{"id":7723,"date":"2019-11-23T20:31:13","date_gmt":"2019-11-23T20:31:13","guid":{"rendered":"http:\/\/putridparrot.com\/blog\/?p=7723"},"modified":"2019-11-23T20:31:13","modified_gmt":"2019-11-23T20:31:13","slug":"println-structs","status":"publish","type":"post","link":"https:\/\/putridparrot.com\/blog\/println-structs\/","title":{"rendered":"println! structs"},"content":{"rendered":"

So we’ve got ourselves a simply little struct<\/p>\n

\r\nstruct Point {\r\n    x: i32,\r\n    y: i32\r\n}\r\n<\/pre>\n
We then decide that we’d like to output the current state of the Point using println!, so we write<\/p>\n
\r\nfn main() {\r\n    let p = Point { x: 20, y: 3 };\r\n\r\n    println!("{}", p);\r\n}\r\n<\/pre>\n
Running this will result in `Point` doesn’t implement `std::fmt::Display`<\/em> and `Point` cannot be formatted with the default formatter<\/em>. In fact we do not really need to implement std::fmt::Display, we can just annotate our struct with #[derive(Debug)]<\/em> and then use the println! formatters (:? or :#?), for example<\/p>\n
\r\n#[derive(Debug)]\r\nstruct Point {\r\n    x: i32,\r\n    y: i32\r\n}\r\n\r\nfn main() {\r\n    let p = Point { x: 20, y: 3 };\r\n\r\n    println!("{:?}", p);\r\n}\r\n<\/pre>\n
The use of :? will result in the following output Point { x: 20, y: 3 }<\/em> whereas :#? will display the values on lines of their own (a “prettier” formatter). Both :? and :#? are debug formatters, hence require the annotation #[derive(Debug)]<\/em> or we can implement std::fmt::Debug, for example<\/p>\n
\r\nimpl std::fmt::Debug for Point {\r\n    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {\r\n        write!(f, "(x is {}, y is {})", self.x, self.y)\r\n    }\r\n}\r\n<\/pre>\n
For situations where we simply want to create our own custom display (not just for Debug), then, as per the original error `Point` doesn’t implement `std::fmt::Display`<\/em>, we would need to implement the std::fmt::Display trait, i.e.<\/p>\n
\r\nimpl std::fmt::Display for Point {\r\n    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {\r\n        write!(f, "(x is {}, y is {})", self.x, self.y)\r\n    }\r\n}\r\n<\/pre>\n
This means we no longer requiring the annotation or special formatters, hence our full code will look like this<\/p>\n
\r\nstruct Point {\r\n    x: i32,\r\n    y: i32\r\n}\r\n\r\nimpl std::fmt::Display for Point {\r\n    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {\r\n        write!(f, "(x is {}, y is {})", self.x, self.y)\r\n    }\r\n}\r\n\r\nfn main() {\r\n    let p = Point { x: 20, y: 3 };\r\n\r\n    println!("{}", p);\r\n}\r\n<\/pre>\n
and as you’d expect our output is now (x is 20, y is 3)<\/em>.<\/p>\n","protected":false},"excerpt":{"rendered":"
So we’ve got ourselves a simply little struct struct Point { x: i32, y: i32 } We then decide that we’d like to output the current state of the Point using println!, so we write fn main() { let p = Point { x: 20, y: 3 }; println!("{}", p); } Running this will result […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[191],"tags":[],"class_list":["post-7723","post","type-post","status-publish","format-standard","hentry","category-rust"],"jetpack_sharing_enabled":true,"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts\/7723","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/comments?post=7723"}],"version-history":[{"count":3,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts\/7723\/revisions"}],"predecessor-version":[{"id":7732,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts\/7723\/revisions\/7732"}],"wp:attachment":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/media?parent=7723"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/categories?post=7723"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/tags?post=7723"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}