17.9 Operators
An operator is a member that defines the meaning of an expression operator
that can be applied to instances of
the class. Operators are declared using operator-declarations:
operator-declaration:
attributesopt operator-modifiers operator-declarator operator-body
operator-modifiers:
operator-modifier
operator-modifiers operator-modifier
operator-modifier:
public
static
extern
operator-declarator:
unary-operator-declarator
binary-operator-declarator
conversion-operator-declarator
unary-operator-declarator:
type operator overloadable-unary-operator ( type identifier )
overloadable-unary-operator: one of
+ - ! ~ ++ -- true false
Chapter 17 Classes
255
binary-operator-declarator:
type operator overloadable-binary-operator ( type identifier , type
identifier )
overloadable-binary-operator: one of
+ - * / % & | ^ << >> == != > < >= <=
conversion-operator-declarator:
implicit operator type ( type identifier )
explicit operator type ( type identifier )
operator-body:
block
;
There are three categories of overloadable operators: Unary operators (§17.9
.1), binary operators (§17.9.2), and
conversion operators (§17.9.3).
When an operator declaration includes an extern modifier, the operator is
said to be an external operator.
Because an external operator provides no actual implementation, its
operator-body consists of a semi-colon. For
all other operators, the operator-body consists of a block, which specifies
the statements to execute when the
operator is invoked. The block of an operator must conform to the rules for
value-returning methods described in
§17.5.8.
The following rules apply to all operator declarations:
? An operator declaration must include both a public and a static modifier.
? The parameter(s) of an operator must be value parameters. It is a
compile-time error for an operator
declaration to specify ref or out parameters.
? The signature of an operator (§17.9.1, §17.9.2, §17.9.3) must differ
from the signatures of all other operators
declared in the same class.
? All types referenced in an operator declaration must be at least as
accessible as the operator itself (§10.5.4).
? It is an error for the same modifier to appear multiple times in an
operator declaration.
Each operator category imposes additional restrictions, as described in the
following sections.
Like other members, operators declared in a base class are inherited by
derived classes. Because operator
declarations always require the class or struct in which the operator is
declared to participate in the signature of
the operator, it is not possible for an operator declared in a derived
class to hide an operator declared in a base
class. Thus, the new modifier is never required, and therefore never
permitted, in an operator declaration.
Additional information on unary and binary operators can be found in §14.2.
Additional information on conversion operators can be found in §13.4.
