Inheritance

In Solidity, inheritance works by means of a keyword, is, which is used to indicate that one contract inherits from another.

Example

contract A {    
    function foo() public {        // do something    }
} 

contract B is A {    
    function bar() public {        // do something else    }
}

• Contract B inherits from the contract A.

• This means that B will have access to the foo() function defined in A. In addition, it will also have its own bar() function.

Import + Inherit

Owned.sol

//SPDX-License-Identifier: MIT
pragma solidity 0.8.3;

contract Owned {
    address owner;

    constructor() {
        owner = msg.sender;
    }

    modifier onlyOwner() {
        require(msg.sender == owner, "You are not the owner");
        _;
    }
}

ModifierExample.sol

//SPDX-License-Identifier: MIT
pragma solidity 0.8.3;

import "./Owned.sol";

contract ModifierExample is Owned {

    mapping(address => uint) public tokenBalance;
    uint tokenPrice = 1 ether;

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

    function createNewToken() public onlyOwner {
        tokenBalance[owner]++;
    }

    function burnToken() public onlyOwner {
        tokenBalance[owner]--;
    }
}

• ModifierExample contract inherits all the functionality of the base contract.

• It means that when you instantiate the contract, a single object code (i.e., a single contract instance) containing all of that functionality is deployed on the network. Any call to a function of the base contract yields a non-external function call.

• On deployment, there is only a single address.