How to deploy a contract and its abstract/interface?

Question

Assume I have a contract A:

pragma solidity ^0.4.4;
contract A{
  struct St{
    uint val;
  }
  function double(uint a)external returns (uint res){
  res= 2*a;
  }
}

Let's call the above lines of code block 1.

I deploy contract A. Then I have a contract B that wants to call a function of contract A:

pragma solidity ^0.4.4;
contract B{
   uint res;
   function call_double(uint val) {
   A a=A(0x1);// 0x1 is the address of contract A after it's deployed.
   res = a.double(val);
   }
 }
 // how to include this contract abstract when deploying contract B?
 contract A{
    function double(uint a)external returns (uint res);
    struct St{
    uint val;
   }
 }

Let's call the above lines of code block 2.

---

Question: How can I include contract A when I'm deploying contract B? In other words, how can I deploy block2?

Please note that when I'm using remix, (after I deploy contract A) I put both contract B and abstract of contract A in one page and deploy B, and everything works fine. But it's not clear to me how it's done when Im manually deploying to a private blockchain using web3. Also, it's not clear which ABI I need when I want to interact with contract B.

Answer 1

You can create an instance of contract A in contract B and then point that instance to the address of contract A.

Deploy contract A:

contract A{
  struct St{
    uint val;
  }
  function double(uint a)external returns (uint res){
  res= 2*a;
  }
}

Now when creating contract B, you can pass the address of contract A. Constructor of B will initialise the instance of A:

contract A{
     function double(uint a)external returns (uint res);
     struct St{
     uint val;
    }
 }

contract B{
    uint res;
    A objOfA;
    function B(address _addressOfContractA){
      objOfA = A(_addressOfContractA);
    }
    function call_double(uint val) {
      res = objOfA.double(val);
    }
}

Answer 2

That's why the interface is needed.

Block2 must be look like below:

interface A{
     function double(uint a)external returns (uint res);
    }
 }

contract B{
    uint res;
    A objOfA;
    function B(address _addressOfContractA){
      objOfA = A(_addressOfContractA);
    }
    function call_double(uint val) {
      res = objOfA.double(val);
    }
}