14.2.2 Operator overloading

All unary and binary operators have predefined implementations that are
automatically available in any
expression. In addition to the predefined implementations, user-defined
implementations can be introduced
by including operator declarations in classes and structs (?7.9).
User-defined operator implementations
always take precedence over predefined operator implementations: Only when
no applicable user-defined
operator implementations exist will the predefined operator implementations
be considered, as described in
?4.2.3 and ?4.2.4.
The overloadable unary operators are:
+ - ! ~ ++ -- true false
[Note: Although true and false are not used explicitly in expressions (and
therefore are not included in
the precedence table in ?4.2.1), they are considered operators because
they are invoked in several
expression contexts: boolean expressions (?4.16) and expressions involving
the conditional (?4.12), and
conditional logical operators (?4.11). end note]
The overloadable binary operators are:
+ - * / % & | ^ << >> == != > < >= <=
Only the operators listed above can be overloaded. In particular, it is not
possible to overload member
access, method invocation, or the =, &&, ||, ?:, checked, unchecked, new,
typeof, as, and
is operators.
When a binary operator is overloaded, the corresponding assignment
operator, if any, is also implicitly
overloaded. [Example: For example, an overload of operator * is also an
overload of operator *=. This is
described further in ?4.13. end example] The assignment operator itself
(=) cannot be overloaded. An
assignment always performs a simple bit-wise copy of a value into a
variable.
Cast operations, such as (T)x, are overloaded by providing user-defined
conversions (?3.4).
Element access, such as a[x], is not considered an overloadable operator.
Instead, user-defined indexing is
supported through indexers (?7.8).
In expressions, operators are referenced using operator notation, and in
declarations, operators are referenced
using functional notation. The following table shows the relationship
between operator and functional
notations for unary and binary operators. In the first entry, op denotes
any overloadable unary prefix
operator. In the second entry, op denotes the unary postfix ++ and --
operators. In the third entry, op
denotes any overloadable binary operator. [Note: For an example of
overloading the ++ and -- operators see
?7.9.1. end note]
Operator notation Functional notation
op x operator op(x)
x op operator op(x)
x op y operator op(x, y)
User-defined operator declarations always require at least one of the
parameters to be of the class or struct
type that contains the operator declaration. [Note: Thus, it is not
possible for a user-defined operator to have
the same signature as a predefined operator. end note]
User-defined operator declarations cannot modify the syntax, precedence, or
associativity of an operator.
[Example: For example, the / operator is always a binary operator, always
has the precedence level
specified in ?4.2.1, and is always left-associative. end example]
[Note: While it is possible for a user-defined operator to perform any
computation it pleases,
implementations that produce results other than those that are intuitively
expected are strongly discouraged.
For example, an implementation of operator == should compare the two
operands for equality and return
an appropriate bool result. end note]
The descriptions of individual operators in ?4.5 through ?4.13 specify
the predefined implementations of
the operators and any additional rules that apply to each operator. The
descriptions make use of the terms
unary operator overload resolution, binary operator overload resolution,
and numeric promotion,
definitions of which are found in the following sections.