Rust ic_cdk: Minimal example of passing argument to `main::install_code` and decoding inside the target canister

Developers Rust
1

The install_code function provided by the Rust CDK has a mechanism to pass arguments to the target canister as part of the InstallCodeArgument as documented here.

Is there a minimal example I can refer that demonstrates how to use this to correctly encode and decode the argument that I wish to pass. I wish to be able to refer to that value in the post_upgrade as documented is possible

The argument that I wish to pass is a custom struct containing some primitive values.

Thoughts?

2

I think you just need to candid encode the struct, e.g. with candid - Rust, but I haven’t done this myself.

3

Thank you. Let me try and come back with findings

4

For someone else stumbling onto this,

The solution is to encode using candid::encode_args as demonstrated in the example

And on the receiving end, use call::arg_data. Specifically, use the turbofish syntax to parse it to the exact shape you expect it to be.

Btw, it also requires them to be in a tuple even if passing a single argument.

This is what my code looks like:

Sender side

let arg = candid::encode_args((version_number,))
                .expect("Failed to serialize the install argument.");

Receiving side

call::arg_data::<(u64, )>().0
5

On the receiving side, if you are using #[ic_cdk_macros::init], you can just add a function parameter, same as with query or update.

6

Oh, that’s quite cool that it’s taken care of by the CDK macro.

We really need to better document these nuggets of information

EDIT: It’s already documented here

7

It is documented in the init macro’s docs :wink:

8

Hey, I’ve played with this before. Here’s how I handled it:

  1. In code, from rust to rust. You can create a struct however you’d like, and pass it in install_code, like so: quickstart_scaling/businesslogic.rs at 3c9a65ee19c0fa458d4ddc8582c76d7bd6dbdbdf · GLicDEV/quickstart_scaling · GitHub

On the receiving side, you get it like so: quickstart_scaling/lifetime.rs at 3c9a65ee19c0fa458d4ddc8582c76d7bd6dbdbdf · GLicDEV/quickstart_scaling · GitHub

  1. For compatibility with dfx, and for automatic did generation, you can append your init function with the proper macro, as described here: How to pass install arguments from dfx?

Hope this helps