src/doc/nomicon/checked-uninit.md

   1 % Checked Uninitialized Memory
   2
   3 Like C, all stack variables in Rust are uninitialized until a value is
   4 explicitly assigned to them. Unlike C, Rust statically prevents you from ever
   5 reading them until you do:
   6
   7 ```rust,ignore
   8 fn main() {
   9     let x: i32;
  10     println!("{}", x);
  11 }
  12 ```
  13
  14 ```text
  15 src/main.rs:3:20: 3:21 error: use of possibly uninitialized variable: `x`
  16 src/main.rs:3     println!("{}", x);
  17                                  ^
  18 ```
  19
  20 This is based off of a basic branch analysis: every branch must assign a value
  21 to `x` before it is first used. Interestingly, Rust doesn't require the variable
  22 to be mutable to perform a delayed initialization if every branch assigns
  23 exactly once. However the analysis does not take advantage of constant analysis
  24 or anything like that. So this compiles:
  25
  26 ```rust
  27 fn main() {
  28     let x: i32;
  29
  30     if true {
  31         x = 1;
  32     } else {
  33         x = 2;
  34     }
  35
  36     println!("{}", x);
  37 }
  38 ```
  39
  40 but this doesn't:
  41
  42 ```rust,ignore
  43 fn main() {
  44     let x: i32;
  45     if true {
  46         x = 1;
  47     }
  48     println!("{}", x);
  49 }
  50 ```
  51
  52 ```text
  53 src/main.rs:6:17: 6:18 error: use of possibly uninitialized variable: `x`
  54 src/main.rs:6   println!("{}", x);
  55 ```
  56
  57 while this does:
  58
  59 ```rust
  60 fn main() {
  61     let x: i32;
  62     if true {
  63         x = 1;
  64         println!("{}", x);
  65     }
  66     // Don't care that there are branches where it's not initialized
  67     // since we don't use the value in those branches
  68 }
  69 ```
  70
  71 Of course, while the analysis doesn't consider actual values, it does
  72 have a relatively sophisticated understanding of dependencies and control
  73 flow. For instance, this works:
  74
  75 ```rust
  76 let x: i32;
  77
  78 loop {
  79     // Rust doesn't understand that this branch will be taken unconditionally,
  80     // because it relies on actual values.
  81     if true {
  82         // But it does understand that it will only be taken once because
  83         // we unconditionally break out of it. Therefore `x` doesn't
  84         // need to be marked as mutable.
  85         x = 0;
  86         break;
  87     }
  88 }
  89 // It also knows that it's impossible to get here without reaching the break.
  90 // And therefore that `x` must be initialized here!
  91 println!("{}", x);
  92 ```
  93
  94 If a value is moved out of a variable, that variable becomes logically
  95 uninitialized if the type of the value isn't Copy. That is:
  96
  97 ```rust
  98 fn main() {
  99     let x = 0;
 100     let y = Box::new(0);
 101     let z1 = x; // x is still valid because i32 is Copy
 102     let z2 = y; // y is now logically uninitialized because Box isn't Copy
 103 }
 104 ```
 105
 106 However reassigning `y` in this example *would* require `y` to be marked as
 107 mutable, as a Safe Rust program could observe that the value of `y` changed:
 108
 109 ```rust
 110 fn main() {
 111     let mut y = Box::new(0);
 112     let z = y; // y is now logically uninitialized because Box isn't Copy
 113     y = Box::new(1); // reinitialize y
 114 }
 115 ```
 116
 117 Otherwise it's like `y` is a brand new variable.