ERC20 Token

Contracts that follow the ERC20 Standard are called ERC20 tokens. They are used to represent fungible assets.

To create an ERC20 conctract, it must implement the following interface:

#[starknet::interface]
trait IERC20<TContractState> {
    fn get_name(self: @TContractState) -> felt252;
    fn get_symbol(self: @TContractState) -> felt252;
    fn get_decimals(self: @TContractState) -> u8;
    fn get_total_supply(self: @TContractState) -> felt252;
    fn balance_of(self: @TContractState, account: ContractAddress) -> felt252;
    fn allowance(
        self: @TContractState, owner: ContractAddress, spender: ContractAddress
    ) -> felt252;
    fn transfer(ref self: TContractState, recipient: ContractAddress, amount: felt252);
    fn transfer_from(
        ref self: TContractState,
        sender: ContractAddress,
        recipient: ContractAddress,
        amount: felt252
    );
    fn approve(ref self: TContractState, spender: ContractAddress, amount: felt252);
    fn increase_allowance(ref self: TContractState, spender: ContractAddress, added_value: felt252);
    fn decrease_allowance(
        ref self: TContractState, spender: ContractAddress, subtracted_value: felt252
    );
}

In Starknet, function names should be written in snake_case. This is not the case in Solidity, where function names are written in camelCase. The Starknet ERC20 interface is therefore slightly different from the Solidity ERC20 interface.

Here's an implementation of the ERC20 interface in Cairo:

#[starknet::contract]
mod erc20 {
    use zeroable::Zeroable;
    use starknet::get_caller_address;
    use starknet::contract_address_const;
    use starknet::ContractAddress;

    #[storage]
    struct Storage {
        name: felt252,
        symbol: felt252,
        decimals: u8,
        total_supply: felt252,
        balances: LegacyMap::<ContractAddress, felt252>,
        allowances: LegacyMap::<(ContractAddress, ContractAddress), felt252>,
    }

    #[event]
    #[derive(Drop, starknet::Event)]
    enum Event {
        Transfer: Transfer,
        Approval: Approval,
    }
    #[derive(Drop, starknet::Event)]
    struct Transfer {
        from: ContractAddress,
        to: ContractAddress,
        value: felt252,
    }
    #[derive(Drop, starknet::Event)]
    struct Approval {
        owner: ContractAddress,
        spender: ContractAddress,
        value: felt252,
    }

    mod Errors {
        const APPROVE_FROM_ZERO: felt252 = 'ERC20: approve from 0';
        const APPROVE_TO_ZERO: felt252 = 'ERC20: approve to 0';
        const TRANSFER_FROM_ZERO: felt252 = 'ERC20: transfer from 0';
        const TRANSFER_TO_ZERO: felt252 = 'ERC20: transfer to 0';
        const BURN_FROM_ZERO: felt252 = 'ERC20: burn from 0';
        const MINT_TO_ZERO: felt252 = 'ERC20: mint to 0';
    }

    #[constructor]
    fn constructor(
        ref self: ContractState,
        recipient: ContractAddress,
        name: felt252,
        decimals: u8,
        initial_supply: felt252,
        symbol: felt252
    ) {
        self.name.write(name);
        self.symbol.write(symbol);
        self.decimals.write(decimals);
        self.mint(recipient, initial_supply);
    }

    #[abi(embed_v0)]
    impl IERC20Impl of super::IERC20<ContractState> {
        fn get_name(self: @ContractState) -> felt252 {
            self.name.read()
        }

        fn get_symbol(self: @ContractState) -> felt252 {
            self.symbol.read()
        }

        fn get_decimals(self: @ContractState) -> u8 {
            self.decimals.read()
        }

        fn get_total_supply(self: @ContractState) -> felt252 {
            self.total_supply.read()
        }

        fn balance_of(self: @ContractState, account: ContractAddress) -> felt252 {
            self.balances.read(account)
        }

        fn allowance(
            self: @ContractState, owner: ContractAddress, spender: ContractAddress
        ) -> felt252 {
            self.allowances.read((owner, spender))
        }

        fn transfer(ref self: ContractState, recipient: ContractAddress, amount: felt252) {
            let sender = get_caller_address();
            self._transfer(sender, recipient, amount);
        }

        fn transfer_from(
            ref self: ContractState,
            sender: ContractAddress,
            recipient: ContractAddress,
            amount: felt252
        ) {
            let caller = get_caller_address();
            self.spend_allowance(sender, caller, amount);
            self._transfer(sender, recipient, amount);
        }

        fn approve(ref self: ContractState, spender: ContractAddress, amount: felt252) {
            let caller = get_caller_address();
            self.approve_helper(caller, spender, amount);
        }

        fn increase_allowance(
            ref self: ContractState, spender: ContractAddress, added_value: felt252
        ) {
            let caller = get_caller_address();
            self
                .approve_helper(
                    caller, spender, self.allowances.read((caller, spender)) + added_value
                );
        }

        fn decrease_allowance(
            ref self: ContractState, spender: ContractAddress, subtracted_value: felt252
        ) {
            let caller = get_caller_address();
            self
                .approve_helper(
                    caller, spender, self.allowances.read((caller, spender)) - subtracted_value
                );
        }
    }

    #[generate_trait]
    impl InternalImpl of InternalTrait {
        fn _transfer(
            ref self: ContractState,
            sender: ContractAddress,
            recipient: ContractAddress,
            amount: felt252
        ) {
            assert(!sender.is_zero(), Errors::TRANSFER_FROM_ZERO);
            assert(!recipient.is_zero(), Errors::TRANSFER_TO_ZERO);
            self.balances.write(sender, self.balances.read(sender) - amount);
            self.balances.write(recipient, self.balances.read(recipient) + amount);
            self.emit(Transfer { from: sender, to: recipient, value: amount });
        }

        fn spend_allowance(
            ref self: ContractState,
            owner: ContractAddress,
            spender: ContractAddress,
            amount: felt252
        ) {
            let allowance = self.allowances.read((owner, spender));
            self.allowances.write((owner, spender), allowance - amount);
        }

        fn approve_helper(
            ref self: ContractState,
            owner: ContractAddress,
            spender: ContractAddress,
            amount: felt252
        ) {
            assert(!spender.is_zero(), Errors::APPROVE_TO_ZERO);
            self.allowances.write((owner, spender), amount);
            self.emit(Approval { owner, spender, value: amount });
        }

        fn mint(ref self: ContractState, recipient: ContractAddress, amount: felt252) {
            assert(!recipient.is_zero(), Errors::MINT_TO_ZERO);
            let supply = self.total_supply.read() + amount; // What can go wrong here?
            self.total_supply.write(supply);
            let balance = self.balances.read(recipient) + amount;
            self.balances.write(recipient, amount);
            self
                .emit(
                    Event::Transfer(
                        Transfer {
                            from: contract_address_const::<0>(), to: recipient, value: amount
                        }
                    )
                );
        }
    }
}

Play with this contract in Remix.

There's several other implementations, such as the Open Zeppelin or the Cairo By Example ones.