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Skeleton and Rust Architecture

In this article, you'll learn about the basic architecture behind the FT contract that you'll develop while following this "Zero to Hero" series. You'll discover the contract's layout and you'll see how the Rust files are structured in order to build a feature-complete smart contract.

New to Rust?

If you are new to Rust and want to dive into smart contract development, our Quick-start guide is a great place to start.

Introduction​

This tutorial presents the code skeleton for the FT smart contract and its file structure. You'll find how all the functions are laid out as well as the missing Rust code that needs to be filled in. Once every file and function has been covered, you'll go through the process of building the mock-up contract to confirm that your Rust toolchain works as expected.

Files structure​

The repository comes with many different folders. Each folder represents a different milestone of this tutorial starting with the skeleton folder and ending with the finished contract folder. If you step into any of these folders, you'll find that they each follow a regular Rust project. The file structure for these smart contracts have:

  • Cargo.toml file to define the code dependencies (similar to package.json in JavaScript and node projects)
  • src folder where all the Rust source files are stored
  • target folder where the compiled wasm will output to.

Source files​

FileDescription
ft_core.rsContains the logic for transferring and controlling FTs. This file represents the implementation of the core standard.
lib.rsHolds the smart contract initialization functions and dictates what information is kept on-chain.
metadata.rsDefines the metadata structure. This file represents the implementation of the metadata extension of the standard.
storage.rsContains the logic for registration and storage. This file represents the implementation of the storage management standard.
skeleton
├── Cargo.lock
├── Cargo.toml
└── src
    ├── ft_core.rs
    ├── lib.rs
    ├── metadata.rs
    └── storage.rs
tip

Explore the code in our GitHub repository.

ft_core.rs​

Core logic that allows you to transfer FTs between users and query for important information.

MethodDescription
ft_transferTransfers a specified amount of FTs to a receiver ID.
ft_transfer_callTransfers a specified amount of FTs to a receiver and attempts to perform a cross-contract call on the receiver’s contract to execute the ft_on_transfer method. The implementation of this ft_on_transfer method is up to the contract writer. You’ll see an example implementation in the marketplace section of this tutorial. Once ft_on_transfer finishes executing, ft_resolve_transfer is called to check if things were successful or not.
ft_total_supplyReturns the total amount of fungible tokens in circulation on the contract.
ft_balance_ofReturns how many fungible tokens a specific user owns.
ft_on_transferMethod that lives on a receiver's contract. It is called when FTs are transferred to the receiver's contract account via the ft_transfer_call method. It returns how many FTs should be refunded back to the sender.
ft_resolve_transferInvoked after the ft_on_transfer is finished executing. This function will refund any FTs not used by the receiver contract and will return the net number of FTs sent to the receiver after the refund (if any).

You'll learn more about these functions in the circulating supply and transfers sections of the tutorial series.

lib.rs​

This file outlines what information the contract stores and keeps track of.

MethodDescription
new_default_metaInitializes the contract with default metadata so the user doesn't have to provide any input. In addition, a total supply is passed in which is sent to the owner.
newInitializes the contract with the user-provided metadata and total supply.
Keep in mind

The initialization functions (new, new_default_meta) can only be called once.

You'll learn more about these functions in the define a token section of the tutorial series.

metadata.rs​

This file is used to outline the metadata for the Fungible Token itself. In addition, you can define a function to view the contract's metadata which is part of the standard's metadata extension.

NameDescription
FungibleTokenMetadataThis structure defines the metadata for the fungible token.
ft_metadataThis function allows users to query for the token's metadata.

You'll learn more about these functions in the define a token section of the tutorial series.

storage.rs​

Contains the registration logic as per the storage management standard.

MethodDescription
storage_depositPayable method that receives an attached deposit of Ⓝ for a given account. This will register the user on the contract.
storage_balance_boundsReturns the minimum and maximum allowed storage deposit required to interact with the contract. In the FT contract's case, min = max.
storage_balance_ofReturns the total and available storage paid by a given user. In the FT contract's case, available is always 0 since it's used by the contract for registration and you can't overpay for storage.
tip

You'll learn more about these functions in the storage section of the tutorial series.

Building the skeleton​

If you haven't cloned the main repository yet, open a terminal and run:

git clone https://github.com/near-examples/ft-tutorial/

Next, build the skeleton contract with the build script found in the 1.skeleton/build.sh file.

cd ft-tutorial/1.skeleton
cargo near build

Since this source is just a skeleton you'll get many warnings about unused code, such as:

   = note: `#[warn(dead_code)]` on by default

warning: constant is never used: `GAS_FOR_RESOLVE_TRANSFER`
 --> src/ft_core.rs:5:1
  |
5 | const GAS_FOR_RESOLVE_TRANSFER: Gas = Gas(5_000_000_000_000);
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: constant is never used: `GAS_FOR_FT_TRANSFER_CALL`
 --> src/ft_core.rs:6:1
  |
6 | const GAS_FOR_FT_TRANSFER_CALL: Gas = Gas(25_000_000_000_000 + GAS_FOR_RESOLVE_TRANSFER.0);
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: `fungible-token` (lib) generated 25 warnings
    Finished release [optimized] target(s) in 1.93s
✨  Done in 2.03s.

Don't worry about these warnings, you're not going to deploy this contract yet. Building the skeleton is useful to validate that your Rust toolchain works properly and that you'll be able to compile improved versions of this FT contract in the upcoming tutorials.

Conclusion​

You've seen the layout of this FT smart contract, and how all the functions are laid out across the different source files. Using yarn, you've been able to compile the contract, and you'll start fleshing out this skeleton in the next section of the tutorial.

Versioning for this article

At the time of this writing, this example works with the following versions:

  • rustc: 1.77.1
  • near-sdk-rs: 5.1.0 (with enabled legacy feature)
  • cargo-near: 0.13.2