Solidity Syntax – What is the Use of Question Mark ‘?’ in Solidity

The super keyword in Solidity gives access to the immediate parent contract from which the current contract is derived. When having a contract A with a function f() that derives from B which also has a function f(), A overrides the f of B. That means that myInstanceOfA.f() will call the version of f that is implemented inside A itself, the original version implemented inside B is not visible anymore. The original function f from B (being A's parent) is thus available inside A via super.f(). Alternatively, one can explicitly specifying the parent of which one wants to call the overridden function because multiple overriding steps are possible as exemplified in the example below:

pragma solidity ^0.4.5;

contract C {
  uint u;
  function f() {
    u = 1;
  }
}

contract B is C {
  function f() {
    u = 2;
  }
}

contract A is B {
  function f() {  // will set u to 3
    u = 3;
  }
  function f1() { // will set u to 2
    super.f();
  }
  function f2() { // will set u to 2
    B.f();
  }
  function f3() { // will set u to 1
    C.f();
  }
}

You can check with the code below that this line:

uint256 roundValue = SafeMath.ceil(_value, basePercent);

If your value _value is higher than basePercent, roundValue will be the number rounded on the hundred, otherwise the _value will be basePercent always.

Example: input = 1--> roundValue = 100

Example: input = 4111--> roundValue = 4200

Code:

Supposing a SafeMath library like the following:

pragma solidity ^0.5.0;

library SafeMath {
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        if (a == 0) {
            return 0;
        }
        uint256 c = a * b;
    assert(c / a == b);
    return c;
    }

    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a / b;
        return c;
    }

    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        assert(b <= a);
        return a - b;
    }

    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        assert(c >= a);
        return c;
    }

    function ceil(uint256 a, uint256 m) internal pure returns (uint256) {
        uint256 c = add(a,m);
        uint256 d = sub(c,1);
        return mul(div(d,m),m);
    }
}

And the onePercent function:

import "./SafeMath.sol";

contract Percentage{
    
    uint256 public basePercent = 100;

    function onePercent(uint256 _value) public view returns (uint256)  {
        uint256 roundValue = SafeMath.ceil(_value, basePercent);
        uint256 onePercent = SafeMath.div(SafeMath.mul(roundValue, basePercent), 10000);
        return onePercent;
    }
}