Testcase: List
Implementing fmt::Display for a structure where the elements must each be
handled sequentially is tricky. The problem is that each write! generates a
fmt::Result. Proper handling of this requires dealing with all the
results. Rust provides the ? operator for exactly this purpose.
Using ? on write! looks like this:
// Try `write!` to see if it errors. If it errors, return
// the error. Otherwise continue.
write!(f, "{}", value)?;
With ? available, implementing fmt::Display for a Vec is
straightforward:
use std::fmt; // Import the `fmt` module. // Define a structure named `List` containing a `Vec`. struct List(Vec<i32>); impl fmt::Display for List { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { // Extract the value using tuple indexing, // and create a reference to `vec`. let vec = &self.0; write!(f, "[")?; // Iterate over `v` in `vec` while enumerating the iteration // index in `index`. for (index, v) in vec.iter().enumerate() { // For every element except the first, add a comma. // Use the ? operator to return on errors. if index != 0 { write!(f, ", ")?; } write!(f, "{}", v)?; } // Close the opened bracket and return a fmt::Result value. write!(f, "]") } } fn main() { let v = List(vec![1, 2, 3]); println!("{}", v); }
Activity
Try changing the program so that the index of each element in the vector is also printed. The new output should look like this:
[0: 1, 1: 2, 2: 3]