Parameters

Parameter passing.

C++

Rust

Java


int multiply(int arg1, int &arg2)
{
    int ret = arg1 * arg2;

    arg1 = ret;
    arg2 = ret;

    return ret;
}

int main(unused int argc, unused char **argv)
{
    int arg1 = 2;
    int arg2 = 3;

    int ret = multiply(arg1, arg2);

    // prints 2 because it's passed by value
    cout << "ARG1: " << arg1 << endl;

    // prints 6 because it's passed by reference
    cout << "ARG2: " << arg2 << endl;

    // prints 6
    cout << "RET: " << ret << endl;

    return 0;
}



fn multiply(mut arg1: isize, arg2: &mut isize) -> (isize)
{
    let ret = arg1 * *arg2;

    arg1 = ret;
    *arg2 = ret;

    return ret;
}

fn main()
{
    let arg1: isize = 2;
    let mut arg2: isize = 3;

    let ret = multiply(arg1, &mut arg2);

    // prints 2 because it's passed by value
    println!("ARG1: {}", arg1);

    // prints 6 because it's passed by mutable reference
    println!("ARG2: {}", arg2);

    // prints 6
    println!("RET: {}", ret);
}



public static int multiply(int arg1, int arg2)
{
    int ret = arg1 * arg2;

    arg1 = ret;
    arg2 = ret;

    return ret;
}

public static void main(String[] args)
{
    int arg1 = 2;
    int arg2 = 3;

    int ret = multiply(arg1, arg2);

    // prints 2 because it's passed by value
    System.out.println("ARG1: " + arg1);

    // prints 3 because it's passed by value
    System.out.println("ARG2: " + arg2);

    // prints 6
    System.out.println("RET: " + ret);


All three languages default to pass-by-value if not otherwise specified.

C++ can specify a parameter as a reference, pointer, or pointer to a pointer. In this example, only pass-by-value and a reference is shown.

With Rust you must specify the mutability of a parameter even though it is passed by value and cannot change. For example, if arg1 isn't marked as mut, then the code will not compile as an attempt to change an immutable value is made. Even though it is marked as mutable, the value doesn't change because it's pass-by-value. The second parameter must be marked as both a reference (&) and mutable (mut) to be able to change it's value.

With Java there is no way to change the value of a primitive like int. Integer is also immutable so it cannot be changed either. One "work-around" is to pass parameters via an array of size one. It is a reference to an array that is passed by value into the function, so the contents of the array can be modified.