Understand the ERC-20 token smart contract

smart contracts

tokens

solidity

getting started

erc-20

beginner

️jdourlens

EthereumDev

April 5, 2020

2 minute read

Tip author 0x19dE91Af973F404EDF5B4c093983a7c6E3EC8ccE

One of the most significant smart contract standards on Ethereum is known as ERC-20, which has emerged as the technical standard used for all smart contracts on the Ethereum blockchain for fungible token implementations.

ERC-20 defines a common list of rules that all fungible Ethereum tokens should adhere to. Consequently, this token standard empowers developers of all types to accurately predict how new tokens will function within the larger Ethereum system. This simplifies and eases developers’ tasks, because they can proceed with their work, knowing that each and every new project won’t need to be redone every time a new token is released, as long as the token follows the rules.

Here is, presented as an interface, the functions an ERC-20 must implement. If you’re not sure about what is an interface: check our article about OOP programming in Solidity.

1pragma solidity ^0.6.0;
2
3interface IERC20 {
4
5    function totalSupply() external view returns (uint256);
6    function balanceOf(address account) external view returns (uint256);
7    function allowance(address owner, address spender) external view returns (uint256);
8
9    function transfer(address recipient, uint256 amount) external returns (bool);
10    function approve(address spender, uint256 amount) external returns (bool);
11    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);
12
13
14    event Transfer(address indexed from, address indexed to, uint256 value);
15    event Approval(address indexed owner, address indexed spender, uint256 value);
16}
17
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Here is a line-by-line explainer of what every function is for. After this we’ll present a simple implementation of the a ERC-20 token.

Getters

1function totalSupply() external view returns (uint256);
2
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Returns the amount of tokens in existence. This function is a getter and does not modify the state of the contract. Keep in mind that there is no floats in Solidity. Therefore most tokens adopt 18 decimals and will return the total supply and other results as followed 1000000000000000000 for 1 token. Not every tokens has 18 decimals and this is something you really need to watch for when dealing with tokens.

1function balanceOf(address account) external view returns (uint256);
2
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Returns the amount of tokens owned by an address (account). This function is a getter and does not modify the state of the contract.

1function allowance(address owner, address spender) external view returns (uint256);
2
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The ERC-20 standard allow an address to give an allowance to another address to be able to retrieve tokens from it. This getter returns the remaining number of tokens that the spender will be allowed to spend on behalf of owner. This function is a getter and does not modify the state of the contract and should return 0 by default.

Functions

1function transfer(address recipient, uint256 amount) external returns (bool);
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Moves the amount of tokens from the function caller address (msg.sender) to the recipient address. This function emits the Transfer event defined later. It returns true if the transfer was possible.

1function approve(address spender, uint256 amount) external returns (bool);
2
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Set the amount of allowance the spender is allowed to transfer from the function caller (msg.sender) balance. This function emits the Approval event. The function returns whether the allowance was successfully set.

1function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);
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Moves the amount of tokens from sender to recipient using the allowance mechanism. amount is then deducted from the caller’s allowance. This function emits the Transfer event.

Events

1event Transfer(address indexed from, address indexed to, uint256 value);
2
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This event is emitted when the amount of tokens (value) is sent from the from address to the to address.

In the case of minting new tokens, the transfer is usually from the 0x00..0000 address while in the case of burning tokens the transfer is to 0x00..0000.

1event Approval(address indexed owner, address indexed spender, uint256 value);
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This event is emitted when the amount of tokens (value) is approved by the owner to be used by the spender.

A basic implementation of ERC-20 tokens

Here is the most simple code to base your ERC-20 token from:

1pragma solidity ^0.6.0;
2
3interface IERC20 {
4
5    function totalSupply() external view returns (uint256);
6    function balanceOf(address account) external view returns (uint256);
7    function allowance(address owner, address spender) external view returns (uint256);
8
9    function transfer(address recipient, uint256 amount) external returns (bool);
10    function approve(address spender, uint256 amount) external returns (bool);
11    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);
12
13
14    event Transfer(address indexed from, address indexed to, uint256 value);
15    event Approval(address indexed owner, address indexed spender, uint256 value);
16}
17
18
19contract ERC20Basic is IERC20 {
20
21    string public constant name = "ERC20Basic";
22    string public constant symbol = "ERC";
23    uint8 public constant decimals = 18;
24
25
26    event Approval(address indexed tokenOwner, address indexed spender, uint tokens);
27    event Transfer(address indexed from, address indexed to, uint tokens);
28
29
30    mapping(address => uint256) balances;
31
32    mapping(address => mapping (address => uint256)) allowed;
33
34    uint256 totalSupply_;
35
36    using SafeMath for uint256;
37
38
39   constructor(uint256 total) public {
40    totalSupply_ = total;
41    balances[msg.sender] = totalSupply_;
42    }
43
44    function totalSupply() public override view returns (uint256) {
45    return totalSupply_;
46    }
47
48    function balanceOf(address tokenOwner) public override view returns (uint256) {
49        return balances[tokenOwner];
50    }
51
52    function transfer(address receiver, uint256 numTokens) public override returns (bool) {
53        require(numTokens <= balances[msg.sender]);
54        balances[msg.sender] = balances[msg.sender].sub(numTokens);
55        balances[receiver] = balances[receiver].add(numTokens);
56        emit Transfer(msg.sender, receiver, numTokens);
57        return true;
58    }
59
60    function approve(address delegate, uint256 numTokens) public override returns (bool) {
61        allowed[msg.sender][delegate] = numTokens;
62        emit Approval(msg.sender, delegate, numTokens);
63        return true;
64    }
65
66    function allowance(address owner, address delegate) public override view returns (uint) {
67        return allowed[owner][delegate];
68    }
69
70    function transferFrom(address owner, address buyer, uint256 numTokens) public override returns (bool) {
71        require(numTokens <= balances[owner]);
72        require(numTokens <= allowed[owner][msg.sender]);
73
74        balances[owner] = balances[owner].sub(numTokens);
75        allowed[owner][msg.sender] = allowed[owner][msg.sender].sub(numTokens);
76        balances[buyer] = balances[buyer].add(numTokens);
77        emit Transfer(owner, buyer, numTokens);
78        return true;
79    }
80}
81
82library SafeMath {
83    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
84      assert(b <= a);
85      return a - b;
86    }
87
88    function add(uint256 a, uint256 b) internal pure returns (uint256) {
89      uint256 c = a + b;
90      assert(c >= a);
91      return c;
92    }
93}
94
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This implementation uses the SafeMath library. Read our tutorial about it if you’d like to learn how the library helps you with handling overflows and underflows in your smart contracts.

Another excellent implementation of the ERC-20 token standard is the OpenZeppelin ERC-20 implementation.

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Understand the ERC-20 token smart contract

Getters

Functions

Events

A basic implementation of ERC-20 tokens