patterns/idioms/concat-format.md

Concatenating strings with format!

Description

It is possible to build up strings using the push and push_str methods on a mutable String, or using its + operator. However, it is often more convenient to use format!, especially where there is a mix of literal and non-literal strings.

Example

fn say_hello(name: &str) -> String {
    // We could construct the result string manually.
    // let mut result = "Hello ".to_owned();
    // result.push_str(name);
    // result.push('!');
    // result

    // But using format! is better.
    format!("Hello {}!", name)
}

Advantages

Using format! is usually the most succinct and readable way to combine strings.

Disadvantages

It is usually not the most efficient way to combine strings - a series of push operations on a mutable string is usually the most efficient (especially if the string has been pre-allocated to the expected size).