Rust Structs and Functions

In Rust, structs and functions are both very popular and it is common to combine them in various ways.

The two most common (and basic) are:

1. Passing structs into a function as an argument.
2. Returning a struct from a function.

In this tutorial, we will cover both of these cases and learn how functions can be helpful when working with structs.

Passing a Struct to a Function

We can pass a struct into a function using the name of the struct.

In the function definition, we use the name of the struct to reference the data type of the parameter:

fn my_function(s: StructName) {}

In this example, s is the parameter and it has a type of StructName.

When we call the function, we provide the struct instance as the argument, which gets passed into the function.

my_function(struct_instance);

In the following example, we use the show_student() function to print out a student’s details in a useful way.

// Declaring a struct called Student:

struct Student {
    name: String,
    age: i32,
    average: i32,
}


// Creating an instance of Student called 'bill':

let bill = Student {
    age: 61,
    name: "Bill".to_string(),
    average: 83,
};


// Creating a function that shows the student details:

fn show_student(s: Student) {
    println!("Name: {} , Age: {} , Average: {} ", s.name, s.age, s.average);
}


// Calling the show_student() function:

show_student(bill);

Standard Output:

Name: Bill , Age: 61 , Average: 83

Returning a Struct From a Function

We can return a struct from a function by specifying the struct name as the type to be returned, in the function definition.

fn my_function() -> StructName {}

The output of this function is a struct. If we want to do something with the struct then we will need to call the function from another function or macro.

We can also save the output as a new variable, and then work with that variable later on. In the following example, we are saving the output of the get_student() function into the variable ‘which_student’.

struct Student {
    name: String,
    age: i32,
    average: i32,
}

let bill = Student {
    name: "Bill".to_string(),
    age: 61,
    average: 83,
};

let steve = Student {
    name: "Steve".to_string(),
    age: 26,
    average: 79,
};

fn get_student(s1: Student, s2: Student) -> Student {
    if s1.name == "Bill" {
        return s1;
    }
    else {
        return s2;
    }
}

    let which_student = get_student(bill, steve);
    println!("{} , {} , {}", which_student.name, which_student.age, which_student.average);