float , etc

• How to overload increment operator in right way?
• How to overload binary operator - to subtract complex numbers?
• Increment ++ and Decrement -- Operators

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Operator overloading.

Operators play an important role in computer programming. By using it, you can change the behavior of an operator based on the types of its arguments.

Operator overloading  (less commonly known as ad-hoc polymorphism) is a specific case of polymorphism (part of the OO nature of the language) in which some or all operators like  + ,  =  or  ==  are treated as polymorphic functions and as such have different behaviors depending on the types of its arguments. Operator overloading is usually only syntactic sugar. It can easily be emulated using function calls.

Consider this operation:

Using operator overloading permits a more concise way of writing it, like this:

(Assuming the  *  operator has higher precedence than  + .)

Operator overloading can provide more than an aesthetic benefit, since the language allows operators to be invoked implicitly in some circumstances. Problems, and critics, to the use of operator overloading arise because it allows programmers to give operators completely free functionality, without an imposition of coherency that permits to consistently satisfy user/reader expectations. Usage of the  <<  operator is an example of this problem.

Will return twice the value of  a  if  a  is an integer variable, but if  a  is an output stream instead this will write "1" to it. Because operator overloading allows the programmer to change the usual semantics of an operator, it is usually considered good practice to use operator overloading with care.

To overload an operator is to provide it with a new meaning for user-defined types. This is done in the same fashion as defining a function. The basic syntax follows (where @ represents a valid operator):

Not all operators may be overloaded, new operators cannot be created, and the precedence, associativity or arity of operators cannot be changed (for example   ! cannot be overloaded as a binary operator). Most operators may be overloaded as either a member function or non-member function, some, however, must be defined as member functions. Operators should only be overloaded where their use would be natural and unambiguous, and they should perform as expected. For example, overloading + to add two complex numbers is a good use, whereas overloading * to push an object onto a vector would not be considered good style.

A simple Message Header

Operators as member functions

Aside from the operators which must be members, operators may be overloaded as member or non-member functions. The choice of whether or not to overload as a member is up to the programmer. Operators are generally overloaded as members when they:

• change the left-hand operand, or
• require direct access to the non-public parts of an object.

When an operator is defined as a member, the number of explicit parameters is reduced by one, as the calling object is implicitly supplied as an operand. Thus, binary operators take one explicit parameter and unary operators none. In the case of binary operators, the left hand operand is the calling object, and no type coercion will be done upon it. This is in contrast to non-member operators, where the left hand operand may be coerced.

Overloadable operators

Arithmetic operators.

• +  (addition)
• -  (subtraction)
• *  (multiplication)
• /  (division)
• %  (modulus)

As binary operators, these involve two arguments which do not have to be the same type. These operators may be defined as member or non-member functions. An example illustrating overloading for the addition of a 2D mathematical vector type follows.

It is good style to only overload these operators to perform their customary arithmetic operation. Because operator has been overloaded as member function, it can access private fields.

Bitwise operators

• ^  (XOR)
• |  (OR)
• &  (AND)
• ~  (complement)
• <<  (shift left, insertion to stream)
• >>  (shift right, extraction from stream)

All of the bitwise operators are binary, except complement, which is unary. It should be noted that these operators have a lower precedence than the arithmetic operators, so if ^ were to be overloaded for exponentiation, x ^ y + z may not work as intended. Of special mention are the shift operators, << and >> . These have been overloaded in the standard library for interaction with streams. When overloading these operators to work with streams the rules below should be followed:

• overload << and >> as friends (so that it can access the private variables with the stream be passed in by references
• (input/output modifies the stream, and copying is not allowed)
• the operator should return a reference to the stream it receives (to allow chaining, cout << 3 << 4 << 5 )

Assignment operator

The assignment operator,  = ,  must be a member function , and is given default behavior for user-defined classes by the compiler, performing an assignment of every member using its assignment operator. This behavior is generally acceptable for simple classes which only contain variables. However, where a class contains references or pointers to outside resources, the assignment operator should be overloaded (as general rule, whenever a destructor and copy constructor are needed so is the assignment operator), otherwise, for example, two strings would share the same buffer and changing one would change the other.

In this case, an assignment operator should perform two duties:

• clean up the old contents of the object
• copy the resources of the other object

For classes which contain raw pointers, before doing the assignment, the assignment operator should check for self-assignment, which generally will not work (as when the old contents of the object are erased, they cannot be copied to refill the object). Self assignment is generally a sign of a coding error, and thus for classes without raw pointers, this check is often omitted, as while the action is wasteful of cpu cycles, it has no other effect on the code.

Another common use of overloading the assignment operator is to declare the overload in the private part of the class and not define it. Thus any code which attempts to do an assignment will fail on two accounts, first by referencing a private member function and second fail to link by not having a valid definition. This is done for classes where copying is to be prevented, and generally done with the addition of a privately declared copy constructor

Relational operators

• ==  (equality)
• !=  (inequality)
• >  (greater-than)
• <  (less-than)
• >=  (greater-than-or-equal-to)
• <=  (less-than-or-equal-to)

All relational operators are binary, and should return either true or false. Generally, all six operators can be based off a comparison function, or each other, although this is never done automatically (e.g. overloading > will not automatically overload < to give the opposite). There are, however, some templates defined in the header <utility> ; if this header is included, then it suffices to just overload operator == and operator < , and the other operators will be provided by the STL.

Logical operators

• !  (NOT)
• &&  (AND)
• ||  (OR)

The logical operators AND are used when evaluating two expressions to obtain a single relational result.The operator corresponds to the boolean logical operation AND, which yields true if operands are true, and false otherwise.The following panel shows the result of operator evaluating the expression.

The  ! operator is unary, && and || are binary. It should be noted that in normal use, && and || have "short-circuit" behavior, where the right operand may not be evaluated, depending on the left operand. When overloaded, these operators get function call precedence, and this short circuit behavior is lost. It is best to leave these operators alone.

If the result of Function1 () is false, then Function2 () is not called.

Both Function3 () and Function4 () will be called no matter what the result of the call is to Function3 () . This is a waste of CPU processing, and worse, it could have surprising unintended consequences compared to the expected "short-circuit" behavior of the default operators. Consider:

Compound assignment operators

• +=  (addition-assignment)
• -=  (subtraction-assignment)
• *=  (multiplication-assignment)
• /=  (division-assignment)
• %=  (modulus-assignment)
• &=  (AND-assignment)
• |=  (OR-assignment)
• ^=  (XOR-assignment)
• <<=  (shift-left-assignment)
• >>=  (shift-right-assignment)

Compound assignment operators should be overloaded as member functions, as they change the left-hand operand. Like all other operators (except basic assignment), compound assignment operators must be explicitly defined, they will not be automatically (e.g. overloading = and + will not automatically overload += ). A compound assignment operator should work as expected: A @= B should be equivalent to A = A @ B . An example of += for a two-dimensional mathematical vector type follows.

Increment and decrement operators

• ++  (increment)
• --  (decrement)

Increment and decrement have two forms, prefix ( ++ i ) and postfix ( i ++ ). To differentiate, the postfix version takes a dummy integer. Increment and decrement operators are most often member functions, as they generally need access to the private member data in the class. The prefix version in general should return a reference to the changed object. The postfix version should just return a copy of the original value. In a perfect world, A += 1 , A = A + 1 , A ++ , ++ A should all leave A with the same value.

Often one operator is defined in terms of the other for ease in maintenance, especially if the function call is complex.

Subscript operator

The subscript operator,  [ ] , is a binary operator which  must be a member function  (hence it takes only one explicit parameter, the index). The subscript operator is not limited to taking an integral index. For instance, the index for the subscript operator for the std::map template is the same as the type of the key, so it may be a string etc. The subscript operator is generally overloaded twice; as a non-constant function (for when elements are altered), and as a constant function (for when elements are only accessed).

Function call operator

The function call operator,  ( ) , is generally overloaded to create objects which behave like functions, or for classes that have a primary operation. The function call operator must be a member function, but has no other restrictions - it may be overloaded with any number of parameters of any type, and may return any type. A class may also have several definitions for the function call operator.

Address of, Reference, and Pointer operators

These three operators, operator &() , operator *() and operator ->() can be overloaded. In general these operators are only overloaded for smart pointers, or classes which attempt to mimic the behavior of a raw pointer. The pointer operator, operator->() has the additional requirement that the result of the call to that operator, must return a pointer, or a class with an overloaded operator ->(). In general A == *& A should be true.

Note that overloading operator& invokes undefined behavior:

These are extremely simplified examples designed to show how the operators can be overloaded and not the full details of a SmartPointer or SmartReference class. In general you won't want to overload all three of these operators in the same class.

Comma operator

The comma operator ,()  ,  can be overloaded. The language comma operator has left to right precedence, the operator ,() has function call precedence, so be aware that overloading the comma operator has many pitfalls.

For non overloaded comma operator, the order of execution will be Function1(), Function2(); With the overloaded comma operator, the compiler can call either Function1(), or Function2() first.

Member Reference operators

The two member access operators, operator ->() and operator ->*() can be overloaded. The most common use of overloading these operators is with defining expression template classes, which is not a common programming technique. Clearly by overloading these operators you can create some very unmaintainable code so overload these operators only with great care.

When the -> operator is applied to a pointer value of type (T *), the language dereferences the pointer and applies the . member access operator (so x -> m is equivalent to (* x ). m ). However, when the -> operator is applied to a class instance, it is called as a unary postfix operator; it is expected to return a value to which the -> operator can again be applied. Typically, this will be a value of type (T *), as in the example under Address of, Reference, and Pointer operators above, but can also be a class instance with operator ->() defined; the language will call operator ->() as many times as necessary until it arrives at a value of type (T *).

Memory management operators

• new  (allocate memory for object)
• new [ ]  (allocate memory for array)
• delete  (deallocate memory for object)
• delete [ ]  (deallocate memory for array)

The memory management operators can be overloaded to customize allocation and deallocation (e.g. to insert pertinent memory headers). They should behave as expected,  new  should return a pointer to a newly allocated object on the heap,  delete  should deallocate memory, ignoring a NULL argument. To overload  new , several rules must be followed:

• new  must be a member function
• the return type must be  void *
• the first explicit parameter must be a  size_t  value

To overload  delete  there are also conditions:

• delete  must be a member function (and cannot be virtual)
• the return type must be  void
• void *, size_t

Conversion operators

Conversion operators enable objects of a class to be either implicitly (coercion) or explicitly (casting) converted to another type. Conversion operators must be member functions, and should not change the object which is being converted, so should be flagged as constant functions. The basic syntax of a conversion operator declaration, and declaration for an int-conversion operator follows.

Notice that the function is declared without a return-type, which can easily be inferred from the type of conversion. Including the return type in the function header for a conversion operator is a syntax error.

Operators which cannot be overloaded

• ?:  (conditional)
• .  (member selection)
• .*  (member selection with pointer-to-member)
• ::  (scope resolution)
• sizeof  (object size information)
• typeid  (object type information)
• static_cast (casting operator)
• const_cast (casting operator)
• reinterpret_cast (casting operator)
• dynamic_cast (casting operator)

To understand the reasons why the language doesn't permit these operators to be overloaded, read "Why can't I overload dot, ::,  sizeof , etc.?" at the Bjarne Stroustrup's C++ Style and Technique FAQ ( http://www.stroustrup.com/bs_faq2.html#overload-dot ).

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Copy assignment operator.

A copy assignment operator is a non-template non-static member function with the name operator = that can be called with an argument of the same class type and copies the content of the argument without mutating the argument.

[ edit ] Syntax

For the formal copy assignment operator syntax, see function declaration . The syntax list below only demonstrates a subset of all valid copy assignment operator syntaxes.

[ edit ] Explanation

The copy assignment operator is called whenever selected by overload resolution , e.g. when an object appears on the left side of an assignment expression.

[ edit ] Implicitly-declared copy assignment operator

If no user-defined copy assignment operators are provided for a class type, the compiler will always declare one as an inline public member of the class. This implicitly-declared copy assignment operator has the form T & T :: operator = ( const T & ) if all of the following is true:

• each direct base B of T has a copy assignment operator whose parameters are B or 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 assignment operator whose parameters are M or const M & or const volatile M & .

Otherwise the implicitly-declared copy assignment operator is declared as T & T :: operator = ( T & ) .

Due to these rules, the implicitly-declared copy assignment operator cannot bind to a volatile lvalue argument.

A class can have multiple copy assignment operators, e.g. both T & T :: operator = ( T & ) and T & T :: operator = ( T ) . If some user-defined copy assignment operators are present, the user may still force the generation of the implicitly declared copy assignment operator with the keyword default . (since C++11)

The implicitly-declared (or defaulted on its first declaration) copy assignment operator has an exception specification as described in dynamic exception specification (until C++17) noexcept specification (since C++17)

Because the copy assignment operator is always declared for any class, the base class assignment operator is always hidden. If a using-declaration is used to bring in the assignment operator from the base class, and its argument type could be the same as the argument type of the implicit assignment operator of the derived class, the using-declaration is also hidden by the implicit declaration.

[ edit ] Implicitly-defined copy assignment operator

If the implicitly-declared copy assignment operator is neither deleted nor trivial, 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++14) . For union types, the implicitly-defined copy assignment copies the object representation (as by std::memmove ). For non-union class types, the operator performs member-wise copy assignment of the object's direct bases and non-static data members, in their initialization order, using built-in assignment for the scalars, memberwise copy-assignment for arrays, and copy assignment operator for class types (called non-virtually).

[ edit ] Deleted copy assignment operator

An implicitly-declared or explicitly-defaulted (since C++11) copy assignment operator 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 non-static data member of a const-qualified non-class type (or possibly multi-dimensional array thereof).
• T has a non-static data member of a reference type.
• T has a potentially constructed subobject of class type M (or possibly multi-dimensional array thereof) such that the overload resolution as applied to find M 's copy assignment operator
• 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 assignment operator

The copy assignment operator for class T is trivial if all of the following is true:

• it is not user-provided (meaning, it is implicitly-defined or defaulted);
• T has no virtual member functions;
• T has no virtual base classes;
• the copy assignment operator selected for every direct base of T is trivial;
• the copy assignment operator selected for every non-static class type (or array of class type) member of T is trivial.

A trivial copy assignment operator makes a copy of the object representation as if by std::memmove . All data types compatible with the C language (POD types) are trivially copy-assignable.

[ edit ] Eligible copy assignment operator

Triviality of eligible copy assignment operators determines whether the class is a trivially copyable type .

[ edit ] Notes

If both copy and move assignment operators are provided, overload resolution selects the move assignment if the argument is an rvalue (either a prvalue such as a nameless temporary or an xvalue such as the result of std::move ), and selects the copy assignment if the argument is an lvalue (named object or a function/operator returning lvalue reference). If only the copy assignment is provided, all argument categories select it (as long as it takes its argument by value or as reference to const, since rvalues can bind to const references), which makes copy assignment the fallback for move assignment, when move is unavailable.

It is unspecified whether virtual base class subobjects that are accessible through more than one path in the inheritance lattice, are assigned more than once by the implicitly-defined copy assignment operator (same applies to move assignment ).

See assignment operator overloading for additional detail on the expected behavior of a user-defined copy-assignment operator.

[ 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 constructor
• 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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C++ Operator Overloading Guidelines

One of the nice features of C++ is that you can give special meanings to operators, when they are used with user-defined classes. This is called operator overloading . You can implement C++ operator overloads by providing special member-functions on your classes that follow a particular naming convention. For example, to overload the + operator for your class, you would provide a member-function named operator+ on your class.

• = (assignment operator)
• + - * (binary arithmetic operators)
• += -= *= (compound assignment operators)
• == != (comparison operators)

Here are some guidelines for implementing these operators. These guidelines are very important to follow, so definitely get in the habit early.

Assignment Operator =

The assignment operator has a signature like this: class MyClass { public: ... MyClass & operator=(const MyClass &rhs); ... } MyClass a, b; ... b = a; // Same as b.operator=(a);

Notice that the = operator takes a const-reference to the right hand side of the assignment. The reason for this should be obvious, since we don't want to change that value; we only want to change what's on the left hand side.

• e = 42 assigns 42 to e , then returns e as the result
• The value of e is then assigned to d , and then d is returned as the result
• The value of d is then assigned to c , and then c is returned as the result

Now, in order to support operator chaining, the assignment operator must return some value. The value that should be returned is a reference to the left-hand side of the assignment.

Notice that the returned reference is not declared const . This can be a bit confusing, because it allows you to write crazy stuff like this: MyClass a, b, c; ... (a = b) = c; // What?? At first glance, you might want to prevent situations like this, by having operator= return a const reference. However, statements like this will work with primitive types. And, even worse, some tools actually rely on this behavior. Therefore, it is important to return a non- const reference from your operator= . The rule of thumb is, "If it's good enough for int s, it's good enough for user-defined data-types."

So, for the hypothetical MyClass assignment operator, you would do something like this: // Take a const-reference to the right-hand side of the assignment. // Return a non-const reference to the left-hand side. MyClass& MyClass::operator=(const MyClass &rhs) { ... // Do the assignment operation! return *this; // Return a reference to myself. } Remember, this is a pointer to the object that the member function is being called on. Since a = b is treated as a.operator=(b) , you can see why it makes sense to return the object that the function is called on; object a is the left-hand side.

But, the member function needs to return a reference to the object, not a pointer to the object. So, it returns *this , which returns what this points at (i.e. the object), not the pointer itself. (In C++, instances are turned into references, and vice versa, pretty much automatically, so even though *this is an instance, C++ implicitly converts it into a reference to the instance.)

Now, one more very important point about the assignment operator:

YOU MUST CHECK FOR SELF-ASSIGNMENT!

This is especially important when your class does its own memory allocation. Here is why: The typical sequence of operations within an assignment operator is usually something like this: MyClass& MyClass::operator=(const MyClass &rhs) { // 1. Deallocate any memory that MyClass is using internally // 2. Allocate some memory to hold the contents of rhs // 3. Copy the values from rhs into this instance // 4. Return *this } Now, what happens when you do something like this: MyClass mc; ... mc = mc; // BLAMMO. You can hopefully see that this would wreak havoc on your program. Because mc is on the left-hand side and on the right-hand side, the first thing that happens is that mc releases any memory it holds internally. But, this is where the values were going to be copied from, since mc is also on the right-hand side! So, you can see that this completely messes up the rest of the assignment operator's internals.

The easy way to avoid this is to CHECK FOR SELF-ASSIGNMENT. There are many ways to answer the question, "Are these two instances the same?" But, for our purposes, just compare the two objects' addresses. If they are the same, then don't do assignment. If they are different, then do the assignment.

So, the correct and safe version of the MyClass assignment operator would be this: MyClass& MyClass::operator=(const MyClass &rhs) { // Check for self-assignment! if (this == &rhs) // Same object? return *this; // Yes, so skip assignment, and just return *this. ... // Deallocate, allocate new space, copy values... return *this; } Or, you can simplify this a bit by doing: MyClass& MyClass::operator=(const MyClass &rhs) { // Only do assignment if RHS is a different object from this. if (this != &rhs) { ... // Deallocate, allocate new space, copy values... } return *this; } Remember that in the comparison, this is a pointer to the object being called, and &rhs is a pointer to the object being passed in as the argument. So, you can see that we avoid the dangers of self-assignment with this check.

• Take a const-reference for the argument (the right-hand side of the assignment).
• Return a reference to the left-hand side, to support safe and reasonable operator chaining. (Do this by returning *this .)
• Check for self-assignment, by comparing the pointers ( this to &rhs ).

Compound Assignment Operators += -= *=

I discuss these before the arithmetic operators for a very specific reason, but we will get to that in a moment. The important point is that these are destructive operators, because they update or replace the values on the left-hand side of the assignment. So, you write: MyClass a, b; ... a += b; // Same as a.operator+=(b) In this case, the values within a are modified by the += operator.

How those values are modified isn't very important - obviously, what MyClass represents will dictate what these operators mean.

The member function signature for such an operator should be like this: MyClass & MyClass::operator+=(const MyClass &rhs) { ... } We have already covered the reason why rhs is a const-reference. And, the implementation of such an operation should also be straightforward.

But, you will notice that the operator returns a MyClass -reference, and a non-const one at that. This is so you can do things like this: MyClass mc; ... (mc += 5) += 3;

Don't ask me why somebody would want to do this, but just like the normal assignment operator, this is allowed by the primitive data types. Our user-defined datatypes should match the same general characteristics of the primitive data types when it comes to operators, to make sure that everything works as expected.

This is very straightforward to do. Just write your compound assignment operator implementation, and return *this at the end, just like for the regular assignment operator. So, you would end up with something like this: MyClass & MyClass::operator+=(const MyClass &rhs) { ... // Do the compound assignment work. return *this; }

As one last note, in general you should beware of self-assignment with compound assignment operators as well. Fortunately, none of the C++ track's labs require you to worry about this, but you should always give it some thought when you are working on your own classes.

Binary Arithmetic Operators + - *

The binary arithmetic operators are interesting because they don't modify either operand - they actually return a new value from the two arguments. You might think this is going to be an annoying bit of extra work, but here is the secret:

Define your binary arithmetic operators using your compound assignment operators.

There, I just saved you a bunch of time on your homeworks.

So, you have implemented your += operator, and now you want to implement the + operator. The function signature should be like this: // Add this instance's value to other, and return a new instance // with the result. const MyClass MyClass::operator+(const MyClass &other) const { MyClass result = *this; // Make a copy of myself. Same as MyClass result(*this); result += other; // Use += to add other to the copy. return result; // All done! } Simple!

Actually, this explicitly spells out all of the steps, and if you want, you can combine them all into a single statement, like so: // Add this instance's value to other, and return a new instance // with the result. const MyClass MyClass::operator+(const MyClass &other) const { return MyClass(*this) += other; } This creates an unnamed instance of MyClass , which is a copy of *this . Then, the += operator is called on the temporary value, and then returns it.

If that last statement doesn't make sense to you yet, then stick with the other way, which spells out all of the steps. But, if you understand exactly what is going on, then you can use that approach.

You will notice that the + operator returns a const instance, not a const reference. This is so that people can't write strange statements like this: MyClass a, b, c; ... (a + b) = c; // Wuh...? This statement would basically do nothing, but if the + operator returns a non- const value, it will compile! So, we want to return a const instance, so that such madness will not even be allowed to compile.

• Implement the compound assignment operators from scratch, and then define the binary arithmetic operators in terms of the corresponding compound assignment operators.
• Return a const instance, to prevent worthless and confusing assignment operations that shouldn't be allowed.

Comparison Operators == and !=

The comparison operators are very simple. Define == first, using a function signature like this: bool MyClass::operator==(const MyClass &other) const { ... // Compare the values, and return a bool result. } The internals are very obvious and straightforward, and the bool return-value is also very obvious.

The important point here is that the != operator can also be defined in terms of the == operator, and you should do this to save effort. You can do something like this: bool MyClass::operator!=(const MyClass &other) const { return !(*this == other); } That way you get to reuse the hard work you did on implementing your == operator. Also, your code is far less likely to exhibit inconsistencies between == and != , since one is implemented in terms of the other.

• C++ Data Types
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Copy Constructor vs Assignment Operator in C++

• How to Create Custom Assignment Operator in C++?
• Assignment Operators In C++
• Why copy constructor argument should be const in C++?
• Advanced C++ | Virtual Copy Constructor
• Move Assignment Operator in C++ 11
• Self assignment check in assignment operator
• Is assignment operator inherited?
• Copy Constructor in C++
• How to Implement Move Assignment Operator in C++?
• Default Assignment Operator and References in C++
• Can a constructor be private in C++ ?
• When is a Copy Constructor Called in C++?
• C++ Assignment Operator Overloading
• std::move in Utility in C++ | Move Semantics, Move Constructors and Move Assignment Operators
• C++ Interview questions based on constructors/ Destructors.
• Assignment Operators in C
• Copy Constructor in Python
• Copy Constructor in Java
• Constructors in Objective-C

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

Consider the following C++ program. 

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

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

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