Lesson 11-Implement

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Applying the impl keyword to a struct in Rust allows you to add methods to it. This provides functionality similar to class structures, making it a very useful feature.

struct User {
  first: String,
  last: String,
  age: u32
}

impl User {
  fn full_name (&self) -> String {
    format!("{} {}", self.first, self.last)
  }
}

To use this feature, a struct must first be defined. Each function within the impl block must have &self as its first argument. This argument provides access to the struct’s data.

Usage and Flexibility

The impl block can contain multiple functions and can be declared multiple times for the same struct. This allows for modular organization of your code.

impl User {
  fn full_name (&self) -> String {
    format!("{} {}", self.first, self.last)
  }
  fn all(&self) -> String {
    format!("{} {} {}", self.first, self.last, self.age)
  }
}

// Alternatively
impl User {
  fn full_name (&self) -> String {
    format!("{} {}", self.first, self.last)
  }
}

impl User {
  fn all(&self) -> String {
    format!("{} {} {}", self.first, self.last, self.age)
  }
}

Restrictions: No Duplicate Function Definitions

It is not permitted to define functions with the same name within the same impl block or across multiple impl blocks for the same struct. This includes attempting to overload functions by changing the parameter list.

impl User {
  fn full_name (&self) -> String {
    format!("{} {}", self.first, self.last)
  }
}

impl User {
  fn full_name (&self) -> String {
    format!("{} {}", self.last, self.first)
  }
}

// Result
   |
8  | /   fn full_name (&self) -> String {
9  | |     format!("{} {}", self.first, self.last)
10 | |   }
   | |___^ duplicate definitions for `full_name`
...
14 | /   fn full_name (&self, n: i32) -> String {
15 | |     format!("{} {}", self.last, self.first)
16 | |   }
   | |___- other definition for `full_name`

Attempting to redefine a function name, even with different parameter types, will result in a compilation error due to the duplicate definition.

impl User {
  fn full_name (&self) -> String {
    format!("{} {}", self.first, self.last)
  }
}

impl User {
  fn full_name (&self, n: i32) -> String {
    format!("{} {}", self.last, self.first)
  }
}

// Result
   |
8  | /   fn full_name (&self) -> String {
9  | |     format!("{} {}", self.first, self.last)
10 | |   }
   | |___^ duplicate definitions for `full_name`
...
14 | /   fn full_name (&self, n: i32) -> String {
15 | |     format!("{} {}", self.last, self.first)
16 | |   }
   | |___- other definition for `full_name`

Additional Arguments

Methods within impl blocks can accept arguments in addition to &self. To pass values to these arguments, you call the function as you would call any other method. The &self argument is implicitly passed.

impl User {
  fn is_older (&self, age:u32) -> bool {
    self.age > age
  }
}

fn main() {
    let user = User { first: "John".to_string(), last: "Doe".to_string(), age: 30 };
    println!("{}", user.is_older(20)); // Correct way to call is_older
}