🥦 kościelniak.pro

Appearance

On this page

variable types

In solidity, there are two locations you can store variables — in storage and in memory.

storage variables are stored permanently on the blockchain. memory variables are temporary, and disappear when the function call ends.

Most of the time Solidity will handle them by default. state-variables are by default storage. Variables declared inside functions are memory.

However, variables have to be declared as storage or memory when dealing with structs and arrays within functions.

solidity
contract SandwichFactory {
  struct Sandwich {
    string name;
    string status;
  }

  Sandwich[] sandwiches;

  function eatSandwich(uint _index) public {
    // You should declare with the `storage` keyword, like:
    Sandwich storage mySandwich = sandwiches[_index];

    // ...in which case `mySandwich` is a pointer to `sandwiches[_index]`
    // in storage, and...

    mySandwich.status = "Eaten!";
    // ...this will permanently change `sandwiches[_index]` on the blockchain.

    // If you just want a copy, you can use `memory`:
    Sandwich memory anotherSandwich = sandwiches[_index + 1];
    // ...in which case `anotherSandwich` will simply be a copy of the
    // data in memory, and...
    anotherSandwich.status = "Eaten!";
    // ...will just modify the temporary variable and have no effect
    // on `sandwiches[_index + 1]`. But you can do this:
    sandwiches[_index + 1] = anotherSandwich;
    // ...if you want to copy the changes back into blockchain storage.
  }
}