Interact with a deployed contract from another contract

Question

Say i have this contract which I have deployed

contract A {
        mapping (string => uint) public someMapping;
}

Now, can I interact with the data of the deployed contractA in some other contractB i.e in contractB can I get the value of someMapping["someString"]?

I am using Truffle along with Ganache and MetaMask.

Answer 1

Yes.

There's a gotcha, though. Contracts can't yet pass around dynamic-length arguments, so strings won't work. I changed to bytes32 to make the simple example work.

With that detail out of the way ...

Contract B must have knowledge of contract's A ABI (the function signatures) and it's address.

The easiest method is to include both files in the same source file. Note that you can deploy B only if A is already out there somewhere and you have knowledge of its address.

B.sol

contract A {
  mapping (bytes32 => uint) public someMapping;
}

contract B {
  A a; // cast "a" as "contract A" defined above

  function B(address addressA) public { // pass address of "A" to the constructor
    a = A(addressA);
  }

  function getAUint(bytes32 key) public view returns(uint) {
    return a.someMapping(key);
  }
}

You will obviously have to extend this example to set values in A somehow so the test is a little more interesting.

If the keys really do need to be strings, you might consider using bytes32 hashes of the strings. This way, every unique string will create a unique fixed-size key. Stringy keys is almost always a sign of going astray in Solidity.

Hope it helps.