12 Overloading [over]

12.4 Overloaded operators [over.oper]

12.4.1 General [over.oper.general]

1
#
A declaration whose declarator-id is an operator-function-id shall declare a function or function template or an explicit instantiation or specialization of a function template.
A function so declared is an operator function.
A function template so declared is an operator function template.
A specialization of an operator function template is also an operator function.
An operator function is said to implement the operator named in its operator-function-id.
operator: one of
new delete new[] delete[] co_await ( ) [ ] -> ->*
~ ! + - * / % ^ &
| = += -= *= /= %= ^= &=
|= == != < > <= >= <=> &&
|| << >> <<= >>= ++ -- ,
[Note 1: 
The operators new[], delete[], (), and [] are formed from more than one token.
The latter two operators are function call ([expr.call]) and subscripting ([expr.sub]).
— end note]
2
#
Both the unary and binary forms of
+ - * &
can be overloaded.
3
#
[Note 2: 
The following operators cannot be overloaded:
. .* :: ?:
nor can the preprocessing symbols # ([cpp.stringize]) and ## ([cpp.concat]).
— end note]
4
#
Operator functions are usually not called directly; instead they are invoked to evaluate the operators they implement ([over.unary] – [over.inc]).
They can be explicitly called, however, using the operator-function-id as the name of the function in the function call syntax ([expr.call]).
[Example 1: complex z = a.operator+(b); // complex z = a+b; void* p = operator new(sizeof(int)*n); — end example]
5
#
The allocation and deallocation functions, operator new, operator new[], operator delete, and operator delete[], are described completely in [basic.stc.dynamic].
The attributes and restrictions found in the rest of [over.oper] do not apply to them unless explicitly stated in [basic.stc.dynamic].
6
#
The co_await operator is described completely in [expr.await].
The attributes and restrictions found in the rest of [over.oper] do not apply to it unless explicitly stated in [expr.await].
7
#
An operator function shall either
  • (7.1)
    be a member function or
  • (7.2)
    be a non-member function that has at least one non-object parameter whose type is a class, a reference to a class, an enumeration, or a reference to an enumeration.
It is not possible to change the precedence, grouping, or number of operands of operators.
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.
Likewise, the meaning of the operators (unary) & and , (comma) can be changed for specific enumeration types.
Operator functions are inherited in the same manner as other base class functions.
8
#
An operator function shall be a prefix unary, binary, function call, subscripting, class member access, increment, or decrement operator function.
9
#
[Note 3: 
The identities among certain predefined operators applied to basic types (for example, ++a ≡ a+=1) need not hold for operator functions.
Some predefined operators, such as +=, require an operand to be an lvalue when applied to basic types; this is not required by operator functions.
— end note]
10
#
An operator function cannot have default arguments ([dcl.fct.default]), except where explicitly stated below.
Operator functions cannot have more or fewer parameters than the number required for the corresponding operator, as described in the rest of [over.oper].
11
#
Operators not mentioned explicitly in subclauses [over.ass] through [over.inc] act as ordinary unary and binary operators obeying the rules of [over.unary] or [over.binary].