The journey into cryptocurrency creation continues. After setting up your digital wallet in the previous guide, the next step is to write the smart contract code that will form the backbone of your token. For many, the ERC-20 standard on the Ethereum blockchain is the ideal starting point. This guide will break down the essential functions you need to understand and implement.
An ERC-20 token contract is built using two primary Solidity files: EIP20.sol and EIP20Interface.sol. The interface file acts as a blueprint or a skeleton, defining the mandatory functions that your token must have to be compatible with the standard. It tells other applications—like wallets and exchanges—what APIs to expect. Let's explore each of these core functions in detail.
Core ERC-20 Token Functions Explained
The transfer Function
The transfer function is the most fundamental operation. It enables the token owner to send a specified amount of their cryptocurrency to another address.
function transfer(address _to, uint256 _value) public returns (bool success);- Purpose: To move tokens from the sender's address to a recipient's address.
Parameters:
_to: The destination wallet address receiving the tokens._value: The number of tokens to be sent.
- Key Mechanism: Internally, the function uses
require(balances[msg.sender] >= _value);to verify that the sender's balance is sufficient before executing the transfer. This is a critical security and validation check.
The transferFrom Function
This function enables delegated transfers. It allows a third-party application (like a decentralized exchange) to transfer tokens on behalf of a user, but only with their prior permission.
function transferFrom(address _from, address _to, uint256 _value) public returns (bool success);- Purpose: For authorized third parties to transfer tokens between accounts.
Parameters:
_from: The address from which tokens are withdrawn._to: The address receiving the tokens._value: The amount of tokens to transfer.
- Key Mechanism: This function checks two things. First, it confirms that the
_fromaddress has enough tokens. Second, it checks the allowance:uint256 allowance = allowed[_from][msg.sender]; require(... && allowance >= _value);. This ensures the third-party caller is authorized to spend up to a specific amount of the owner's funds.
The balanceOf Function
A simple view function that provides information about a wallet's holdings without modifying the blockchain state.
function balanceOf(address _owner) public view returns (uint256 balance);- Purpose: To query the token balance of any given Ethereum address.
Parameters:
_owner: The address whose balance you want to check.
- Use Case: Essential for wallets and explorers to display how many tokens an account holds.
The approve Function
This function is the prerequisite for transferFrom. It grants a specific address permission to spend a set amount of your tokens.
function approve(address _spender, uint256 _value) public returns (bool success);- Purpose: To authorize another address to withdraw tokens from your account.
Parameters:
_spender: The address being granted spending privileges._value: The maximum total amount of tokens the spender can transfer. This allowance can be used in multiple transactions until the total spent reaches this limit.
The allowance Function
This view function checks the remaining amount of tokens that a _spender is still permitted to withdraw from a _owner's account.
function allowance(address _owner, address _spender) public view returns (uint256 remaining);- Purpose: To verify the current spending allowance granted by one account to another.
Parameters:
_owner: The address that owns the tokens._spender: The address that has been granted spending rights.
- Use Case: Important for dApp interfaces to show users how much of their allowance has been used.
Understanding these five functions provides a solid foundation for how ERC-20 tokens operate. They govern the movement, authorization, and querying of tokens, forming the basis of most token-based interactions on Ethereum. For a deeper dive into the variables and events that complete an ERC-20 contract, you can explore more strategies for advanced smart contract development.
Frequently Asked Questions
What is the main purpose of the ERC-20 standard?
ERC-20 is a technical standard used for all smart contracts on the Ethereum blockchain that implement a token. It defines a common list of rules that all Ethereum tokens must adhere to, ensuring they can be easily exchanged and interacted with across different applications like wallets and exchanges.
What is the critical difference between transfer and transferFrom?
The transfer function is used to send tokens directly from your own address. In contrast, transferFrom is used by an approved third-party address to transfer tokens on your behalf, but only up to a limit you have预先 approved using the approve function.
Why is the approve function necessary?
The approve function is essential for enabling decentralized financial applications. It allows users to grant permission to smart contracts (e.g., for a decentralized exchange or lending platform) to access a specific amount of their tokens without giving the contract full control over their entire balance, enhancing security.
How can I check how much I've allowed a dApp to spend?
You can check your remaining allowance for any specific spender address by calling the allowance function. You will need to input your address as the _owner and the dApp's contract address as the _spender.
What happens if a transfer transaction fails?
If a condition like an insufficient balance causes a transfer to fail, the built-in require statement will trigger. This will revert the entire transaction, meaning no state changes (like token deductions) will occur, and the user will only lose the gas fee for the failed transaction.
Are these functions enough to create a full token?
While these functions form the core logic of a token, a complete ERC-20 contract also requires state variables (like balances and allowed mappings) and standard events (like Transfer and Approval) to log activity on the blockchain. The interface defines the required functions, but the implementation is built around them.