How to import a contract in another one without using import but using the address in different files?

Question

How to import a contract in another one without using import but using the address of that particular contract and accessing its functions and variables in different files in Solidity?

Answer 1

Here's how an example of 2 contracts in 1 file can be split into 2 files:

C1.sol has:

contract C1 {
    function f1() returns(uint) {
        return(10);
    }
}

C2.sol has:

contract C1 { function f1() returns(uint) {} }
contract C2 {
    function f2(address addrC1) returns(uint) {
        C1 c1 = C1(addrC1);
        return c1.f1();
    }
}

C2.sol does need to have a stub (abstract contract code) of C1, so that the compiler knows the functions/interface of C1.

Answer 2

When you instantiate a contract with its address as parameter, then you have access to this very contract:

contract C1 {

  function call (address myContractsAddress, uint aParameter) returns(uint){
    C2 c2= new C2(myContractsAddress);
    return c2.aFunction(aParameter);
  }
}

contract C2 {
  function aFunction(uint aParameter) returns(uint) {
    return 1;
  }
}

Answer 3

If you know the address and function signature of the other contract, you can call based on it's address. The solidity docs have an example:

address nameReg = 0x72ba7d8e73fe8eb666ea66babc8116a41bfb10e2;
nameReg.call(bytes4(sha3("fun(uint256)")), a);

Note that the last line generates the hex of the function signature which you could precompute...

nameReg.call('0x7a9839c2', a);

Answer 4

That is not possible. Importing contracts is exactly the way to inform the compiler that functions the other address have. A pure address may only contains EVM bytecode but it is impossible to recreate high-level API out of it.

There are ideas to publish contracts' interface descriptions next to the blockchain but nothing of that has been done yet.