How can I create a function with a variable number of arguments in Rust?
Like this Java code:
void foo(String... args) {
for (String arg : args) {
System.out.println(arg);
}
}
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3 Answers
In general, you can't - Rust does not support variadic functions, except when interoperating with C code that uses varargs.
In this case, since all of your arguments are the same type, you can accept a slice:
fn foo(args: &[&str]) {
for arg in args {
println!("{}", arg);
}
}
fn main() {
foo(&["hello", "world", "I", "am", "arguments"]);
}
Beyond that, you can explicitly accept optional arguments:
fn foo(name: &str, age: Option<u8>) {
match age {
Some(age) => println!("{} is {}.", name, age),
None => println!("Who knows how old {} is?", name),
}
}
fn main() {
foo("Sally", Some(27));
foo("Bill", None);
}
If you need to accept many arguments, optional or not, you can implement a builder:
struct Arguments<'a> {
name: &'a str,
age: Option<u8>,
}
impl<'a> Arguments<'a> {
fn new(name: &'a str) -> Arguments<'a> {
Arguments {
name: name,
age: None
}
}
fn age(self, age: u8) -> Self {
Arguments {
age: Some(age),
..self
}
}
}
fn foo(arg: Arguments) {
match arg.age {
Some(age) => println!("{} is {}.", arg.name, age),
None => println!("Who knows how old {} is?", arg.name),
}
}
fn main() {
foo(Arguments::new("Sally").age(27));
foo(Arguments::new("Bill"));
}
Comments
In the general case where your types may differ, you can use a macro:
macro_rules! print_all {
($($args:expr),*) => {{
$(
println!("{}", $args);
)*
}}
}
fn main() {
print_all!(1, 2, "Hello");
}
As another example, if you want to fold over the arguments, you can do something like this:
macro_rules! sum {
($($args:expr),*) => {{
let result = 0;
$(
let result = result + $args;
)*
result
}}
}
fn main() {
assert_eq!(sum!(1, 2, 3), 6);
}
See the Rust book's simplified implementation of vec![...] for another example.
Comments
fn variable_func<T>(_vargs: &[T]) {}
fn main() {
variable_func(&[1]);
variable_func(&[1, 2]);
variable_func(&["A", "B", "C"]);
}
5 Comments
creativcoder
@Shepmaster Compiles now. Yes, i agree on the allocation part. Is there a similar way without allocation ?
creativcoder
ah yes, the macro is really not needed here. I will modify the answer according to your comment.
qiuxiafei
what if i want the function take the ownership of the array?
kbolino
@qiuxiafei unless you can hard-code the size of the array (thus making it not variadic), or you write a variant for every
[T; N] you want to support, you'll have to give it a Vec<T>qiuxiafei
@kbolino yes, i thinks Vec<T> is the only solution.