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Constants

Constants are values that are bound to a name and cannot change. They are always immutable. In Rust, constants are declared with the const keyword. Unlike variables declared with the let keyword, constants must be annotated with their type.

Constants are valid for the entire length of the transaction. They are essentially inlined wherever they are used, meaning that their value is copied directly into whatever context invokes them.

Since their value is hardcoded, they can save on gas cost as their value does not need to be fetched from storage.

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src/lib.rs​

// Only run this as a WASM if the export-abi feature is not set.
#![cfg_attr(not(any(feature = "export-abi", test)), no_main)]
extern crate alloc;

use alloc::vec;
use alloc::vec::Vec;

use stylus_sdk::alloy_primitives::Address;
use stylus_sdk::prelude::*;
use stylus_sdk::storage::StorageAddress;

const OWNER: &str = "0xd8dA6BF26964aF9D7eEd9e03E53415D37aA96045";

#[storage]
#[entrypoint]
pub struct Contract {
    owner: StorageAddress,
}

#[public]
impl Contract {
    pub fn init(&mut self) -> Result<(), Vec<u8>> {
        // Parse the const &str as a local Address variable
        let owner_address = Address::parse_checksummed(OWNER, None).expect("Invalid address");

        // Save the result as the owner
        self.owner.set(owner_address);

        Ok(())
    }
    pub fn owner(&self) -> Result<Address, Vec<u8>> {
        let owner_address = self.owner.get();

        Ok(owner_address)
    }
}

Cargo.toml​

[package]
name = "stylus_constants_example"
version = "0.1.7"
edition = "2021"
license = "MIT OR Apache-2.0"
keywords = ["arbitrum", "ethereum", "stylus", "alloy"]

[dependencies]
alloy-primitives = "0.7.6"
alloy-sol-types = "0.7.6"
mini-alloc = "0.4.2"
stylus-sdk = "0.6.0"
hex = "0.4.3"

[dev-dependencies]
tokio = { version = "1.12.0", features = ["full"] }
ethers = "2.0"
eyre = "0.6.8"

[features]
export-abi = ["stylus-sdk/export-abi"]

[lib]
crate-type = ["lib", "cdylib"]

[profile.release]
codegen-units = 1
strip = true
lto = true
panic = "abort"
opt-level = "s"