How to destroy a token or list of token from a another contract?

Question

[Edit 14-03-2018] I want to create a contract A that can generate a random number of Token A and another contract B that receive and destroy one or many of this token to generate a new Token B.

It's like sending one CryptosKitty to another contract B, burn the kitty, and generate a Dog with the kitty's gen.

Thanks for the help.

Answer 1

In case contract A allows token holders to burn their tokens, you just need to call corresponding method of contract A from contract B like this:

contract B is ERC721 {
  uint nextDogID = 0;
  A a = ...;

  function catToDog (uint _catID) public (returns _dogID) {
    _dogID = nextDogID++;
    _mint (msg.sender, _dogID);
    copyGene (_catID, _dogID);
    require (a.transferFrom (msg.sender, address (this), _catID));
    require (a.burn (_catID)); // Here we burn the cat token!
  }
}

If A does not allow burning, just send "burned" tokens to some dead address:

contract B is ERC721 {
  uint nextDogID = 0;
  IERC721 a = ...;

  function catToDog (uint _catID) public (returns _dogID) {
    _dogID = nextDogID++;
    _mint (msg.sender, _dogID);
    copyGene (_catID, _dogID);

    // Here we send the cat token to dead address effectively burning it
    require (a.transferFrom (
      msg.sender,
      0x000000000000000000000000000000000000dead,
      _catID));
  }
}

Answer 2

You don't actually need to "receive" anything. The contract B just needs to be able to burn some tokens which are listed in contract A.

Here's some example code:

contract A {
    uint tokens = 10;
    // which address is allowed to burn tokens
    address allowedBurnerAddress = 0x1234;

    // allowed contract (B) can call this to burn tokens
    function burn(uint amount) public {
        require(msg.sender == allowedBurnerAddress);
        tokens -= amount;
    }
}

contract B {
    // address for contract A
    address burnContract = 0x0987;

    // call this to burn tokens which exist in contract A
    function PerformBurn() public {
        // add some requirements for who can call this function

        A original = A(burnContract);
        original.burn(5);
    }
}