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Strings

String​

  • roughly speaking, a wrapper of Vec<u8>
  • Sized

str​

  • string slice
  • not Sized
    • DST; dynamically sized type
    • can't allocate to stack 
      let mut s = String::with_capacity(5);
      s.push_str("Hello");
      let t: str = s[1..];

      // error[E0277]: the size for values of type `str` cannot be known at compilation time
      //   |
      // 8 |     let t: str = s[1..];
      //   |         ^ doesn't have a size known at compile-time
      //   |
      //   = help: the trait `Sized` is not implemented for `str`
      //   = note: all local variables must have a statically known size
      //   = help: unsized locals are gated as an unstable feature
      // help: consider borrowing here
      //   |
      // 8 |     let t: &str = s[1..];
      //   |            +

&str​

  • fat reference
    • reference storing metadata; a pointer and length of the str
  • Sized

Example​

let mut s = String::with_capacity(5);
s.push_str("Hello");
let t: &str = &s[1..];
                                                                                               
              ┌──────────────────────────────────────────┐                                     
              │                                          │                                     
              │        s: String                         │               t: &str               
       ┌──────▼──────┬────────────┬────────────┐   ┌─────┼─────┐   ┌───────────┬───────────┐   
 Stack │   pointer   │   length   │  capacity  │   │  pointer  │   │  pointer  │  length   │   
       │      │      │     5      │     5      │   │           │   │     │     │    4      │   
       └──────┼──────┴────────────┴────────────┘   └───────────┘   └─────┼─────┴───────────┘   
              │                                     &s: &String          │                     
              │                                                          │                     
              │        str                                               │                     
              │ ┌───────────────┐                                        │                     
              │                                                          │                     
            ┌─▼─┬───┬───┬───┬───┐                                        │                     
       Heap │ H │ e │ l │ l │ o │                                        │                     
            └───┴─▲─┴───┴───┴───┘                                        │                     
                  │                                                      │                     
                  │                                                      │                     
                  └──────────────────────────────────────────────────────┘

References​