Field access expressions - The Rust Reference (original) (raw)

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The Rust Reference

Field access expressions

A field expression is a place expression that evaluates to the location of a field of a struct or union.

When the operand is mutable, the field expression is also mutable.

The syntax for a field expression is an expression, called the container operand, then a ., and finally an identifier.

Field expressions cannot be followed by a parenthetical comma-separated list of expressions, as that is instead parsed as a method call expression. That is, they cannot be the function operand of a call expression.

Note

Wrap the field expression in a parenthesized expression to use it in a call expression.

#![allow(unused)]
fn main() {
struct HoldsCallable<F: Fn()> { callable: F }
let holds_callable = HoldsCallable { callable: || () };

// Invalid: Parsed as calling the method "callable"
// holds_callable.callable();

// Valid
(holds_callable.callable)();
}

Examples:

mystruct.myfield;
foo().x;
(Struct {a: 10, b: 20}).a;
(mystruct.function_field)() // Call expression containing a field expression

Automatic dereferencing

If the type of the container operand implements Deref or DerefMut depending on whether the operand is mutable, it is automatically dereferenced as many times as necessary to make the field access possible. This process is also called autoderef for short.

Borrowing

The fields of a struct or a reference to a struct are treated as separate entities when borrowing. If the struct does not implement Drop and is stored in a local variable, this also applies to moving out of each of its fields. This also does not apply if automatic dereferencing is done through user-defined types other than Box.

#![allow(unused)]
fn main() {
struct A { f1: String, f2: String, f3: String }
let mut x: A;
x = A {
    f1: "f1".to_string(),
    f2: "f2".to_string(),
    f3: "f3".to_string()
};
let a: &mut String = &mut x.f1; // x.f1 borrowed mutably
let b: &String = &x.f2;         // x.f2 borrowed immutably
let c: &String = &x.f2;         // Can borrow again
let d: String = x.f3;           // Move out of x.f3
}