Multiple Contracts

Since the beginning of the course, you've been writing one contract at a time. The contracts you've engineered starts to get bigger and bigger.

Declaration

To that end, Solidity allows you to write different contracts in the same file and make them interact with each other. You just have to declare the second contract below the first one.

pragma solidity ^0.4.24;

contract SpaceMuffin {
  /* No code in here */
}

// As simple as it can be
contract SpaceCookie {
  /* Still, no code required */
}

pragma solidity ^0.4.24;
// No validation required

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Variables

Interestingly, contract can be stored as variables. Once declared, method can be called on them. It can be done as follow

contract Position {
  uint public x = 0;

  function plusOne() view public returns (uint) {
    return x + 1;
  }
}

contract Map {
  Position origin = new Position();

  function plusOne() view public returns (uint) {
    return origin.plusOne();
  }
}

SpaceCookie industry would like to spy on the number of SpaceMuffin fans. Help them by completing the following exercise.

pragma solidity ^0.4.24;

contract SpaceMuffin {
  uint private fans = 1;

  function getFans() view public returns (uint) {
    return fans;
  }
}

contract SpaceCookie {
  // You're doing such an evil work
  SpaceMuffin muffin = new SpaceMuffin();
  
  function spy() view public returns (uint) {
    // A truly evil work
    return muffin.getFans();
  }
}

pragma solidity ^0.4.24;
// TODO: validation

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Your help to SpaceCookie industry has been greatly appreciated by their supporters. Let's see if you have what it takes to go away from the one and only SpaceMuffin.