In this blog post, let's explore key differences between String and str and when to use what.
String is a heap-allocated, growable data structure.
str is an immutable, fixed-length sequence of characters stored in memory (heap/stack).
String
A String can be created either using String::from method or .to_string() method as below:
// Creating new String variable s1 using String::from method let mut s1: String = String::from("Sivakumar"); println!("{}", s1); // Creating new String variable s2 using to_string() method let s2: String = "Sivakumar".to_string(); println!("{}", s2); // Calling len() and capacity() methods on string variable println!("s1 variable's Length: {}, Capacity: {}", s1.len(), s1.capacity()); // Calling len() and capacity() methods on string variable println!("s2 variable's Length: {}, Capacity: {}", s2.len(), s2.capacity()); // Adding more data to existing string variable s1.push_str(" is working in Singapore!!!"); println!("{}", s1); When we run the above code, we'll get following output:
Sivakumar Sivakumar s1 variable's Length: 9, Capacity: 9 s2 variable's Length: 9, Capacity: 9 Sivakumar is working in Singapore!!! str
str variable can be created just by declaring a string literal as below and it will always be immutable.
// str typed variable can be created as below let str1 = "Sivakumar"; println!("{}", str1); // Calling len() method on str variable println!("str1 variable's Length: {}", str1.len()); When we run the above code, we'll get following output:
Sivakumar s1 variable's Length: 9 If you notice above, rust won't allow you to declare str variable with its specific type. Instead, rust will infer the data type for you.
And also, there is no capacity() method available on str variable.
If you try to declare str as mutable, you'll end up getting warning as below
// str typed variable can be created as below let mut str1 = "Sivakumar"; println!("{}", str1); // Calling len() method on str variable println!("str1 variable's Length: {}", str1.len()); warning: variable does not need to be mutable --> src\main.rs:23:9 | 23 | let mut str1 = "Sivakumar"; | ----^^^^ | | | help: remove this `mut` | = note: `#[warn(unused_mut)]` on by default warning: 1 warning emitted Finished dev [unoptimized + debuginfo] target(s) in 0.88s Running `target\debug\example.exe` Sivakumar str1 variable's Length: 9 When we have a function which accepts slice of characters, you can pass String referenced variable as an argument.
fn print_len(s: &str) { println!("Length of variable: {}", s.len()); } // str typed variable can be created as below let str1 = "Sivakumar"; println!("{}", str1); // Creating new String variable s1 using String::from method let s1: String = String::from("Sivakumar"); println!("{}", s1); print_len(str1); print_len(&s1); When you run the above code, you will get following output:
Sivakumar Sivakumar Length of variable: 9 Length of variable: 9 But, when you have a method which accepts, String as argument, passing str variable will result in compiler error
fn print_len(s: String) { println!("Length of variable: {}", s.len()); } // str typed variable can be created as below let str1 = "Sivakumar"; println!("{}", str1); // Creating new String variable s1 using String::from method let s1: String = String::from("Sivakumar"); println!("{}", s1); print_len(str1); print_len(s1); Output:
error[E0308]: mismatched types --> src\main.rs:34:15 | 34 | print_len(str1); | ^^^^ | | | expected struct `std::string::String`, found `&str` | help: try using a conversion method: `str1.to_string()` error[E0308]: mismatched types --> src\main.rs:36:15 | 36 | print_len(&s1); | ^^^ | | | expected struct `std::string::String`, found `&std::string::String` | help: consider removing the borrow: `s1` error: aborting due to 2 previous errors For more information about this error, try `rustc --explain E0308`. error: could not compile `example`. To learn more, run the command again with --verbose. Please feel free to share your comments.
Happy reading!!!
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