Why an address stored in one contract looks different when it's called and seen by other contract?

Question

I'm learning Solidity and using remix IDE.

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My ultimate goal is to store list addresses in one contract and then access them via another contract. But for the sake of simplicity here I consider only one address.

Problem: The address stored in T1 contract is changed when I call it and see it in T2.

pragma solidity ^0.4.4;
contract T1{
   address public a;
   function T1(){
   a=msg.sender;// here I want to store only the owner
  }
} 


pragma solidity ^0.4.4;
import "./T1.sol";

contract T2{
   address public b;
   T1 t1=new T1();
   function T2(){
     b= t1.a(); // here I want to retrive whatever's stored in a, but it 
   //changes
  }
}

Now if you check b, the address is different than I stored in a.

Question: How can I solve the above problem?

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Note: I'd like to store the address of people who call contract T1, so I think I have to use msg.sender, not a fixed one, like 0x123.

Answer 1

When you do T1 t1=new T1();, you are creating a new contract instance. Apart from the fact that it happens to have been created with the same source code, this isn't connected to your original T1 instance whose address you are hoping to retrieve.

Instead, you want to access the existing contract instance. To do this, you will need to let T2 know the address of T1, for example as an argument to the constructor. Then you can do something like T1 t1 = T1(t1address) and you'll get the existing contract instance, which will have the a that was set earlier when it was mined.