Function std::io::read_to_string

1.65.0 · source ·
pub fn read_to_string<R: Read>(reader: R) -> Result<String>
Expand description

Read all bytes from a reader into a new String.

This is a convenience function for Read::read_to_string. Using this function avoids having to create a variable first and provides more type safety since you can only get the buffer out if there were no errors. (If you use Read::read_to_string you have to remember to check whether the read succeeded because otherwise your buffer will be empty or only partially full.)


The downside of this function’s increased ease of use and type safety is that it gives you less control over performance. For example, you can’t pre-allocate memory like you can using String::with_capacity and Read::read_to_string. Also, you can’t re-use the buffer if an error occurs while reading.

In many cases, this function’s performance will be adequate and the ease of use and type safety tradeoffs will be worth it. However, there are cases where you need more control over performance, and in those cases you should definitely use Read::read_to_string directly.

Note that in some special cases, such as when reading files, this function will pre-allocate memory based on the size of the input it is reading. In those cases, the performance should be as good as if you had used Read::read_to_string with a manually pre-allocated buffer.


This function forces you to handle errors because the output (the String) is wrapped in a Result. See Read::read_to_string for the errors that can occur. If any error occurs, you will get an Err, so you don’t have to worry about your buffer being empty or partially full.


fn main() -> io::Result<()> {
    let stdin = io::read_to_string(io::stdin())?;
    println!("Stdin was:");