Modules 101 question

1

Explaining Rust’s Modules: Answering once and for all, “Where should we put these files?”

After reading this piece. I'm still trying to answer “Where should I put mod files?”

For the directory structure

my_project
|-- src
       -- main.rs
|-- target 
|-- .gitignore
|-- Cargo.lock
|-- Cargo.toml

? Where do I put file my_module.rs, so that all subsequent cargo-created files main.rs can access my_module.rs using the statement ' mod my_module ;' ?

2

Here's an example with four files:

// src/main.rs
pub mod my_module1;
pub mod my_module2;

fn main() {
    my_module1::func1();
}

// src/my_module1.rs
use crate::my_module2::func2;

pub fn func1() {
    func2();
}

pub fn func4() {
    println!("Hello world!");
}

// src/my_module2/mod.rs
mod my_submodule;

pub fn func2() {
    my_submodule::func3();
}

// src/my_module2/my_submodule.rs
use crate::my_module1;

pub fn func2() {
    my_module1::func4();
}

To be clear, each file should be mentioned by a mod statement exactly once, except for main.rs which should be mentioned exactly zero times. Any other accesses to the file should go through a use statement.

3

Many thanks,,,What you refer to as src/main.rs is clearly under my_project/src/main.rs

? Do I place my_module1.rs and my_module2.rs under
my_project1 /src /my_module1.rs and
my_project2 /src /my_module2.rs
.. to ensure they are available to all future created cargo projects ?

To reduce directory clutter, is it possible to store all module files under a single directory " my_modules / src/ which contains
my_mod1.rs, my_mod2.rs, ...etc

4

So you have some code that you want to re-use across multiple Cargo projects? The way to do this is to make a library:

cargo new --lib my_library

Instead of a src/main.rs, this will have a src/lib.rs as its “root” module. Any types/functions/submodules in this root module that are marked pub will be usable from within other Cargo projects. For example:

// my_library/src/lib.rs
pub fn do_stuff() {
   println!("Hi");
}

To use this code from one of your binary projects like my_project, you can put the following in my_project/Cargo.toml:

[dependencies]
my_library = { path = "../my_library" }

(The path value can be either an absolute path or a relative path to the my_library directory.)

Now, everywhere within my_project, you can access public items from the my_library crate, just like you can from other libraries:

// my_project/src/main.rs
fn main() {
    my_library::do_stuff();
}
5

Looks like I should forget about my using modules and switch over to libraries instead.

6

Note that you can still use modules to organize code within each crate (whether it's a binary or library). But a library is the way to share code used by multiple crates.

For example, you could put all your common code into a single library, divided into different modules. And then all your binary projects could depend on that library.

7

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