Are state variables copied during inheritance?

Question

I am testing. Looks to me like state variables are copied during inheritance.

Also when I use new, I am unsure what happens to state variables.

I have d13 state as car - bool. It is updated by d13. When I update using d12 I do not have a new state definition in d12 but it looks like a new state variable is defined in d12 (see storage display).

When I use d14, it looks like another new state variable is defined but I cannot even find that one.

Does anyone know how state variables are inherited and held in storage? What is the point of a design which copies the state variable definition into the new contract?

Code -

contract d13 {
    bool public car;
    function setCar() public returns (bool) {
        car=true;
        return car;
    }

    function removeCar() public returns (bool) {
        car=false;
        return car;
    }
    function showMemory() pure returns (uint) {
        uint ms;
        assembly {
           ms := msize()
        }
        return ms;
    }
}


contract d12 is d13 {
    function isCar() public view returns (bool) {
        return car; 
    }

    function newCar() public {
        car=true;
    }

    function oldCar() public {
        car=false;
    }
    function showMemory() pure returns (uint) {
        uint ms;
        assembly {
           ms := msize()
        }
        return ms;
    }
}

contract d14 {
    function isCar() view returns (bool) {
        return (new d13()).car();
    }

    function setCar() returns (bool) {
        d13 d13instance = new d13();
        d13instance.setCar();
        return (d13instance.car());
    }
    function showMemory() pure returns (uint) {
        uint ms;
        assembly {
           ms := msize()
        }
        return ms;
    }

}

Geth -

> eth.getStorageAt(d13con.address,0)
"0x0000000000000000000000000000000000000000000000000000000000000001"
> d13con.car()
true
> d13con.removeCar()
"0x01973e61dbbf3b44994b016122afc1bd6d94ab2476042aed8902eeca01b6ed80"
> d13con.car()
false
> d12con.isCar()
true
> d12con.oldCar()
"0x09bebed58d84621ac18ae6820634f7678be7b8514dd029b5685ec514f552342d"
> eth.pendingTransactions.length
1
> eth.pendingTransactions.length
1
> eth.pendingTransactions.length
0
> d12con.isCar()
false
> eth.getStorageAt(d13con.address,0)
"0x0000000000000000000000000000000000000000000000000000000000000000"
> d13con.setCar()
"0x9b425c7640c06dc8faf66976ba07ef53c7d8a2a8c362ae75756192c78076a65c"
> eth.pendingTransactions.length
0
> d13con.car()
true
> d12con.isCar()
false
> eth.getStorageAt(d12con.address,0)
"0x0000000000000000000000000000000000000000000000000000000000000000"
> d12con.newCar()
"0x300d0c641711436bc60add13b7e101e8acc22305704de4a16bd466db61dce37a"
> d12con.isCar()
false
> eth.pendingTransactions.length
0
> eth.getStorageAt(d12con.address,0)
"0x0000000000000000000000000000000000000000000000000000000000000001"
> d12con.isCar()
true
> eth.getStorageAt(d13con.address,0)
"0x0000000000000000000000000000000000000000000000000000000000000001"
> d14con.isCar()
false
> eth.getStorageAt(d14con.address,0)
"0x0000000000000000000000000000000000000000000000000000000000000000"
> d14con.setCar()
"0x71a896c7822a0413d64df03e96fdce1e77f61425e3a86124948dc99cda2718f6"
> eth.pendingTransactions.length
0
> d14con.isCar()
false
> eth.getStorageAt(d14con.address,0)
"0x0000000000000000000000000000000000000000000000000000000000000000"

I set a contract instance as d12con etc for d12.

Answer 1

I tested this and it seems that the inheriting contract basically takes a complete copy of the inherited contract.

Hence d12 actually updates the car state variable in d12 but it's not explicitly defined in d12.