Solidity – Basics of Interface

Interfaces are the same as abstract contracts created by using an interface keyword, also known as a pure abstract contract. Interfaces do not have any definition or any state variables, constructors, or any function with implementation, they only contain function declarations i.e. functions in interfaces do not have any statements. Functions of Interface can be only of type external. They can inherit from other interfaces, but they can’t inherit from other contracts. An interface can have enum, structs which can be accessed using interface name dot notation.

Example: In the below example, the contract thisContract implements an interface InterfaceExample and implements all the interface functions.

Solidity

// Solidity program to
// demonstrate the working
// of the interface

pragma solidity 0.4.19;

// A simple interface
interface InterfaceExample{

    // Functions having only
    // declaration not definition
    function getStr(
    ) public view returns(string memory);
    function setValue(
      uint _num1, uint _num2) public;
    function add(
    ) public view returns(uint);
}

// Contract that implements interface
contract thisContract is InterfaceExample{

    // Private variables
    uint private num1;
    uint private num2;

    // Function definitions of functions
    // declared inside an interface
    function getStr(
    ) public view returns(string memory){
        return "GeeksForGeeks";
    }

     // Function to set the values
    // of the private variables
    function setValue(
      uint _num1, uint _num2) public{
        num1 = _num1;
        num2 = _num2;
    }

    // Function to add 2 numbers
    function add(
    ) public view returns(uint){
        return num1 + num2;
    }

}

contract call{

    //Creating an object
    InterfaceExample obj;

    function call() public{
        obj = new thisContract();
    }

    // Function to print string
    // value and the sum value
    function getValue(
    ) public returns(uint){
        obj.getStr;
        obj.setValue(10, 16);
        return obj.add();
    }
}

Output :