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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.



Assignment operators.

Assignment operators modify the value of the object.

[ edit ] Definitions

Copy assignment replaces the contents of the object a with a copy of the contents of b ( b is not modified). For class types, this is performed in a special member function, described in copy assignment operator .

For non-class types, copy and move assignment are indistinguishable and are referred to as direct assignment .

Compound assignment replace the contents of the object a with the result of a binary operation between the previous value of a and the value of b .

[ edit ] Assignment operator syntax

The assignment expressions have the form

  • ↑ target-expr must have higher precedence than an assignment expression.
  • ↑ new-value cannot be a comma expression, because its precedence is lower.

[ edit ] Built-in simple assignment operator

For the built-in simple assignment, the object referred to by target-expr is modified by replacing its value with the result of new-value . target-expr must be a modifiable lvalue.

The result of a built-in simple assignment is an lvalue of the type of target-expr , referring to target-expr . If target-expr is a bit-field , the result is also a bit-field.

[ edit ] Assignment from an expression

If new-value is an expression, it is implicitly converted to the cv-unqualified type of target-expr . When target-expr is a bit-field that cannot represent the value of the expression, the resulting value of the bit-field is implementation-defined.

If target-expr and new-value identify overlapping objects, the behavior is undefined (unless the overlap is exact and the type is the same).

In overload resolution against user-defined operators , for every type T , the following function signatures participate in overload resolution:

For every enumeration or pointer to member type T , optionally volatile-qualified, the following function signature participates in overload resolution:

For every pair A1 and A2 , where A1 is an arithmetic type (optionally volatile-qualified) and A2 is a promoted arithmetic type, the following function signature participates in overload resolution:

[ edit ] Built-in compound assignment operator

The behavior of every built-in compound-assignment expression target-expr   op   =   new-value is exactly the same as the behavior of the expression target-expr   =   target-expr   op   new-value , except that target-expr is evaluated only once.

The requirements on target-expr and new-value of built-in simple assignment operators also apply. Furthermore:

  • For + = and - = , the type of target-expr must be an arithmetic type or a pointer to a (possibly cv-qualified) completely-defined object type .
  • For all other compound assignment operators, the type of target-expr must be an arithmetic type.

In overload resolution against user-defined operators , for every pair A1 and A2 , where A1 is an arithmetic type (optionally volatile-qualified) and A2 is a promoted arithmetic type, the following function signatures participate in overload resolution:

For every pair I1 and I2 , where I1 is an integral type (optionally volatile-qualified) and I2 is a promoted integral type, the following function signatures participate in overload resolution:

For every optionally cv-qualified object type T , the following function signatures participate in overload resolution:

[ edit ] Example

Possible output:

[ edit ] Defect reports

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

[ edit ] See also

Operator precedence

Operator overloading

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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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Overview of Operator Overloading


Until now, we have mostly performed manipulations on objects by calling member functions of the objects. This member function-call notation is cumbersome for certain kinds of classes (such as mathematical classes). Also, many common manipulations in C++ are typically performed with operators (e.g., input and output, comparisons, etc.).

We can use C++'s large set of built-in operators to specify common object manipulations. Enabling C++'s operators to work with objects is called operator overloading .

We have already used a couple of overloaded operators. For example, operator << is used for multiple purposes in C++: as the stream-insertion operator and as the left-shift operator. Similarly, >> is also overloaded; it is used both as the stream-extraction operator and as the right-shift operator.

Fundamentals of Operator Overloading

C++ enables the programmer to overload most operators to be sensitive to the context in which they are used. The compiler generates an appropriate function or member function call based on the operator's use.

To overload an operator, write a function definition; the function name must be the keyword operator followed by the symbol for the operator being overloaded.

To use an operator on class objects, that operator must be overloaded - with two exceptions. The assignment operator ( = ) may be used with two objects of the same class to perform a default memberwise assignment without overloading. The address operator ( & ) can can be used with objects of any class without overloading; it returns the address of the object in memory.

The point of operator overloading is to provide the same concise expressive power for user-defined data types that C++ provides with its rich collection of operators that work on built-in types.

Operator overloading is not automatic - the programmer must write operator overloading functions to perform the desired operations. Sometimes these functions are best made member functions; sometimes they are best as friend functions.

Restrictions on Operator Overloading

Here is the list of C++ operators that can be overloaded:

There are a handful of C++ operators that cannot be overloaded:

Some other restrictions that apply:

The precedence and associativity of an operator (i.e., whether the operator is applied right-to-left or left-to-right) cannot be changed by overloading. Parentheses can be used to force the order of evaluation of overloaded operators in an expression.

It is not possible to change the number of operands that an operator takes: overloaded unary operators remain unary operators, overloaded binary operators remain binary operators. C++'s only ternary operator, ?: , cannot be overloaded.

It is not possible to define new operator symbols for new operators; only the existing operators can be overloaded.

The meaning of how an operator works on built-in data types cannot be changed by overloading. The programmer cannot, for example, change the meaning of how + adds two integers. Operator overloading works only with objects of user-defined types (i.e. classes or structs) or with a mixture of an object of a user-defined type and and a built-in type.

There is no implicit overloading. Overloading an operator such as + does not automatically overload += .

Overloaded Operators as Standalone Functions vs. Member Functions

The C++ standard specifies that overloaded operators () , [] , -> and any assignment operator must be member functions of the class for which they are overloaded.

For operators other than those, overloaded operator functions can be member functions or standalone functions that are not part of a class.

When an operator function is implemented as a member, the leftmost operand must be a class object (or a reference to a class object) of the operator's class.

If the left operand of an overloaded operator is an object of a C++ standard library class or is not an object at all, then the operator function must be implemented as a standalone function. A standalone operator function is usually made a friend of the class to improve performance by avoiding the overhead of calls to accessor and mutator methods.

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 .
  • The overload of operator -> must either return a raw pointer or return an object (by reference or by value), for which operator -> is in turn overloaded.
  • It is not possible to change the precedence, grouping, or number of operands of operators.

[ edit ] Canonical implementations

Other than 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. 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 pre-increment/pre-decrement 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 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 built-in 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 l-value c [ i ] = v ; and r-value v = c [ i ] ; usage, operator[] may return a proxy. see for example std::bitset::operator[] .

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

CS107: C++ Programming

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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What Is C++ Overloading Operators And How To Use IT

Get ready to navigate the seas of C++ operator overloading. This essential tool for C++ developers can make your code more intuitive and easier to use. Learn the syntax, best practices, common pitfalls, and frequently asked questions in this comprehensive guide.


  • Operator overloading in C++ enables you to redefine the behavior of standard operators for custom data types, offering powerful customization and expressive code.
  • The article highlights the importance of const-correctness when overloading operators, ensuring the preservation of object states and preventing unintended side effects.
  • Understanding the concept of friend functions allows you to access private class members when overloading operators, enhancing encapsulation and maintainability.
  • Smart pointers play a pivotal role in safe memory management during operator overloading, reducing the risk of memory leaks and improving code robustness.

In the vast landscape of C++, operator overloading stands as a pillar of efficiency and elegance. This feature, when used judiciously, enables us to extend the logic of built-in types to user-defined types, creating code that's both expressive and intuitive. Yet, without the right guidance, it can also lead us into a labyrinth of complexity. Today, we're tackling this nuanced subject head-on.

an assignment operator might be overloaded to

Understanding Operator Overloading

Syntax of operator overloading, unary operator overloading, overloading binary operators, overloading assignment operators, overloading the stream operators, operator overloading best practices, common pitfalls and solutions, frequently asked questions.

Operator overloading is a crucial aspect of C++, offering an intuitive way to work with user-defined data types. In essence, this feature allows us to give operators additional meanings when applied to specific classes. Overloading operators effectively leads to syntactic sugar that mimics built-in type behaviors, thus improving code readability and efficiency.

Consider a simple class named 'Vector' that represents a 2D mathematical vector. Without operator overloading, adding two Vector objects might look like this:

However, wouldn't it be more intuitive to use the '+' operator like we do with primitive data types?

That's the primary purpose of operator overloading – to extend the language syntax to user-defined types, making them behave just like built-in types.

However, there are a few operators that cannot be overloaded, including scope (::), sizeOf, member selector (.), and member pointer selector (.*).

Remember that while overloading operators can improve code readability, it should be done wisely.

Let's explore the syntax for operator overloading in C++. Fundamentally, operator overloading is about defining new behaviors for existing operators when they're applied to objects of user-defined classes. We accomplish this by implementing an operator function that gets invoked when the corresponding operator is used.

For instance, if we want to overload the '+' operator for our Vector class, the overloaded function could look something like this:

In the function declaration above, the keyword 'operator' is followed by the symbol of the operator being overloaded ('+'). The function returns a new Vector whose components are the sums of the corresponding components of the two operand vectors.

However, there are cases when overloading as a non-member function is necessary or advantageous. This is particularly true when the left operand of a binary operator isn't an object of our class. For example, to support scalar multiplication with the scalar on the left (like 3 * v ), we'd have to overload the '*' operator as a non-member function:

Note: that the function definition is now outside the class, but still has access to its private members because it's declared as a friend inside the class.

Unary operators are those that act upon a single operand. Common unary operators in C++ include increment (++) and decrement (--), among others. Overloading unary operators in C++ follows a similar syntax and process to overloading binary operators.

For instance, let's assume we want to overload the increment operator (++) for our Vector class to increase both components by 1. The overloaded operator function might look like this:

In the function definition above, the 'operator' keyword is followed by the symbol of the operator being overloaded (++). This function increments the x and y components of the Vector and returns a reference to the vector itself, allowing for chaining operations .

The post-increment/decrement version is overloaded by adding an extra int parameter to the function signature. The int isn't used; it's only a marker distinguishing pre- and post-increment/decrement:

In the example above, the operator++ function takes an unused int parameter, indicating this is a post-increment operation. It first saves the current state, increments the components, and then returns the original state, following the semantics of post-increment.

Binary operators are those that act upon two operands. In C++, common binary operators include arithmetic (+, -, *, /), comparison (==, !=, >, <, >=, <=), and assignment (=), among others.

Overloading Comparison Operators

Overloading assignment operator.

Let's revisit our Vector class to illustrate overloading of the addition operator (+) as a binary operator:

In this example, the operator+ function receives a reference to another Vector and returns a new Vector that is the component-wise sum of the current Vector and the argument Vector. This is an instance of overloading binary operators .

Overloading comparison operators for custom classes can enhance readability and allow the use of these classes in algorithms and data structures that require comparisons.

As an example, we could overload the equality operator (==) for our Vector class as follows:

This implementation of operator== checks if both components of the two vectors are equal and returns the result.

The assignment operator (=) has a special role in C++, and overloading it requires careful consideration. This is particularly true for classes that manage resources, like dynamic memory. However, for simple classes like Vector, overloading can be straightforward:

In this code, we first check for self-assignment (v1 = v1), which could lead to problems in classes managing resources. We then copy the components from the right-hand side vector to the left-hand side vector.

When you write a class, if you don't define an assignment operator, C++ generates one for you. This default assignment operator performs a shallow copy, which might be incorrect for classes managing resources. Here's a simple example for our Vector class:

In this example, operator= copies the values from the input Vector into the current Vector and then returns a reference to the current Vector, allowing chain assignments like v1 = v2 = v3 .

When dealing with resource-managing classes, there's a crucial distinction between copy assignment and move assignment .

Copy assignment involves constructing a new object as a copy of an existing object, while move assignment involves stealing resources from a temporary object that's about to be destroyed (often referred to as an "rvalue").

Here's a simple implementation of copy assignment and move assignment operators for a hypothetical resource-managing class:

In this code, copy assignment makes a new copy of the resource, while move assignment transfers the existing resource, leaving the source object in a safe-to-destruct state.

Stream operators, namely the insertion operator (<<) and the extraction operator (>>), are commonly overloaded in C++ to enable easy output and input of user-defined types. They are typically overloaded for standard streams like std::cout and std::cin .

Overloading the insertion operator (<<) allows us to directly output the contents of an object. For example, for our Vector class, we might implement it as follows:

In this example, the operator<< function outputs the x and y components of the Vector in a specific format. It's declared as a friend function so it can access private members of Vector. It returns a reference to the output stream, allowing chaining of output operations .

Similarly, we can overload the extraction operator (>>) to input the contents of an object. It might look like this for our Vector class:

This function reads in two numbers from the input stream and assigns them to the x and y components of the Vector. Again, the function returns a reference to the input stream, enabling chained input operations .

Operator overloading, when done correctly, can make your C++ code cleaner, more intuitive, and easier to read. However, if used improperly, it can also make your code difficult to understand and debug. Here are some best practices for operator overloading .

Keep It Natural

Be consistent, return type matters, use friend function wisely, self-assignment, overload symmetric operators as non-members.

When overloading an operator, aim to maintain the intuitive meaning of the operator. For example, overloading the addition operator (+) for a Matrix class should result in matrix addition, not subtraction or multiplication.

Ensure consistency between related operators . If you overload the equality operator (==), it's usually a good idea to also overload the inequality operator (!=).

Pay attention to the return type of overloaded operators. For example, assignment and arithmetic operators usually return a reference to the object they are modifying, allowing chain operations.

The friend keyword can grant a function or another class access to a class's private and protected members. While necessary in some cases, such as overloading stream operators, be cautious of overusing it as it can break encapsulation .

Always handle self-assignment correctly in overloaded assignment operators. Failing to do so can lead to hard-to-detect bugs.

When overloading symmetric operators (like arithmetic operators), it's often beneficial to do it as non-member functions (typically as friend functions) to preserve symmetry .

This ensures that expressions like v1 + v2 and v2 + v1 are both valid, assuming v1 and v2 are Vector objects.


If we want users to like our software, we should design it to behave like a likable person.

Alan Cooper

Source: Techvify Software

Lastly , always remember: overload operators judiciously . Overloading an operator when the operation is counterintuitive or when it doesn't make the code easier to read or maintain can lead to confusing code.

While operator overloading can be a powerful tool in C++, it can also lead to some common pitfalls if not used correctly. Understanding these can help you avoid them and write more efficient and safer code.

Pitfall 1: Overloading The Wrong Operator

Pitfall 2: inconsistent operator overloads, pitfall 3: ignoring self-assignment, pitfall 4: misuse of friend keyword, pitfall 5: forgetting return types.

A common mistake is overloading an operator that does not intuitively match the intended operation. This can lead to code misinterpretation and bugs. The solution? Stick to the natural semantics of operators. For example, use '+' for addition or concatenation, not for subtraction or any other unintuitive operation.

When overloading relational operators (like == and !=), it's important to maintain consistency. If you overload one, overload the others too. Neglecting to do so may lead to unexpected results .

Not handling self-assignment in your overloaded assignment operator can lead to serious bugs . Always include a check to handle this scenario in your implementation.

Overuse of the friend keyword can lead to broken encapsulation and issues in large projects. Use it only when necessary, and prefer member functions whenever possible.

Incorrect or missing return types in overloaded operators can prevent chaining and lead to unexpected behavior . Always specify the return type in your overloaded operator to match the expected behavior of the operator.

When should I overload an operator?

You should consider overloading an operator when it will make your code more intuitive and easier to read and understand. For example, overloading the '+' operator for a Vector class to implement vector addition would be a good use of operator overloading.

Can operators be overloaded for primitive types?

No, operators can only be overloaded for user-defined types (like classes and structs). You cannot overload an operator for primitive types such as int, char, float, etc.

What does 'friend' keyword do in operator overloading?

The 'friend' keyword allows an external function to access the private and protected members of a class. This is useful when overloading certain operators, like the stream operators (<< and >>), that need to be implemented as non-member functions but still need access to private members of the class.

Can I overload an operator without making it a member function?

Yes, an operator can be overloaded as a non-member function using the 'friend' keyword. This is particularly useful for overloading operators where symmetry between the left and right operands is desirable, like arithmetic and comparison operators.

Let’s test your knowledge!

Which Operator Cannot Be Overloaded in C++?

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There are various ways to overload Operators in C++ by implementing any of the following types of functions:

1) Member Function

2) Non-Member Function

3) Friend Function

List of operators that can be overloaded are:  

Example 1: Overloading ++ Operator  

This function is called when the ++ operator operates on the object of the overload class (object i in this case). In the program, void operator ++ () operator function is defined (inside overload class). This function increments the value of count by 1 for i object. Example 2: Overloading ++ operator i.e. pre and post increment operator  

Example 3: Overloading [ ] operator  

Example 4 : Overloading -> operator  

List of operators that cannot be overloaded

1) Scope Resolution Operator  (::)    

2) Ternary or Conditional Operator (?:)   

3) Member Access or Dot operator  (.)    

4) Pointer-to-member Operator (.*)  

5) Object size Operator ( sizeof ) 

6) Object type Operator(typeid) 

7) static_cast (casting operator)

8) const_cast (casting operator)

9) reinterpret_cast (casting operator)

10) dynamic_cast (casting operator)

Example 5: Overloading this .(dot) operator Dot (.) operator can’t be overloaded, so it will generate an error. 

Output: Error

This program will generate an error. Similarly, the above operators will also generate an error, if overloaded.

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