Customizes the C++ operators for operands of user-defined types.
Syntax
Operator functions are
with special function names:
operatorop(1) operatornew
operatornew [](2) operatordelete
operatordelete [](3) operatorco_await(4) (since C++20)op- any of the following operators:+-*/%^&|~!=<>+=-=*=/=%=^=&=|=<<>>>>=<<===!=<=>=<=>(since C++20)&&||++--,->*->()[]1) An overloaded punctuation operator.
The behaviors of non-punctuation operators are described in their own respective pages. Unless otherwise specified, the remaining description in this page does not apply to these functions.
Explanation
When an operator appears in an
, and at least one of its operands has a
or an
, then
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 Given std::strings;, s="abc"; calls s.operator=("abc")a(b...) (a).operator()(b...) cannot be non-member Given std::random_devicer;, auton=r(); calls r.operator()()a[b...] (a).operator[](b...) cannot be non-member Given std::map<int,int>m;, m[1]=2; calls m.operator[](1)a-> (a).operator->( ) cannot be non-member Given std::unique_ptr<S>p;, p->bar() calls p.operator->()a@ (a).operator@ (0) operator@ (a, 0) Given std::vector<int>::iteratori;, 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.
In addition, for comparison operators ==, !=, <, >, <=, >=, <=>, overload resolution also considers the
operator== or operator<=>.
(since C++20)Overloaded operators (but not the built-in operators) can be called using function notation:
std::stringstr="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 sequencingStatic overloaded operators
Overloaded operators that are member functions can be declared
. However, this is only allowed for operator() and operator[].
Such operators can be called using function notation. However, when these operators appear in expressions, they still require an object of class type.
structSwapThem{template<typenameT>staticvoidoperator()(T&lhs,T&rhs){std::ranges::swap(lhs,rhs);}template<typenameT>staticvoidoperator[](T&lhs,T&rhs){std::ranges::swap(lhs,rhs);}};inlineconstexprSwapThemswap_them{};voidfoo(){inta=1,b=2;swap_them(a,b);// OKswap_them[a,b];// OKSwapThem{}(a,b);// OKSwapThem{}[a,b];// OKSwapThem::operator()(a,b);// OKSwapThem::operator[](a,b);// OKSwapThem(a,b);// error, invalid constructionSwapThem[a,b];// error}(since C++23)Restrictions
An operator function must have at least one function parameter or implicit object parameter whose type is a class, a reference to a class, an enumeration, or a reference to an enumeration.
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.
It is not possible to change the precedence, grouping, or number of operands of operators.
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.
The overloads of operators && and || lose short-circuit evaluation.
&&, ||, and , lose their special
when overloaded and behave like regular function calls even when they are used without function-call notation.
(until C++17)Canonical implementations
Besides the restrictions above, the language puts no other constraints on what the overloaded operators do, or on the return type (it does not participate in overload resolution), but in general, overloaded operators are expected to behave as similar as possible to the built-in operators: operator+ is expected to add, rather than multiply its arguments, operator= is expected to assign, etc. The related operators are expected to behave similarly (operator+ and operator+= do the same addition-like operation). The return types are limited by the expressions in which the operator is expected to be used: for example, assignment operators return by reference to make it possible to write a=b=c=d, because the built-in operators allow that.
Commonly overloaded operators have the following typical, canonical forms:
Assignment operator
The assignment operator operator= has special properties: see
and
for details.
The canonical copy-assignment operator is expected to
, and to return the lhs by reference:
// copy assignmentT&operator=(constT&other){// Guard self assignmentif(this==&other)return*this;// assume *this manages a reusable resource, such as a heap-allocated buffer mArrayif(size!=other.size)// resource in *this cannot be reused{temp=newint[other.size];// allocate resource, if throws, do nothingdelete[]mArray;// release resource in *thismArray=temp;size=other.size;}std::copy(other.mArray,other.mArray+other.size,mArray);return*this;}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
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:
// move assignmentT&operator=(T&&other)noexcept{// Guard self assignmentif(this==&other)return*this;// delete[]/size=0 would also be okdelete[]mArray;// release resource in *thismArray=std::exchange(other.mArray,nullptr);// leave other in valid statesize=std::exchange(other.size,0);return*this;}(since C++11)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
or
), 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.
// copy assignment (copy-and-swap idiom)T&T::operator=(Tother)noexcept// call copy or move constructor to construct other{std::swap(size,other.size);// exchange resources between *this and otherstd::swap(mArray,other.mArray);return*this;}// destructor of other is called to release the resources formerly managed by *thisThis form automatically provides
, but prohibits resource reuse.
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.
std::ostream&operator<<(std::ostream&os,constT&obj){// write obj to streamreturnos;}std::istream&operator>>(std::istream&is,T&obj){// read obj from streamif(/* T could not be constructed */)is.setstate(std::ios::failbit);returnis;}These operators are sometimes implemented as
.
Function call operator
When a user-defined class overloads the function call operator operator(), it becomes a
type.
An object of such a type can be used in a function call expression:
// An object of this type represents a linear function of one variable a * x + b.structLinear{doublea,b;doubleoperator()(doublex)const{returna*x+b;}};intmain(){Linearf{2,1};// Represents function 2x + 1.Linearg{-1,0};// Represents function -x.// f and g are objects that can be used like a function.doublef_0=f(0);doublef_1=f(1);doubleg_0=g(0);}Many standard library
accept
to customize behavior. There are no particularly notable canonical forms of operator(), but to illustrate the usage:
Run this code
#include<algorithm>#include<iostream>#include<vector>structSum{intsum=0;voidoperator()(intn){sum+=n;}};intmain(){std::vector<int>v={1,2,3,4,5};Sums=std::for_each(v.begin(),v.end(),Sum());std::cout<<"The sum is "<<s.sum<<'\n';}Output:
The sum is 15 Increment and decrement
When the postfix increment or decrement operator appears in an expression, the corresponding user-defined function (operator++ or operator--) is called with an integer argument 0. Typically, it is declared as Toperator++(int) or Toperator--(int), where the argument is ignored. The postfix increment and decrement operators are usually implemented in terms of the prefix versions:
structX{// prefix incrementX&operator++(){// actual increment takes place herereturn*this;// return new value by reference}// postfix incrementXoperator++(int){Xold=*this;// copy old valueoperator++();// prefix incrementreturnold;// return old value}// prefix decrementX&operator--(){// actual decrement takes place herereturn*this;// return new value by reference}// postfix decrementXoperator--(int){Xold=*this;// copy old valueoperator--();// prefix decrementreturnold;// return old value}};Although the canonical implementations of the prefix increment and decrement operators return by reference, as with any operator overload, the return type is user-defined; for example the overloads of these operators for
return by value.
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:
classX{public:X&operator+=(constX&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 lookupfriendXoperator+(Xlhs,// passing lhs by value helps optimize chained a+b+cconstX&rhs)// otherwise, both parameters may be const references{lhs+=rhs;// reuse compound assignmentreturnlhs;// return the result by value (uses move constructor)}};Comparison operators
Standard library algorithms such as
and containers such as
expect operator< to be defined, by default, for the user-provided types, and expect it to implement strict weak ordering (thus satisfying the
requirements). An idiomatic way to implement strict weak ordering for a structure is to use lexicographical comparison provided by
:
struct Record { std::string name; unsigned int floor; double weight; friend bool operator<(const Record& l, const Record& r) { return std::tie(l.name, l.floor, l.weight) < std::tie(r.name, r.floor, r.weight); // keep the same order } };Typically, once operator< is provided, the other relational operators are implemented in terms of operator<.
inline bool operator< (const X& lhs, const X& rhs) { /* do actual comparison */ } inline bool operator> (const X& lhs, const X& rhs) { return rhs < lhs; } inline bool operator<=(const X& lhs, const X& rhs) { return !(lhs > rhs); } inline bool operator>=(const X& lhs, const X& rhs) { return !(lhs < rhs); }Likewise, the inequality operator is typically implemented in terms of operator==:
inline bool operator==(const X& lhs, const X& rhs) { /* do actual comparison */ } inline bool operator!=(const X& lhs, const X& rhs) { return !(lhs == rhs); }When three-way comparison (such as
or
) is provided, all six two-way comparison operators may be expressed through that:
inline bool operator==(const X& lhs, const X& rhs) { return cmp(lhs,rhs) == 0; } inline bool operator!=(const X& lhs, const X& rhs) { return cmp(lhs,rhs) != 0; } inline bool operator< (const X& lhs, const X& rhs) { return cmp(lhs,rhs) < 0; } inline bool operator> (const X& lhs, const X& rhs) { return cmp(lhs,rhs) > 0; } inline bool operator<=(const X& lhs, const X& rhs) { return cmp(lhs,rhs) <= 0; } inline bool operator>=(const X& lhs, const X& rhs) { return cmp(lhs,rhs) >= 0; }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:
struct T { value_t& operator[](std::size_t idx) { return mVector[idx]; } const value_t& operator[](std::size_t idx) const { return mVector[idx]; } };Alternatively, they can be expressed as a single member function template using an
explicit object member functions
:
struct T { decltype(auto) operator[](this auto& self, std::size_t idx) { return self.mVector[idx]; } };(since C++23)If the value type is known to be a scalar type, the const variant should return by value.
Where direct access to the elements of the container is not wanted or not possible or distinguishing between lvalue c[i] = v; and rvalue v = c[i]; usage, operator[] may return a proxy. See for example
.
operator[] can only take one subscript. In order 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;.
(until C++23)operator[] can take any number of subscripts. For example, an operator[] of a 3D array class declared as T& operator[](std::size_t x, std::size_t y, std::size_t z); can directly access the elements.
Run this code
#include <array> #include <cassert> #include <iostream> template<typename T, std::size_t Z, std::size_t Y, std::size_t X> struct Array3d { std::array<T, X * Y * Z> m{}; constexpr T& operator[](std::size_t z, std::size_t y, std::size_t x) // C++23 { assert(x < X and y < Y and z < Z); return m[z * Y * X + y * X + x]; } }; int main() { Array3d<int, 4, 3, 2> v; v[3, 2, 1] = 42; std::cout << "v[3, 2, 1] = " << v[3, 2, 1] << '\n'; }Output:
v[3, 2, 1] = 42(since C++23)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.
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 to boolean type (see
for the standard library example), and the expected behavior of operator! is to return the value opposite of operator bool.
(until C++11)Since the built-in operator ! performs
, user-defined classes that are intended to be used in boolean contexts could provide only operator bool and need not overload operator!.
(since C++11)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&, it is unspecified whether the operator has the built-in meaning or the operator function is called. Because this operator may be overloaded, generic libraries use
to obtain addresses of objects of user-defined types. The best known example of a canonical overloaded operator& is the Microsoft class
. An example of this operator's use in EDSL can be found in
.
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
.
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 operator, in
,
, and other libraries. The database access library
also overloads operator,.
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 a
, and in fact is used in that capacity by actors in
. It is more common in EDSLs such as
.
Notes
Feature-test macro ValueStdFeature
(C++23)static operator()
__cpp_multidimensional_subscript
(C++23)static operator[]Keywords
Example
Run this code
#include <iostream> class Fraction { // or C++17's std::gcd constexpr int gcd(int a, int b) { return b == 0 ? a : gcd(b, a % b); } int n, d; public: constexpr Fraction(int n, int d = 1) : n(n / gcd(n, d)), d(d / gcd(n, d)) {} constexpr int num() const { return n; } constexpr int den() const { return d; } constexpr 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(); } constexpr bool operator==(const Fraction& lhs, const Fraction& rhs) { return lhs.num() == rhs.num() && lhs.den() == rhs.den(); } constexpr bool operator!=(const Fraction& lhs, const Fraction& rhs) { return !(lhs == rhs); } constexpr Fraction operator*(Fraction lhs, const Fraction& rhs) { return lhs *= rhs; } int main() { constexpr 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'; static_assert(f3 == f2 * 10); }Output:
3/8 * 1/2 = 3/16 1/2 * 5/1 = 5/2 2 * 3/8 = 3/4Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
C++98 the non-member prefix increment operator could only have a parameter
of class type, enumeration type, or a reference type to such types no type requirement
C++23 explicit object member operator functions could only have no parameter
of class type, enumeration type, or a reference type to such types prohibited See also
Common operators
a = b a += b a -= b a *= b a /= b a %= b a &= b a |= b a ^= b a <<= b a >>= b++a --a a++ a--+a -a a + b a - b a * b a / b a % b ~a a & b a | b a ^ b a << b a >> b!a a && b a || ba == b a != b a < b a > b a <= b a >= b a <=> ba[...] *a &a a->b a.b a->*b a.*bfunction calla(...)
commaa, b
conditionala ? b : c
Special operators
converts one type to another related type
converts within inheritance hierarchies
adds or removes
-qualifiers
converts type to unrelated type
converts one type to another by a mix of static_cast, const_cast, and reinterpret_cast
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
(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)
produces a reflection value from a grammatical construct (since C++26)
External links
on StackOverflow C++ FAQ