src/doc/reference/src/expressions/field-expr.md

   1 # Field access expressions
   2
   3 > **<sup>Syntax</sup>**\
   4 > _FieldExpression_ :\
   5 > &nbsp;&nbsp; [_Expression_] `.` [IDENTIFIER]
   6
   7 A _field expression_ consists of an expression followed by a single dot and an [identifier], when not immediately followed by a parenthesized expression-list (the latter is always a [method call expression]).
   8 A field expression denotes a field of a [struct] or [union].
   9 To call a function stored in a struct, parentheses are needed around the field expression.
  10
  11 <!-- ignore: needs lots of support code -->
  12 ```rust,ignore
  13 mystruct.myfield;
  14 foo().x;
  15 (Struct {a: 10, b: 20}).a;
  16 mystruct.method();          // Method expression
  17 (mystruct.function_field)() // Call expression containing a field expression
  18 ```
  19
  20 A field access is a [place expression] referring to the location of that field.
  21 When the subexpression is [mutable], the field expression is also mutable.
  22
  23 Also, if the type of the expression to the left of the dot is a pointer, it is automatically dereferenced as many times as necessary to make the field access possible.
  24 In cases of ambiguity, we prefer fewer autoderefs to more.
  25
  26 Finally, the fields of a struct or a reference to a struct are treated as separate entities when borrowing.
  27 If the struct does not implement [`Drop`](../special-types-and-traits.md#drop) and is stored in a local variable, this also applies to moving out of each of its fields.
  28 This also does not apply if automatic dereferencing is done though user defined types other than [`Box`](../special-types-and-traits.html#boxt).
  29
  30 ```rust
  31 struct A { f1: String, f2: String, f3: String }
  32 let mut x: A;
  33 # x = A {
  34 #     f1: "f1".to_string(),
  35 #     f2: "f2".to_string(),
  36 #     f3: "f3".to_string()
  37 # };
  38 let a: &mut String = &mut x.f1; // x.f1 borrowed mutably
  39 let b: &String = &x.f2;         // x.f2 borrowed immutably
  40 let c: &String = &x.f2;         // Can borrow again
  41 let d: String = x.f3;           // Move out of x.f3
  42 ```
  43
  44 [_Expression_]: ../expressions.md
  45 [IDENTIFIER]: ../identifiers.md
  46 [method call expression]: method-call-expr.md
  47 [struct]: ../items/structs.md
  48 [union]: ../items/unions.md
  49 [place expression]: ../expressions.md#place-expressions-and-value-expressions
  50 [mutable]: ../expressions.md#mutability