[over.oper] # 12 Overloading [[over]](./#over) ## 12.4 Overloaded operators [over.oper] ### [12.4.1](#general) General [[over.oper.general]](over.oper.general) [1](#general-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3338) A declaration whose [*declarator-id*](dcl.decl.general#nt:declarator-id "9.3.1 General [dcl.decl.general]") is an [*operator-function-id*](#nt:operator-function-id "12.4.1 General [over.oper.general]") shall declare a function or function template or an explicit instantiation or specialization of a function template[.](#general-1.sentence-1) A function so declared is an [*operator function*](#def:function,operator "12.4.1 General [over.oper.general]")[.](#general-1.sentence-2) A function template so declared is an [*operator function template*](#def:function,operator,template "12.4.1 General [over.oper.general]")[.](#general-1.sentence-3) A specialization of an operator function template is also an operator function[.](#general-1.sentence-4) An operator function is said to[*implement*](#def:operator,implementation "12.4.1 General [over.oper.general]") the operator named in its[*operator-function-id*](#nt:operator-function-id "12.4.1 General [over.oper.general]")[.](#general-1.sentence-5) [operator-function-id:](#nt:operator-function-id "12.4.1 General [over.oper.general]") operator [*operator*](#nt:operator "12.4.1 General [over.oper.general]") [operator:](#nt:operator "12.4.1 General [over.oper.general]") one of new delete new[] delete[] co_await ( ) [ ] -> ->* ~ ! + - * / % ^ & | = += -= *= /= %= ^= &= |= == != < > <= >= <=> && || << >> <<= >>= ++ -- , [*Note [1](#general-note-1)*: The operatorsnew[],delete[],(), and[] are formed from more than one token[.](#general-1.sentence-6) The latter two operators are [function call](expr.call "7.6.1.3 Function call [expr.call]") and [subscripting](expr.sub "7.6.1.2 Subscripting [expr.sub]")[.](#general-1.sentence-7) — *end note*] [2](#general-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3382) Both the unary and binary forms of + - * & can be overloaded[.](#general-2.sentence-1) [3](#general-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3389) [*Note [2](#general-note-2)*: The following operators cannot be overloaded: . .* :: ?: nor can the preprocessing symbols# ([[cpp.stringize]](cpp.stringize "15.7.3 The # operator")) and## ([[cpp.concat]](cpp.concat "15.7.4 The ## operator"))[.](#general-3.sentence-1) — *end note*] [4](#general-4) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3402) Operator functions are usually not called directly; instead they are invoked to evaluate the operators they implement ([[over.unary]](#over.unary "12.4.2 Unary operators") – [[over.inc]](#over.inc "12.4.7 Increment and decrement"))[.](#general-4.sentence-1) They can be explicitly called, however, using the[*operator-function-id*](#nt:operator-function-id "12.4.1 General [over.oper.general]") as the name of the function in the function call syntax ([[expr.call]](expr.call "7.6.1.3 Function call"))[.](#general-4.sentence-2) [*Example [1](#general-example-1)*: complex z = a.operator+(b); // complex z = a+b;void* p = operator new(sizeof(int)*n); — *end example*] [5](#general-5) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3416) The allocation and deallocation functions,operator new,operator new[],operator delete, andoperator delete[], are described completely in [[basic.stc.dynamic]](basic.stc.dynamic "6.8.6.5 Dynamic storage duration")[.](#general-5.sentence-1) The attributes and restrictions found in the rest of [over.oper] do not apply to them unless explicitly stated in [[basic.stc.dynamic]](basic.stc.dynamic "6.8.6.5 Dynamic storage duration")[.](#general-5.sentence-2) [6](#general-6) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3427) The co_await operator is described completely in [[expr.await]](expr.await "7.6.2.4 Await")[.](#general-6.sentence-1) The attributes and restrictions found in the rest of [over.oper] do not apply to it unless explicitly stated in [[expr.await]](expr.await "7.6.2.4 Await")[.](#general-6.sentence-2) [7](#general-7) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3433) An operator function shall 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[.](#general-7.sentence-1) It is not possible to change the precedence, grouping, or number of operands of operators[.](#general-7.sentence-2) The meaning of the operators =, (unary) &, and , (comma), predefined for each type, can be changed for specific class types by defining operator functions that implement these operators[.](#general-7.sentence-3) Likewise, the meaning of the operators (unary) & and , (comma) can be changed for specific enumeration types[.](#general-7.sentence-4) Operator functions are inherited in the same manner as other base class functions[.](#general-7.sentence-5) [8](#general-8) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3452) An operator function shall be a prefix unary, binary, function call, subscripting, class member access, increment, or decrement operator function[.](#general-8.sentence-1) [9](#general-9) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3457) [*Note [3](#general-note-3)*: The identities among certain predefined operators applied to fundamental types (for example,++a ≡a+=1) need not hold for operator functions[.](#general-9.sentence-1) Some predefined operators, such as+=, require an operand to be an lvalue when applied to fundamental types; this is not required by operator functions[.](#general-9.sentence-2) — *end note*] [10](#general-10) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3471) An operator function cannot have [default arguments](dcl.fct.default "9.3.4.7 Default arguments [dcl.fct.default]"), except where explicitly stated below[.](#general-10.sentence-1) Operator functions cannot have more or fewer parameters than the number required for the corresponding operator, as described in the rest of [over.oper][.](#general-10.sentence-2) [11](#general-11) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3480) Operators not mentioned explicitly in subclauses [[over.assign]](#over.assign "12.4.3.2 Simple assignment") through [[over.inc]](#over.inc "12.4.7 Increment and decrement") act as ordinary unary and binary operators obeying the rules of [[over.unary]](#over.unary "12.4.2 Unary operators") or [[over.binary]](#over.binary "12.4.3 Binary operators")[.](#general-11.sentence-1) ### [12.4.2](#over.unary) Unary operators [[over.unary]](over.unary) [1](#over.unary-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3491) A [*prefix unary operator function*](#def:operator_function,prefix_unary "12.4.2 Unary operators [over.unary]") is a function named operator@ for a prefix [*unary-operator*](expr.unary.general#nt:unary-operator "7.6.2.1 General [expr.unary.general]") @ ([[expr.unary.op]](expr.unary.op "7.6.2.2 Unary operators")) that is either a non-static member function ([[class.mfct]](class.mfct "11.4.2 Member functions")) with no non-object parameters or a non-member function with one parameter[.](#over.unary-1.sentence-1) For a [*unary-expression*](expr.unary.general#nt:unary-expression "7.6.2.1 General [expr.unary.general]") of the form @ [*cast-expression*](expr.cast#nt:cast-expression "7.6.3 Explicit type conversion (cast notation) [expr.cast]"), the operator function is selected by overload resolution ([[over.match.oper]](over.match.oper "12.2.2.3 Operators in expressions"))[.](#over.unary-1.sentence-2) If a member function is selected, the expression is interpreted as [*cast-expression*](expr.cast#nt:cast-expression "7.6.3 Explicit type conversion (cast notation) [expr.cast]") . operator @ () Otherwise, if a non-member function is selected, the expression is interpreted as operator @ ( [*cast-expression*](expr.cast#nt:cast-expression "7.6.3 Explicit type conversion (cast notation) [expr.cast]") ) [*Note [1](#over.unary-note-1)*: The operators ++ and -- ([[expr.pre.incr]](expr.pre.incr "7.6.2.3 Increment and decrement")) are described in [[over.inc]](#over.inc "12.4.7 Increment and decrement")[.](#over.unary-1.sentence-3) — *end note*] [2](#over.unary-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3517) [*Note [2](#over.unary-note-2)*: The unary and binary forms of the same operator have the same name[.](#over.unary-2.sentence-1) Consequently, a unary operator can hide a binary operator from an enclosing scope, and vice versa[.](#over.unary-2.sentence-2) — *end note*] ### [12.4.3](#over.binary) Binary operators [[over.binary]](over.binary) #### [12.4.3.1](#over.binary.general) General [[over.binary.general]](over.binary.general) [1](#over.binary.general-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3530) A [*binary operator function*](#def:operator_function,binary "12.4.3.1 General [over.binary.general]") is a function named operator@ for a binary operator @ that is either a non-static member function ([[class.mfct]](class.mfct "11.4.2 Member functions")) with one non-object parameter or a non-member function with two parameters[.](#over.binary.general-1.sentence-1) For an expression x @ y with subexpressions x and y, the operator function is selected by overload resolution ([[over.match.oper]](over.match.oper "12.2.2.3 Operators in expressions"))[.](#over.binary.general-1.sentence-2) If a member function is selected, the expression is interpreted as x . operator @ ( y ) Otherwise, if a non-member function is selected, the expression is interpreted as operator @ ( x , y ) [2](#over.binary.general-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3550) An [*equality operator function*](#def:operator_function,equality "12.4.3.1 General [over.binary.general]") is an operator function for an equality operator ([[expr.eq]](expr.eq "7.6.10 Equality operators"))[.](#over.binary.general-2.sentence-1) A [*relational operator function*](#def:operator_function,relational "12.4.3.1 General [over.binary.general]") is an operator function for a relational operator ([[expr.rel]](expr.rel "7.6.9 Relational operators"))[.](#over.binary.general-2.sentence-2) A [*three-way comparison operator function*](#def:operator_function,three-way_comparison "12.4.3.1 General [over.binary.general]") is an operator function for the three-way comparison operator ([[expr.spaceship]](expr.spaceship "7.6.8 Three-way comparison operator"))[.](#over.binary.general-2.sentence-3) A [*comparison operator function*](#def:operator_function,comparison "12.4.3.1 General [over.binary.general]") is an equality operator function, a relational operator function, or a three-way comparison operator function[.](#over.binary.general-2.sentence-4) #### [12.4.3.2](#over.assign) Simple assignment [[over.assign]](over.assign) [1](#over.assign-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3566) A [*simple assignment operator function*](#def:operator_function,simple_assignment "12.4.3.2 Simple assignment [over.assign]") is a binary operator function named operator=[.](#over.assign-1.sentence-1) A simple assignment operator function shall be a non-static member function[.](#over.assign-1.sentence-2) [*Note [1](#over.assign-note-1)*: Because only standard conversion sequences are considered when converting to the left operand of an assignment operation ([[over.best.ics]](over.best.ics "12.2.4.2 Implicit conversion sequences")), an expression x = y with a subexpression x of class type is always interpreted as x.operator=(y)[.](#over.assign-1.sentence-3) — *end note*] [2](#over.assign-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3577) [*Note [2](#over.assign-note-2)*: Since a copy assignment operator is implicitly declared for a class if not declared by the user ([[class.copy.assign]](class.copy.assign "11.4.6 Copy/move assignment operator")), a base class assignment operator function is always hidden by the copy assignment operator function of the derived class[.](#over.assign-2.sentence-1) — *end note*] [3](#over.assign-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3585) [*Note [3](#over.assign-note-3)*: Any assignment operator function, even the copy and move assignment operators, can be virtual[.](#over.assign-3.sentence-1) For a derived class D with a base class B for which a virtual copy/move assignment has been declared, the copy/move assignment operator in D does not overrideB's virtual copy/move assignment operator[.](#over.assign-3.sentence-2) [*Example [1](#over.assign-example-1)*: struct B {virtual int operator= (int); virtual B& operator= (const B&);};struct D : B {virtual int operator= (int); virtual D& operator= (const B&);}; D dobj1; D dobj2; B* bptr = &dobj1;void f() { bptr->operator=(99); // calls D​::​operator=(int)*bptr = 99; // ditto bptr->operator=(dobj2); // calls D​::​operator=(const B&)*bptr = dobj2; // ditto dobj1 = dobj2; // calls implicitly-declared D​::​operator=(const D&)} — *end example*] — *end note*] ### [12.4.4](#over.call) Function call [[over.call]](over.call) [1](#over.call-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3622) A [*function call operator function*](#def:operator_function,function_call "12.4.4 Function call [over.call]") is a function named operator() that is a member function with an arbitrary number of parameters[.](#over.call-1.sentence-1) It may have default arguments[.](#over.call-1.sentence-2) For an expression of the form [*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") ( [*expression-list*](expr.post.general#nt:expression-list "7.6.1.1 General [expr.post.general]")opt ) where the [*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") is of class type, the operator function is selected by overload resolution ([[over.call.object]](over.call.object "12.2.2.2.3 Call to object of class type"))[.](#over.call-1.sentence-3) If a surrogate call function is selected, let e be the result of invoking the corresponding conversion operator function on the [*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]"); the expression is interpreted as e ( [*expression-list*](expr.post.general#nt:expression-list "7.6.1.1 General [expr.post.general]")opt ) Otherwise, the expression is interpreted as [*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") . operator () ( [*expression-list*](expr.post.general#nt:expression-list "7.6.1.1 General [expr.post.general]")opt ) ### [12.4.5](#over.sub) Subscripting [[over.sub]](over.sub) [1](#over.sub-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3650) A [*subscripting operator function*](#def:operator_function,subscripting "12.4.5 Subscripting [over.sub]") is a member function named operator[] with an arbitrary number of parameters[.](#over.sub-1.sentence-1) It may have default arguments[.](#over.sub-1.sentence-2) For an expression of the form [*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") [ [*expression-list*](expr.post.general#nt:expression-list "7.6.1.1 General [expr.post.general]")opt ] the operator function is selected by overload resolution ([[over.match.oper]](over.match.oper "12.2.2.3 Operators in expressions"))[.](#over.sub-1.sentence-3) If a member function is selected, the expression is interpreted as [*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") . operator [] ( [*expression-list*](expr.post.general#nt:expression-list "7.6.1.1 General [expr.post.general]")opt ) [2](#over.sub-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3667) [*Example [1](#over.sub-example-1)*: struct X { Z operator[](std::initializer_list); Z operator[](auto...);}; X x; x[{1,2,3}] = 7; // OK, meaning x.operator[]({1,2,3}) x[1,2,3] = 7; // OK, meaning x.operator[](1,2,3)int a[10]; a[{1,2,3}] = 7; // error: built-in subscript operator a[1,2,3] = 7; // error: built-in subscript operator — *end example*] ### [12.4.6](#over.ref) Class member access [[over.ref]](over.ref) [1](#over.ref-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3687) A [*class member access operator function*](#def:operator_function,class_member_access "12.4.6 Class member access [over.ref]") is a function named operator-> that is a non-static member function taking no non-object parameters[.](#over.ref-1.sentence-1) For an expression of the form [*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") -> templateopt [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") the operator function is selected by overload resolution ([[over.match.oper]](over.match.oper "12.2.2.3 Operators in expressions")), and the expression is interpreted as ( [*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") . operator -> () ) -> templateopt [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") ### [12.4.7](#over.inc) Increment and decrement [[over.inc]](over.inc) [1](#over.inc-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3708) An [*increment operator function*](#def:operator_function,increment "12.4.7 Increment and decrement [over.inc]") is a function named operator++[.](#over.inc-1.sentence-1) If this function is a non-static member function with no non-object parameters, or a non-member function with one parameter, it defines the prefix increment operator++ for objects of that type[.](#over.inc-1.sentence-2) If the function is a non-static member function with one non-object parameter (which shall be of typeint) or a non-member function with two parameters (the second of which shall be of typeint), it defines the postfix increment operator++ for objects of that type[.](#over.inc-1.sentence-3) When the postfix increment is called as a result of using the++ operator, theint argument will have value zero[.](#over.inc-1.sentence-4)[107](#footnote-107 "Calling operator++ explicitly, as in expressions like a.operator++(2), has no special properties: The argument to operator++ is 2.") [*Example [1](#over.inc-example-1)*: struct X { X& operator++(); // prefix ++a X operator++(int); // postfix a++}; struct Y { }; Y& operator++(Y&); // prefix ++b Y operator++(Y&, int); // postfix b++void f(X a, Y b) {++a; // a.operator++(); a++; // a.operator++(0);++b; // operator++(b); b++; // operator++(b, 0); a.operator++(); // explicit call: like ++a; a.operator++(0); // explicit call: like a++;operator++(b); // explicit call: like ++b;operator++(b, 0); // explicit call: like b++;} — *end example*] [2](#over.inc-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L3765) A [*decrement operator function*](#def:operator_function,decrement "12.4.7 Increment and decrement [over.inc]") is a function named operator-- and is handled analogously to an increment operator function[.](#over.inc-2.sentence-1) [107)](#footnote-107)[107)](#footnoteref-107) Callingoperator++ explicitly, as in expressions likea.operator++(2), has no special properties: The argument tooperator++ is2[.](#footnote-107.sentence-1)