What actually happens to contract storage?

Question

Supose I Have a contract like

contract foo{

    bytes32[] bar;

    function set(bytes32 _foo) {
        bar.push(_foo);
    }
}

So if i deploy the contract what will happen 1st. And for every time when i call set() will the whole bar array need to be writen in chain or only the newest element will be writen in chain as a transaction. Now if i hav a getter for bar then how it will return the all element of that array?

Answer 1

So if i deploy the contract what will happen 1st.

This contract has no constructor, so at deployment nothing much happens - the contract code is copied into the blockchain, that's all. No storage is allocated at this point.

And for every time when i call set() will the whole bar array need to be writen in chain or only the newest element will be writen in chain as a transaction

Only the newest element is written; the previous elements remain untouched.

Two things happen when you call set:

1. An additional storage location is written to with your input value _foo.

   You will be charged 20000 gas for setting the element to a non-zero value.

2. The length of the array is updated. Solidity maintains this as a separate record in storage for you.

   The first time you call set, intially setting this length record will cost 20000 gas; subsequent calls only update it and cost 5000 gas.

Now if i hav a getter for bar then how it will return the all element of that array?

You can create a getter automatically by declaring bar like this:

bytes32[] public bar;

This creates a "constant" function bar(uint n) which returns element n from the array. If you need to know how long the array is you will need to write a function for that:

function getLength() returns (uint) {
    return bar.length;
}

If you want to get the whole array returned with a single call, you can add your own getter function:

function returnAll() returns (bytes32[]) {
    return bar;
}