...by Daniel Szego
"Simplicity is the ultimate sophistication."
Leonardo da Vinci

Sunday, December 30, 2018

Solidity Tips and Tricks - struct at local function hack

Solidity has a lot of surprising and actually shitty characteristics. On thing is that a struct can be defined both in memory and in storage as well similarly to arrays. The problem is however that if you define your struct in a local function body, it overwrites the storage variables of the contract itself. On top unfortunately, if you create a struct in a function body without explicitly marking if it is a storage or a memory one, it is created automatically as a storage. So the following example:

contract StructHack {
    uint public myNum = 0;
    struct TestStruct {
        uint structNum;
    function hackStruct() {
        TestStruct test;
        test.structNum = 22;

Surprisingly, if you deploy the contract and call the function hackStruct, the myNum value will be initialized to 22.