Overload Assignment Operator Pythongb

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


Overloaded operators are functions with special function names:

op (1)
type (2)


suffix-identifier (5) (since C++11)
op - any of the following 38(until C++20)39(since C++20) operators:+-*/%^&|~!=<>+=-=*=/=%=^=&=|=<<>>>>=<<===!=<=>=<=>(since C++20)&&||++--,->*->()[]

1) overloaded operator;

2)user-defined conversion function;

3)allocation function;

4)deallocation function;

5)user-defined literal.

[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:

Expression As member function As non-member function Example
@a (a).operator@ ( ) operator@ (a) !std::cin calls std::cin.operator!()
a@b (a).operator@ (b) operator@ (a, b) std::cout<<42 calls std::cout.operator<<(42)
a=b (a).operator= (b) cannot be non-member std::string s; s ="abc"; calls s.operator=("abc")
a(b...) (a).operator()(b...) cannot be non-member std::random_device r;auto n = r(); calls r.operator()()
a[b] (a).operator[](b) cannot be non-member std::map<int, int> m; m[1]=2; calls m.operator[](1)
a-> (a).operator-> ( ) cannot be non-member auto p =std::make_unique<S>(); p->bar() calls p.operator->()
a@ (a).operator@ (0) operator@ (a, 0) std::vector<int>::iterator i = v.begin(); i++ calls i.operator++(0)

in this table, is a placeholder representing all matching operators: all prefix operators in @a, all postfix operators other than -> in a@, all infix operators other than = in a@b

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:


  • 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 lose short-circuit evaluation.
  • 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.
  • , , and (comma) lose their special sequencing properties when overloaded and behave like regular function calls even when they are used without function-call notation.
(until C++17)

[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.

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

The canonical move assignment is expected to leave the moved-from object in valid state (that is, a state with class invariants intact), and either do nothing or at least leave the object in a valid state on self-assignment, and return the lhs by reference to non-const, and be noexcept:

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

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

[edit]Stream extraction and insertion

The overloads of and 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 ( 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 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 . 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 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, and expect it to implement strict weak ordering (thus satisfying the concept). An idiomatic way to implement strict weak ordering for a structure is to use lexicographical comparison provided by std::tie:

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

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

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

All six relational operators are automatically generated by the compiler if the three-way comparison operator operator<=> is defined, and that operator, in turn, is generated by the compiler if it is defined as defaulted:

See default comparisons for details.

struct Record {std::string name;unsignedint floor;double weight;auto operator<=>(const Record&)=default;};// records can now be compared with ==, !=, <, <=, >, and >=
(since C++20)

[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 lvalue c[i]= v; and rvalue 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]Bitwise arithmetic operators

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

[edit]Boolean negation operator

The operator operator! is commonly overloaded by the user-defined classes that are intended to be used in boolean contexts. Such classes also provide a user-defined conversion function explicit operator bool() (see std::basic_ios for the standard library example), and the expected behavior of operator! is to return the value opposite of operator bool.

[edit]Rarely overloaded operators

The following operators are rarely overloaded:

  • The address-of operator, operator&. If the unary & is applied to an lvalue of incomplete type and the complete type declares an overloaded operator&, the behavior is undefined(until C++11) it is implementation-defined whether the overloaded operator is used(since C++11). Because this operator may be overloaded, generic libraries use std::addressof to obtain addresses of objects of user-defined types. The best known example of a canonical overloaded operator& is the Microsoft class CComPtr. An example of its use in EDSL can be found in boost.spirit.
  • The boolean logic operators, operator&& and operator||. Unlike the built-in versions, the overloads cannot implement short-circuit evaluation. Also unlike the built-in versions, they do not sequence their left operand before the right one.(until C++17) In the standard library, these operators are only overloaded for std::valarray.
  • The comma operator, operator,. Unlike the built-in version, the overloads do not sequence their left operand before the right one.(until C++17) Because this operator may be overloaded, generic libraries use expressions such as a,void(),b instead of a,b to sequence execution of expressions of user-defined types. The boost library uses in boost.assign, boost.spirit, and other libraries. The database access library SOCI also overloads .
  • The member access through pointer to member operator->*. There are no specific downsides to overloading this operator, but it is rarely used in practice. It was suggested that it could be part of smart pointer interface, and in fact is used in that capacity by actors in boost.phoenix. It is more common in EDSLs such as cpp.react.


Run this code


#include <iostream>   class Fraction {int gcd(int a, int b){return b ==0? a : gcd(b, a % b);}int n, d;public: Fraction(int n, int d =1): n(n/gcd(n, d)), d(d/gcd(n, d)){}int num()const{return n;}int den()const{return d;} Fraction& operator*=(const Fraction& rhs){int new_n = n * rhs.n/gcd(n * rhs.n, d * rhs.d); d = d * rhs.d/gcd(n * rhs.n, d * rhs.d); n = new_n;return*this;}};std::ostream& operator<<(std::ostream& out, const Fraction& f){return out << f.num()<<'/'<< f.den();}bool operator==(const Fraction& lhs, const Fraction& rhs){return lhs.num()== rhs.num()&& lhs.den()== rhs.den();}bool operator!=(const Fraction& lhs, const Fraction& rhs){return!(lhs == rhs);} Fraction operator*(Fraction lhs, const Fraction& rhs){return lhs *= rhs;}   int main(){ Fraction f1(3, 8), f2(1, 2), f3(10, 2);std::cout<< f1 <<" * "<< f2 <<" = "<< f1 * f2 <<'\n'<< f2 <<" * "<< f3 <<" = "<< f2 * f3 <<'\n'<<2<<" * "<< f1 <<" = "<<2* f1 <<'\n';}
3/8 * 1/2 = 3/16 1/2 * 5/1 = 5/2 2 * 3/8 = 3/4

[edit]Defect reports

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

DR Applied to Behavior as published Correct behavior
CWG 1458 C++11 taking address of incomplete type that overloads address-of was undefined behavior the behavior is only unspecified

[edit]See Also

Common operators
assignment increment
arithmetic logical comparison member

a = b
a += b
a -= b
a *= b
a /= b
a %= b
a &= b
a |= b
a ^= b
a <<= b
a >>= b


a + b
a - b
a * b
a / b
a % b
a & b
a | b
a ^ b
a << b
a >> b

a && b
a || b

a == b
a != b
a < b
a > b
a <= b
a >= b
a <=> b


a, b

Special operators

converts one type to another related type
converts within inheritance hierarchies
adds or removes cv qualifiers
converts type to unrelated type
C-style cast converts one type to another by a mix of , , and
creates objects with dynamic storage duration
destructs objects previously created by the new expression and releases obtained memory area
queries the size of a type
queries the size of a parameter pack(since C++11)
queries the type information of a type
checks if an expression can throw an exception (since C++11)
queries alignment requirements of a type (since C++11)


  1. ↑Operator Overloading on StackOverflow C++ FAQ
std::string str ="Hello, "; str.operator+=("world");// same as str += "world"; operator<<(operator<<(std::cout, str) , '\n');// same as std::cout << str << '\n';// (since C++17) except for sequencing
// assume the object holds reusable storage, such as a heap-allocated buffer mArray T& operator=(const T& other)// copy assignment{if(this !=&other){// self-assignment check expectedif(other.size!= size){// storage cannot be reused delete[] mArray;// destroy storage in this size =0; mArray = nullptr;// preserve invariants in case next line throws mArray = new int[other.size];// create storage in this size = other.size;}std::copy(other.mArray, other.mArray+ other.size, mArray);}return*this;}
T& operator=(T&& other)noexcept// move assignment{if(this !=&other){// no-op on self-move-assignment (delete[]/size=0 also ok) delete[] mArray;// delete this storage mArray =std::exchange(other.mArray, nullptr);// leave moved-from in valid state size =std::exchange(other.size, 0);}return*this;}
T& T::operator=(T arg)noexcept// copy/move constructor is called to construct arg{ swap(arg);// resources are exchanged between *this and argreturn*this;}// destructor of arg is called to release the resources formerly held by *this
std::ostream& operator<<(std::ostream& os, const T& obj){// write obj to streamreturn os;}std::istream& operator>>(std::istream& is, T& obj){// read obj from streamif(/* T could not be constructed */) is.setstate(std::ios::failbit);return is;}
struct Sum {int sum; Sum(): sum(0){}void operator()(int n){ sum += n;}}; Sum s =std::for_each(v.begin(), v.end(), Sum());
struct X { X& operator++(){// actual increment takes place herereturn*this;} X operator++(int){ X tmp(*this);// copy operator++();// pre-incrementreturn tmp;// return old value}};
class X {public: X& operator+=(const X& rhs)// compound assignment (does not need to be a member,{// but often is, to modify the private members)/* addition of rhs to *this takes place here */return*this;// return the result by reference}   // friends defined inside class body are inline and are hidden from non-ADL lookupfriend X operator+(X lhs, // passing lhs by value helps optimize chained a+b+cconst X& rhs)// otherwise, both parameters may be const references{ lhs += rhs;// reuse compound assignmentreturn lhs;// return the result by value (uses move constructor)}};
struct Record {std::string name;unsignedint floor;double weight;friendbool operator<(const Record& l, const Record& r){returnstd::tie(l.name, l.floor, l.weight)<std::tie(r.name, r.floor, r.weight);// keep the same order}};
inlinebool operator<(const X& lhs, const X& rhs){/* do actual comparison */}inlinebool operator>(const X& lhs, const X& rhs){return rhs < lhs;}inlinebool operator<=(const X& lhs, const X& rhs){return!(lhs > rhs);}inlinebool operator>=(const X& lhs, const X& rhs){return!(lhs < rhs);}
inlinebool operator==(const X& lhs, const X& rhs){/* do actual comparison */}inlinebool operator!=(const X& lhs, const X& rhs){return!(lhs == rhs);}
inlinebool operator==(const X& lhs, const X& rhs){return cmp(lhs,rhs)==0;}inlinebool operator!=(const X& lhs, const X& rhs){return cmp(lhs,rhs)!=0;}inlinebool operator<(const X& lhs, const X& rhs){return cmp(lhs,rhs)<0;}inlinebool operator>(const X& lhs, const X& rhs){return cmp(lhs,rhs)>0;}inlinebool operator<=(const X& lhs, const X& rhs){return cmp(lhs,rhs)<=0;}inlinebool operator>=(const X& lhs, const X& rhs){return cmp(lhs,rhs)>=0;}
struct T { value_t& operator[](std::size_t idx){return mVector[idx];}const value_t& operator[](std::size_t idx)const{return mVector[idx];}};

A move assignment operator of class is a non-template non-static member function with the name operator= that takes exactly one parameter of type T&&, const T&&, volatile T&&, or constvolatile T&&.


class_nameclass_name ( class_name ) (1) (since C++11)
class_nameclass_name ( class_name ) = default; (2) (since C++11)
class_nameclass_name ( class_name ) = delete; (3) (since C++11)


  1. Typical declaration of a move assignment operator.
  2. Forcing a move assignment operator to be generated by the compiler.
  3. Avoiding implicit move assignment.

The move assignment operator is called whenever it is selected by overload resolution, e.g. when an object appears on the left-hand side of an assignment expression, where the right-hand side is an rvalue of the same or implicitly convertible type.

Move assignment operators typically "steal" the resources held by the argument (e.g. pointers to dynamically-allocated objects, file descriptors, TCP sockets, I/O streams, running threads, etc.), rather than make copies of them, and leave the argument in some valid but otherwise indeterminate state. For example, move-assigning from a std::string or from a std::vector may result in the argument being left empty. This is not, however, a guarantee. A move assignment is less, not more restrictively defined than ordinary assignment; where ordinary assignment must leave two copies of data at completion, move assignment is required to leave only one.

[edit]Implicitly-declared move assignment operator

If no user-defined move assignment operators are provided for a class type (struct, class, or union), and all of the following is true:

  • there are no user-declared copy constructors;
  • there are no user-declared move constructors;
  • there are no user-declared copy assignment operators;
  • there are no user-declared destructors;
  • the implicitly-declared move assignment operator would not be defined as deleted,
(until C++14)

then the compiler will declare a move assignment operator as an member of its class with the signature .

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

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

Because some assignment operator (move or copy) 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]Deleted implicitly-declared move assignment operator

The implicitly-declared or defaulted move assignment operator for class is defined as deleted if any of the following is true:

  • has a non-static data member that is const;
  • has a non-static data member of a reference type;
  • has a non-static data member that cannot be move-assigned (has deleted, inaccessible, or ambiguous move assignment operator);
  • has direct or virtual base class that cannot be move-assigned (has deleted, inaccessible, or ambiguous move assignment operator);
  • has a non-static data member or a direct or virtual base without a move assignment operator that is not trivially copyable;
  • has a direct or indirect virtual base class.
(until C++14)

A deleted implicitly-declared move assignment operator is ignored by overload resolution.

(since C++14)

[edit]Trivial move assignment operator

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

  • It is not user-provided (meaning, it is implicitly-defined or defaulted);
  • has no virtual member functions;
  • has no virtual base classes;
  • the move assignment operator selected for every direct base of is trivial;
  • the move assignment operator selected for every non-static class type (or array of class type) member of is trivial;
  • has no non-static data members of volatile-qualified type.
(since C++14)

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

[edit]Implicitly-defined move assignment operator

If the implicitly-declared move 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.

For union types, the implicitly-defined move assignment operator copies the object representation (as by std::memmove).

For non-union class types (class and struct), the move assignment operator performs full member-wise move assignment of the object's direct bases and immediate non-static members, in their declaration order, using built-in assignment for the scalars, memberwise move-assignment for arrays, and move assignment operator for class types (called non-virtually).

As with copy assignment, 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 move assignment operator:

struct V { V& operator=(V&& other){// this may be called once or twice// if called twice, 'other' is the just-moved-from V subobjectreturn*this;}};struct A :virtual V {};// operator= calls V::operator=struct B :virtual V {};// operator= calls V::operator=struct C : B, A {};// operator= calls B::operator=, then A::operator=// but they may only called V::operator= once   int main(){ C c1, c2; c2 = std::move(c1);}
(since C++14)


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 move assignment operator (same applies to copy assignment).

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


Run this code


#include <string>#include <iostream>#include <utility>   struct A {std::string s; A(): s("test"){} A(const A& o): s(o.s){std::cout<<"move failed!\n";} A(A&& o): s(std::move(o.s)){} A& operator=(const A& other){ s = other.s;std::cout<<"copy assigned\n";return*this;} A& operator=(A&& other){ s = std::move(other.s);std::cout<<"move assigned\n";return*this;}};   A f(A a){return a;}   struct B : A {std::string s2;int n;// implicit move assignment operator B& B::operator=(B&&)// calls A's move assignment operator// calls s2's move assignment operator// and makes a bitwise copy of n};   struct C : B { ~C(){}// destructor prevents implicit move assignment};   struct D : B { D(){} ~D(){}// destructor would prevent implicit move assignment D& operator=(D&&)=default;// force a move assignment anyway };   int main(){ A a1, a2;std::cout<<"Trying to move-assign A from rvalue temporary\n"; a1 = f(A());// move-assignment from rvalue temporarystd::cout<<"Trying to move-assign A from xvalue\n"; a2 = std::move(a1);// move-assignment from xvalue   std::cout<<"Trying to move-assign B\n"; B b1, b2;std::cout<<"Before move, b1.s = \""<< b1.s<<"\"\n"; b2 = std::move(b1);// calls implicit move assignmentstd::cout<<"After move, b1.s = \""<< b1.s<<"\"\n";   std::cout<<"Trying to move-assign C\n"; C c1, c2; c2 = std::move(c1);// calls the copy assignment operator   std::cout<<"Trying to move-assign D\n"; D d1, d2; d2 = std::move(d1);}
Trying to move-assign A from rvalue temporary move assigned Trying to move-assign A from xvalue move assigned Trying to move-assign B Before move, b1.s = "test" move assigned After move, b1.s = "" Trying to move-assign C copy assigned Trying to move-assign D move assigned

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