85 lines
1.8 KiB
Markdown
85 lines
1.8 KiB
Markdown
% Checked Uninitialized Memory
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Like C, all stack variables in Rust are uninitialized until a
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value is explicitly assigned to them. Unlike C, Rust statically prevents you
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from ever reading them until you do:
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```rust
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fn main() {
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let x: i32;
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println!("{}", x);
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}
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```
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```text
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src/main.rs:3:20: 3:21 error: use of possibly uninitialized variable: `x`
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src/main.rs:3 println!("{}", x);
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^
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```
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This is based off of a basic branch analysis: every branch must assign a value
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to `x` before it is first used. Interestingly, Rust doesn't require the variable
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to be mutable to perform a delayed initialization if every branch assigns
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exactly once. However the analysis does not take advantage of constant analysis
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or anything like that. So this compiles:
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```rust
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fn main() {
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let x: i32;
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if true {
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x = 1;
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} else {
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x = 2;
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}
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println!("{}", x);
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}
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```
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but this doesn't:
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```rust
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fn main() {
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let x: i32;
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if true {
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x = 1;
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}
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println!("{}", x);
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}
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```
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```text
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src/main.rs:6:17: 6:18 error: use of possibly uninitialized variable: `x`
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src/main.rs:6 println!("{}", x);
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```
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while this does:
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```rust
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fn main() {
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let x: i32;
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if true {
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x = 1;
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println!("{}", x);
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}
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// Don't care that there are branches where it's not initialized
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// since we don't use the value in those branches
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}
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```
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If a value is moved out of a variable, that variable becomes logically
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uninitialized if the type of the value isn't Copy. That is:
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```rust
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fn main() {
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let x = 0;
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let y = Box::new(0);
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let z1 = x; // x is still valid because i32 is Copy
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let z2 = y; // y is now logically uninitialized because Box isn't Copy
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}
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```
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However reassigning `y` in this example *would* require `y` to be marked as
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mutable, as a Safe Rust program could observe that the value of `y` changed.
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Otherwise the variable is exactly like new.
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