Solidity – Constructors

A constructor is a special method in any object-oriented programming language which gets called whenever an object of a class is initialized. It is totally different in case of Solidity, Solidity provides a constructor declaration inside the smart contract and it invokes only once when the contract is deployed and is used to initialize the contract state. A default constructor is created by the compiler if there is no explicitly defined constructor.

Creating a constructor

A Constructor is defined using a constructor keyword without any function name followed by an access modifier. It’s an optional function which initializes state variables of the contract. A constructor can be either internal or public, an internal constructor marks contract as abstract. 

Syntax:

constructor() <Access Modifier> {          
} 

Example: In the below example, in the contract constructorExample, a constructor is created to initialize the state variable str. 

Solidity

// Solidity program to demonstrate
// creating a constructor
pragma solidity ^0.5.0;

// Creating a contract
contract constructorExample {

    // Declaring state variable
    string str;

    // Creating a constructor
    // to set value of 'str'
    constructor() public {
        str = "GeeksForGeeks";
    }

    // Defining function to
    // return the value of 'str'
    function getValue(
    ) public view returns (
      string memory) {
        return str;
    }
}

Output :

Constructor in Inheritance

In case if a constructor is not defined then the default constructor is called, but if the constructor is defined in a parent contract and having some arguments then the child contract should also provide the required parameters to the constructor. If the child contract is not passing any parameter to the parent’s constructor the child contract will become an abstract contract. There are two ways of calling a parent contract’s constructor: 

1. Direct Initialization: In the below example, the direct initialization method is used to initialize the constructor of the parent class.

Solidity

// Solidity program to demonstrate
// Constructor in Inheritance
pragma solidity ^0.5.0;

// Creating a contract
contract Base {

   // Declaring variable
   uint data;

   // Defining a constructor
   constructor(uint _data) public {
      data = _data;
   }

   // Defining function
   function Print(
   ) public  returns(string memory){
       return "Direct Initialization";
   }

}

// Child contract inheriting
// the parent contract 'Base'
contract Derived is Base(2){

    // Defining a constructor
    constructor() public {}

   // Defining function to access
   // variable of parent contract
   function getData(
   ) external returns(uint){
       uint result = data ** 2;
       return result;
   }
}

// Caller contract
contract caller{

   // Creating an object of child contract
    Derived c = new Derived();

   // Accessing functions of parent
   // and child contract using
   // object of child contract
    function getResult() public returns(uint){
        c.Print();
        return c.getData();
    }
}

Output :

2. Indirect Initialization: In the below example, the indirect initialization using Base(string(abi.encodePacked(_info, _info))) is done to initialize the constructor of the base class.

Solidity

// Solidity program to demonstrate
// Indirect Initialization
pragma solidity ^0.5.0;

// Creating a contract
contract Base {

   // Declaring state variable
   string str;

   // Defining a constructor
   constructor(
     string memory _str) public {
      str = _str;
   }

   // Defining a function
   function Print(
   ) public returns(string memory){
       return "Indirect Initialization";
   }
}

// Child contract inheriting
// parent contract 'Base'
contract Derived is Base {

   // Defining a constructor
   constructor(
     string memory _info) Base(
     string(abi.encodePacked(
       _info, _info))) public {}

   // Defining function to
   // return value of parent
   // contract variable 'str'
   function getStr(
   ) public view returns(string memory){
       return str;
   }
}

// Caller contract
contract caller {

    // Creating an object of
    // child contract
    Derived c
      = new Derived("GeeksForGeeks");

    //Defining a function to access
    // functions of the parent
    //contract and child contract
    function getResult() public view{
        c.Print();
        c.getStr();
    }
}

Output :

Need of Constructors

Constructors are very useful in a smart contract, a parameter value can be defined at the run time and can also restrict the method call. Constructor overloading is not supported in Solidity, it only allows one constructor at a time.

Example: In the below example, the contract constructorExample consists of a constructor to demonstrate the need for the constructor.

Solidity

// Solidity program to demonstrate
// Need of constructors
pragma solidity ^0.5.0;

// Creating a contract
contract constructorExample {

    // Declaring state variable
    string str;
    address private owner
      = 0x62Ab98A0efB752C48bd82836D0b4335018B9B97e;

    // Defining constructor
    constructor(string memory string) public {
        if(msg.sender == owner){
            str = string;
        }
    }

    // Defining function to
    // return value of 'str'
    function getValue() public view returns (
      string memory) {
        return str;
    }
}

Output :