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【以太坊開發】發幣指南--進階篇

參考:https://www.ethereum.org/token

pragma solidity ^0.4.16;

interface tokenRecipient { function receiveApproval(address _from, uint256 _value, address _token, bytes _extraData) external; }

contract TokenERC20 {
    // Public variables of the token
    string public name;
    string public symbol;
    uint8 public
decimals = 18; // 18 decimals is the strongly suggested default, avoid changing it uint256 public totalSupply; // This creates an array with all balances mapping (address => uint256) public balanceOf; mapping (address => mapping (address => uint256)) public allowance; // This generates a public event on the blockchain that will notify clients
event Transfer(address indexed from, address indexed to, uint256 value); // This generates a public event on the blockchain that will notify clients event Approval(address indexed _owner, address indexed _spender, uint256 _value); // This notifies clients about the amount burnt event Burn
(address indexed from, uint256 value); /** * Constructor function * * Initializes contract with initial supply tokens to the creator of the contract */ function TokenERC20( uint256 initialSupply, string tokenName, string tokenSymbol ) public { totalSupply = initialSupply * 10 ** uint256(decimals); // Update total supply with the decimal amount balanceOf[msg.sender] = totalSupply; // Give the creator all initial tokens name = tokenName; // Set the name for display purposes symbol = tokenSymbol; // Set the symbol for display purposes } /** * Internal transfer, only can be called by this contract */ function _transfer(address _from, address _to, uint _value) internal { // Prevent transfer to 0x0 address. Use burn() instead require(_to != 0x0); // Check if the sender has enough require(balanceOf[_from] >= _value); // Check for overflows require(balanceOf[_to] + _value >= balanceOf[_to]); // Save this for an assertion in the future uint previousBalances = balanceOf[_from] + balanceOf[_to]; // Subtract from the sender balanceOf[_from] -= _value; // Add the same to the recipient balanceOf[_to] += _value; emit Transfer(_from, _to, _value); // Asserts are used to use static analysis to find bugs in your code. They should never fail assert(balanceOf[_from] + balanceOf[_to] == previousBalances); } /** * Transfer tokens * * Send `_value` tokens to `_to` from your account * * @param _to The address of the recipient * @param _value the amount to send */ function transfer(address _to, uint256 _value) public returns (bool success) { _transfer(msg.sender, _to, _value); return true; } /** * Transfer tokens from other address * * Send `_value` tokens to `_to` on behalf of `_from` * * @param _from The address of the sender * @param _to The address of the recipient * @param _value the amount to send */ function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) { require(_value <= allowance[_from][msg.sender]); // Check allowance allowance[_from][msg.sender] -= _value; _transfer(_from, _to, _value); return true; } /** * Set allowance for other address * * Allows `_spender` to spend no more than `_value` tokens on your behalf * * @param _spender The address authorized to spend * @param _value the max amount they can spend */ function approve(address _spender, uint256 _value) public returns (bool success) { allowance[msg.sender][_spender] = _value; emit Approval(msg.sender, _spender, _value); return true; } /** * Set allowance for other address and notify * * Allows `_spender` to spend no more than `_value` tokens on your behalf, and then ping the contract about it * * @param _spender The address authorized to spend * @param _value the max amount they can spend * @param _extraData some extra information to send to the approved contract */ function approveAndCall(address _spender, uint256 _value, bytes _extraData) public returns (bool success) { tokenRecipient spender = tokenRecipient(_spender); if (approve(_spender, _value)) { spender.receiveApproval(msg.sender, _value, this, _extraData); return true; } } /** * Destroy tokens * * Remove `_value` tokens from the system irreversibly * * @param _value the amount of money to burn */ function burn(uint256 _value) public returns (bool success) { require(balanceOf[msg.sender] >= _value); // Check if the sender has enough balanceOf[msg.sender] -= _value; // Subtract from the sender totalSupply -= _value; // Updates totalSupply emit Burn(msg.sender, _value); return true; } /** * Destroy tokens from other account * * Remove `_value` tokens from the system irreversibly on behalf of `_from`. * * @param _from the address of the sender * @param _value the amount of money to burn */ function burnFrom(address _from, uint256 _value) public returns (bool success) { require(balanceOf[_from] >= _value); // Check if the targeted balance is enough require(_value <= allowance[_from][msg.sender]); // Check allowance balanceOf[_from] -= _value; // Subtract from the targeted balance allowance[_from][msg.sender] -= _value; // Subtract from the sender's allowance totalSupply -= _value; // Update totalSupply emit Burn(_from, _value); return true; } }

approveallowance的用法

賬戶A有1000個ETH,想允許B賬戶隨意呼叫100個ETH。A賬戶按照以下形式呼叫approve函式approve(B,100)。當B賬戶想用這100個ETH中的10個ETH給C賬戶時,則呼叫transferFrom(A, C, 10)。這時呼叫allowance(A, B)可以檢視B賬戶還能夠呼叫A賬戶多少個token。

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