# 【合約互動】

我們可以使用 `address()` 將 `contract` 型態轉為 `address` 型態。

如果今天要使用同一個檔案裏面其他合約的函數，可以使用以下方法：

```solidity
contract A {
  function foo() view external returns(uint) {...}
}

contract B {
  function callFoo(address addrA) external {
    uint result = A(addrA).foo();
  }
}
```

甚至可以在一個合約裡面宣告其他合約。

```solidity
contract A {
  constructor(uint a) {...}
  function foo() external {...}
}

contract B {
  function createA(uint a) external {
    A AInstance = new A(a); //pass constructor argument(s) if any
  }
}
```

#### Modifier, Inheritance, Importing

```solidity
pragma solidity ^0.8.11;

contract Owned {
    address owner;
    constructor() public {
        owner = msg.sender;
    }
    
    modifier onlyOwner() {
        require(msg.sender == owner, "You are not allowed");
        _;
    }    
}

contract InheritanceModifierExample is Owned {

    mapping(address => uint) public tokenBalance;


    uint tokenPrice = 1 ether;

    constructor() public {
        tokenBalance[owner] = 100;
    }

    function createNewToken() public onlyOwner{
        // require statement is originally here
        tokenBalance[owner]++;
    }

    function burnToken() public onlyOwner{
        // require statement is originally here
        tokenBalance[owner]--;
    }

    function purchaseToken() public payable {
        require((tokenBalance[owner] * tokenPrice) / msg.value > 0, "not enoug
        h tokens");
        tokenBalance[owner] -= msg.value / tokenPrice;
        tokenBalance[msg.sender] += msg.value / tokenPrice;
    }

    function sendToken(address _to, uint _amount) public {
        require(tokenBalance[msg.sender] >= _amount, "Not enough tokens");
        assert(tokenBalance[_to] + _amount >= tokenBalance[_to]);
        assert(tokenBalance[msg.sender] - _amount <= tokenBalance[msg.sender])
        ;
        tokenBalance[msg.sender] -= _amount;
        tokenBalance[_to] += _amount;
    }
}
```

Or we can clip the code to two parts(solidity file, Owned.sol and Modifier.sol)

* Owned.sol

```solidity

contract Owned {
    address owner;
    constructor() public {
        owner = msg.sender;
    }
    
    modifier onlyOwner() {
        require(msg.sender == owner, "You are not allowed");
        _;
    }    
}
```

* Modifier.sol

```solidity
pragma solidity ^0.8.11;

import "./Owned.sol"

contract InheritanceModifierExample is Owned {

    mapping(address => uint) public tokenBalance;

    uint tokenPrice = 1 ether;

    constructor() public {
        tokenBalance[owner] = 100;
    }

    function createNewToken() public onlyOwner{
        // require statement is originally here
        tokenBalance[owner]++;
    }

    function burnToken() public onlyOwner{
        // require statement is originally here
        tokenBalance[owner]--;
    }

    function purchaseToken() public payable {
        require((tokenBalance[owner] * tokenPrice) / msg.value > 0, "not enoug
        h tokens");
        tokenBalance[owner] -= msg.value / tokenPrice;
        tokenBalance[msg.sender] += msg.value / tokenPrice;
    }

    function sendToken(address _to, uint _amount) public {
        require(tokenBalance[msg.sender] >= _amount, "Not enough tokens");
        assert(tokenBalance[_to] + _amount >= tokenBalance[_to]);
        assert(tokenBalance[msg.sender] - _amount <= tokenBalance[msg.sender])
        ;
        tokenBalance[msg.sender] -= _amount;
        tokenBalance[_to] += _amount;
    }
}
```


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