B - Operators - The Rust Programming Language

## Appendix B: Operators

### Unary operator expressions

Rust defines the following unary operators. They are all written as prefix operators, before the expression they apply to.

- `-` : Negation. Signed integer types and floating-point types support negation. It is an error to apply negation to unsigned types; for example, the compiler rejects `-1u32`.
- `*` : Dereference. When applied to a pointer, it denotes the pointed-to location. For pointers to mutable locations, the resulting value can be assigned to. On non-pointer types, it calls the `deref` method of the `std::ops::Deref` trait, or the `deref_mut` method of the `std::ops::DerefMut` trait (if implemented by the type and required for an outer expression that will or could mutate the dereference), and produces the result of dereferencing the `&` or `&mut` borrowed pointer returned from the overload method.
- `!` : Logical negation. On the boolean type, this flips between `true` and `false`. On integer types, this inverts the individual bits in the two's complement representation of the value.
- `&` and `&mut` : Borrowing. When applied to a value, these operators produce a reference (pointer) to that value. The value is also placed into a borrowed state for the duration of the reference. For a shared borrow (`&`), this implies that the value may not be mutated, but it may be read or shared again. For a mutable borrow (`&mut`), the value may not be accessed in any way until the borrow expires.

### Binary operator expressions

Binary operators expressions are given in order of operator precedence.

#### Arithmetic operators

Binary arithmetic expressions are syntactic sugar for calls to built-in traits, defined in the `std::ops` module of the `std` library. This means arithmetic operators can be overridden for user-defined types. The default meaning of the operators on standard types is given here.

- `+` : Addition and array/string concatenation. Calls the `add` method on the `std::ops::Add` trait.
- `-` : Subtraction. Calls the `sub` method on the `std::ops::Sub` trait.
- `*` : Multiplication. Calls the `mul` method on the `std::ops::Mul` trait.
- `/` : Quotient. Calls the `div` method on the `std::ops::Div` trait.
- `%` : Remainder. Calls the `rem` method on the `std::ops::Rem` trait.

Note that Rust does not have a built-in operator for exponential (power) calculation; see the `pow` method on the numeric types.

#### Bitwise operators

Like the arithmetic operators, bitwise operators are syntactic sugar for calls to methods of built-in traits. This means bitwise operators can be overridden for user-defined types. The default meaning of the operators on standard types is given here. Bitwise `&`, `|` and `^` applied to boolean arguments are equivalent to logical `&&`, `||` and `!=` evaluated in non-lazy fashion.

- `&` : Bitwise AND. Calls the `bitand` method of the `std::ops::BitAnd` trait.
- `|` : Bitwise inclusive OR. Calls the `bitor` method of the `std::ops::BitOr` trait.
- `^` : Bitwise exclusive OR. Calls the `bitxor` method of the `std::ops::BitXor` trait.
- `<<` : Left shift. Calls the `shl` method of the `std::ops::Shl` trait.
- `>>` : Right shift (arithmetic). Calls the `shr` method of the `std::ops::Shr` trait.

#### Lazy boolean operators

The operators `||` and `&&` may be applied to operands of boolean type. The `||` operator denotes logical 'or', and the `&&` operator denotes logical 'and'. They differ from `|` and `&` in that the right-hand operand is only evaluated when the left-hand operand does not already determine the result of the expression. That is, `||` only evaluates its right-hand operand when the left-hand operand evaluates to `false`, and `&&` only when it evaluates to `true`.

#### Comparison operators

Comparison operators are, like the arithmetic operators and bitwise operators, syntactic sugar for calls to built-in traits. This means that comparison operators can be overridden for user-defined types. The default meaning of the operators on standard types is given here.

- `==` : Equal to. Calls the `eq` method on the `std::cmp::PartialEq` trait.
- `!=` : Unequal to. Calls the `ne` method on the `std::cmp::PartialEq` trait.
- `<` : Less than. Calls the `lt` method on the `std::cmp::PartialOrd` trait.
- `>` : Greater than. Calls the `gt` method on the `std::cmp::PartialOrd` trait.
- `<=` : Less than or equal. Calls the `le` method on the `std::cmp::PartialOrd` trait.
- `>=` : Greater than or equal. Calls the `ge` method on the `std::cmp::PartialOrd` trait.

#### Type cast expressions

A type cast expression is denoted with the binary operator `as`.

Executing an `as` expression casts the value on the left-hand side to the type on the right-hand side.

An example of an `as` expression:

    # #![allow(unused_variables)]
    #fn main() {
    # fn sum(values: &[f64]) -> f64 { 0.0 }
    # fn len(values: &[f64]) -> i32 { 0 }

    fn average(values: &[f64]) -> f64 {
        let sum: f64 = sum(values);
        let size: f64 = len(values) as f64;
        sum / size
    }

    #}

Some of the conversions which can be done through the `as` operator can also be done implicitly at various points in the program, such as argument passing and assignment to a `let` binding with an explicit type. Implicit conversions are limited to "harmless" conversions that do not lose information and which have minimal or no risk of surprising side-effects on the dynamic execution semantics.

#### Assignment expressions

An assignment expression consists of a pattern followed by an equals sign (`=`) and an expression.

Evaluating an assignment expression either copies or moves its right-hand operand to its left-hand operand.

    # let mut x = 0;
    # let y = 0;
    x = y;

#### Compound assignment expressions

The `+`, `-`, `*`, `/`, `%`, `&`, `|`, `^`, `<<`, and `>>` operators may be composed with the `=` operator. The expression `lval OP= val` is equivalent to `lval = lval OP val`. For example, `x = x + 1` may be written as `x += 1`.

Any such expression always has the `unit` type.

#### Operator precedence

The precedence of Rust binary operators is ordered as follows, going from strong to weak:

    as :
    * / %
    + -
    << >>
    &
    ^
    |
    == != < > <= >=
    &&
    ||
    .. ...
    <-
    =

Operators at the same precedence level are evaluated left-to-right. Unary operators have the same precedence level and are stronger than any of the binary operators.