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I have this very simple contract:

    pragma solidity ^0.4.18;
    contract C{
    uint public a = 9;
    uint[] public data;

    function f(uint _a) public{
        a = _a;
        uint[] y;
        y.push(2900);
    }
}

I understand that local variables of type array (uint[] y) references storage, but I don't understand why y.push(2900) modifies storage variable named a, and no matter what value I push to y it always increments variable a with one unit. Thanks you

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3

You are declaring y as a storage but you left unitialized so y points to the same slot than a.

The first 32 bytes of an array in storage is the array length, and after that follows the data.

So the first a = _a will set the array length. And then y.push(2900) will append a new value to the array incrementing its length.

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  • Why does "so y points to the same slot as a" happens? Should not it be considered as a bug of solidity compiler? As per as I understand the usual convention in most programming language compiler design is that memory location of different variables should not overlap unless explicitly done so? – sourav Mar 11 '18 at 3:46
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    @sourav Solidity is a ver simplistic language and allocation of storage variables is static, first variable gets slot 0, and so on. If you do not initialize a storage variable it will refer to slot 0. New versions of solc display a warning when used like that. – Ismael Mar 11 '18 at 4:35
  • Thanks @Ismael. One question does in solidity all uninitialized variable refer to slot 0 or it refers to last assigned slot in the storage. For example, if I define 2 variables uint a = 1; uint b = 2; and then declare a variable like y in the above question. which slot y will refer to? 0 or one which is the last index allocated(1 in my example) – sourav Mar 11 '18 at 7:25
  • @sourav Uninitialized storage variables will point to slot 0. – Ismael Mar 11 '18 at 19:32

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