Contract in Solidity

Contract in Solidity is similar to a Class in C++. A Contract have following properties.

• Constructor − A special function declared with constructor keyword which will be executed once per contract and is invoked when a contract is created.

• State Variables − Variables per Contract to store the state of the contract.

• Functions − Functions per Contract which can modify the state variables to alter the state of a contract.

Visibility Quantifiers

Following are various visibility quantifiers for functions/state variables of a contract.

• external − External functions are meant to be called by other contracts. They cannot be used for internal call. To call external function within contract this.function_name() call is required. State variables cannot be marked as external.

• public − Public functions/ Variables can be used both externally and internally. For public state variable, Solidity automatically creates a getter function.

• internal − Internal functions/ Variables can only be used internally or by derived contracts.

• private − Private functions/ Variables can only be used internally and not even by derived contracts.

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; }
}
//External Contract
contract D {
   function readData() public returns(uint) {
      C c = new C();
      c.updateData(7);         
      return c.getData();
   }
}
//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(); }
}