Unexpected behavior with storage keyword?

Question

Could someone explain to me what exactly happens when I declare a storage pointer?

This example highlights the issue that I have encountered:

contract minimum {

  uint256[] a = [1,2,3,4];

  function createOffersAsList() public returns (uint256[]) {

    uint256[] storage b;
    uint256 i = 0;

    Logger("Before loop", b); 
    // What I expected it to return: '"Before loop", ""'
    // What it actually returns: '"Before loop", "1, 2, 3, 4"'
    
    while (i<4) {
        b.push(a[i]);
        i++;
    }
    Logger("After loop", b);
    return b;
    // What I expected it to return: '"After loop", "1, 2, 3, 4"'
    // What it actually returns: '"After loop", "1, 2, 3, 4, 1, 2, 3, 4"'
  }

  event Logger(string s, uint256[] u);
}

It seems to me, that when I declare b that it is automatically assigned to the storage of a. Why is that? I would have thought that it is just assigned to some free space?

And if I need to use a local dynamically sized array, how would this look like?

Thanks in advance and best regards, carbee

Answer 1

When you declare a storage variable, it's essentially a reference to some location in storage. Until you assign it to something, it points to location 0, which also happens to be the location of the first declared state variable (in this case a). You're basically using an uninitialized pointer.

See http://solidity.readthedocs.io/en/develop/types.html#data-location.

For your second question, you can allocate variable-length arrays in memory using the new keyword, but you can't resize them after creation.

See http://solidity.readthedocs.io/en/develop/types.html#allocating-memory-arrays.