Moves in Function Calls

When you pass a value to a function, the value is assigned to the function parameter. This transfers ownership:

fn say_hello(name: String) {
    println!("Hello {name}")
}

fn main() {
    let name = String::from("Alice");
    say_hello(name);
    // say_hello(name);
}
  • With the first call to say_hello, main gives up ownership of name. Afterwards, name cannot be used anymore within main.
  • The heap memory allocated for name will be freed at the end of the say_hello function.
  • main can retain ownership if it passes name as a reference (&name) and if say_hello accepts a reference as a parameter.
  • Alternatively, main can pass a clone of name in the first call (name.clone()).
  • Rust makes it harder than C++ to inadvertently create copies by making move semantics the default, and by forcing programmers to make clones explicit.