Inheritance is a way to extend functionality of a contract. Solidity supports both single as well as multiple inheritance. Following are the key highlighsts.

• A derived contract can access all non-private members including internal methods and state variables. But using this is not allowed.

• Function overriding is allowed provided function signature remains same. In case of difference of output parameters, compilation will fail.

• We can call a super contract”s function using super keyword or using super contract name.

• In case of multiple inheritance, function call using super gives preference to most derived contract.

Example

pragma solidity ^0.5.0;

contract C {
   //private state variable
   uint private data;
   
   //public state variable
   uint public info;

   //constructor
   constructor() public {
      info = 10;
   }
   //private function
   function increment(uint a) private pure returns(uint) { return a + 1; }
   
   //public function
   function updateData(uint a) public { data = a; }
   function getData() public view returns(uint) { return data; }
   function compute(uint a, uint b) internal pure returns (uint) { return a + b; }
}
//Derived Contract
contract E is C {
   uint private result;
   C private c;
   constructor() public {
      c = new C();
   }  
   function getComputedResult() public {      
      result = compute(3, 5); 
   }
   function getResult() public view returns(uint) { return result; }
   function getData() public view returns(uint) { return c.info(); }
}

Run the above program using steps provided in Solidity First Application chapter. Run various method of Contracts. For E.getComputedResult() followed by E.getResult() shows −

Output

0: uint256: 8