Background

In Rust, let us cover how to print out the contents of the struct data structure.

In this post we will use the fmt::Debug trait.

Definition Statement

Rust-Lang.org

Debug

Link

All types which want to use std::fmt formatting traits require an implementation to be printable.

Automatic implementations are only provided for types such as in the std library.

All others must be manually implemented somehow.

The fmt::Debug trait makes this very straightforward.

All types can derive (automatically create) the fmt::Debug implementation.

This is not true for fmt::Display which must be manually implemented.

Formatting Code

Type	Format Specifier	Sample	Sample Explanation
Print	{:?}	{0:?}	0 means the first variable
Pretty Print	{:#?}	{1:#?}	1 means the second variable

Code

Original

	/*
	Specify that structure implements the Debug trait
	i.e. we will be able to print structure contents by using {:?} specifier in our print statements
	*/
	//#[derive(Debug)]
	struct Person
	{
	name: String
	, age: u8
	}


	fn main()
	{

	// Create struct with field init shorthand

	//String Literals ( static )
	let nameHardCoded = "Peter 1";

	let age1 = 27;

	let objPeter1 = Person
	{
	name:nameHardCoded.to_string()
	, age:age1
	};


	//Debug Basic Print Person 1
	println!
	(
	"{0:?}"
	, objPeter1
	);

	println!("");
	println!("");


	//Debug Pretty Print Person 1
	println!
	(
	"{0:#?}"
	, objPeter1
	);


	println!("");
	println!("");

	/*

	//Print Person 1 along with other data
	println!
	(
	"variable contents \r\n \r\n {0:#?} \r\n \r\n variable name {1} \r\n \r\n age {2}"
	, objPeter1
	, "objPeter1"
	, 30
	);

	*/

	}

view raw

rustdatastructurestructprintdebugfailure.rs

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Output

Output – Image

Output – Text

error[E0277]: `Person` doesn't implement `Debug`
   --&amp;gt; main.rs:113:11
    |
113 |         , objPeter2
    |           ^^^^^^^^^ `Person` cannot be formatted using `{:?}`
    |
    = help: the trait `Debug` is not implemented for `Person`
    = note: add `#[derive(Debug)]` to `Person` or manually `impl Debug for Person`
    = note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)
error: aborting due to previous error
For more information about this error, try `rustc --explain E0277`

Explanation

The format string {:?} relies on the Debug trait being implemented.

Here are the option for implementing it:-

#[derive(Debug)]
- Prefix the data type with the #[derive(Debug)] keyword
impl Debug for Person
- Add an implementation keyword

Revision

Outline

Enable Debug implementation
- #[derive(Debug)]

Enable Debug Implementation

Code

	/*
	Specify that structure implements the Debug trait
	i.e. we will be able to print structure contents by using {:?} specifier in our print statements
	*/
	#[derive(Debug)]
	struct Person
	{
	name: String
	, age: u8
	}


	fn main()
	{

	// Create struct with field init shorthand

	//String Literals ( static )
	let nameHardCoded = "Peter 1";

	let age1 = 27;

	let objPeter1 = Person
	{
	name:nameHardCoded.to_string()
	, age:age1
	};


	//Debug Basic Print Person 1
	println!
	(
	"{0:?}"
	, objPeter1
	);

	println!("");
	println!("");


	//Debug Pretty Print Person 1
	println!
	(
	"{0:#?}"
	, objPeter1
	);


	println!("");
	println!("");


	}

view raw

rustdatastructurestructprintdebugsuccess.rs

hosted with ❤ by GitHub

Output

Output – Image

Explanation

The format string {:?} and {:#?} relies on the Debug trait being implemented.

Here are the option for implementing it:-

#[derive(Debug)]
- Prefix the data type with the #[derive(Debug)] keyword
- We remove the comments that prefixed the #[derive(Debug)] keyword
Format Specifiers
- {:?}
  - Print
- {:#?}
  - Pretty Print

Online Software Development

OnlineGDB

rustdatastructurestructprintdebug.rs
Link

References

Rust By Example
- Module std::fmt
  Link
- Debug
  Link

One thought on “Rust:- Data Structure – struct – print – debug implementation ( fmt::Debug )”

Rust:- Data Structure – struct – print – display implementation ( fmt::Display ) | Learning in the Open says:

November 23, 2022 at 3:34 pm

[…] Rust:- Data Structure – struct – print – debug implementation ( fmt::Debug ) Link […]

Background

Definition Statement

Rust-Lang.org

Debug

Formatting Code

Code

Original

Output

Output – Image

Output – Text

Explanation

Revision

Outline

Enable Debug Implementation

Code

Output

Output – Image

Explanation

Online Software Development

OnlineGDB

References

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