solidity – Casting Between Contract Classes in Solidity

contract-developmentsolidity

I wonder how do you cast between classes in solidity.

For example, if I have these classes:

contract Token {
    function getFive() public returns (uint) {
        return 5;
    }
}

contract MyToken is Token {
    function getSix() public returns (uint) {
        return 6;
    }
}

And I have this variable in other contract:

contract TokenSale {
    Token token;

    function TokenSale(address tokenAddress) {
        token = Token(tokenAddress);
    }
}

contract MyTokenSale is TokenSale {
    function MyTokenSale(address tokenAddress) TokenSale(tokenAddress) {
        // Here I want to call to MyToken's getSix funciton
    }
}

How can I downcast the token variable in MyTokenSale to call getSix function, assuming I can't change TokenSale's code?

As an OOP language, I'm sure that Solidity allows you to do it somehow, but for some reason I couldn't find it in the docs.

Thanks!

Best Answer

This is the syntax you would use:

MyToken myToken = MyToken(address(token));

You have to do it like this because Solidity isn't actually an OOP language, it just has some features that make it feel kinda like it is.

Related Solutions

Solidity Interfaces – Array of Contract Interfaces

Yes it is possible, yes every contract has it's own address, but think about it like a memory reference rather than simply a direct call to an address. Here's an example:

pragma solidity ^0.4.0;

interface Letter {
    function f() 
        public 
        returns (uint);
}

contract A is Letter {
    function f() 
        public 
        returns (uint) {
        return 1;
    }
}

contract B is A {
}

contract C is Letter {
    function f() 
        public 
        returns (uint) {
        return 2;
    }
}

contract Alphabet {
    Letter[] letters;

    event printer(uint);

    // Add our own new contracts
    function Alphabet() 
        public {
        letters.push(new A());
        letters.push(new B());
        letters.push(new C());
    }

    // Include existing contracts
    function alphabetFromAddresses(address _addrA, address _addrB, 
                                   address _addrC)
        public{
        letters.push(A(_addrA));
        letters.push(B(_addrB));
        letters.push(C(_addrC));
    }

    function getLetters() 
        public {
        for(uint i = 0; i < letters.length; i++) {
            printer(letters[i].f());
        }
    }
}

You can add your own new instances which all get assigned their own contract addresses on creation, you can also reference existing contracts as long as they are implementations of your contracts or interfaces.

Solidity – Handling Unimplemented Functions in Contracts

The problem is that totalSupply() implementation is missing. Seems like autogenerated getters do not count. So you need to explicitly create the function.

contract StandardToken is Token {
    uint256 _totalSupply;

    function totalSupply() constant returns (uint256 totalSupply) {
        totalSupply = _totalSupply;
    }
}

Best Answer

Related Solutions

Solidity Interfaces – Array of Contract Interfaces

Solidity – Handling Unimplemented Functions in Contracts

Related Topic