Chapter 4: Static Properties and Const Variables

Static Properties

A property decorated with the keyword static is a static property, which must be initialized when the property is declared. In the function of the contract, it can be accessed through the contract name and the property name (with a dot in the middle). The contract prefix can be omitted if used in the same contract where it is defined.

contract Test {
    static int x = 12;

    public function unlock(int y) {
        Test.x = y;
        // using prefix
        int z = Test.x + y;
        // without prefix
        z = x;
    }
}

Const Variables

A variable declared using the const keyword cannot be changed once it is initialized, as shown below:

contract Test {
    const int x;

    constructor(int x) {
        // initialize a const property
        this.x = x; // good
    }

    public function equal(const int y) {
        y = 1; // <-- error

        const int a = 36;
        a = 11; // <-- error

    }
}

A property can be declared using both. This is typically used to define constants.

contract Test {
    static const int PERCENT = 100;
}

Put it to the test

Add the following static properties to the TicTacToe contract and decorate it with const.

  1. Add N, type is int, value is 9. Indicates the byte length of the storage state of the contract. Tic-tac-toe game has a total of 9 board positions.
  2. Add EMPTY, the type is int, and the value is 0. Indicates that the board position has not yet been placed
  3. Add ALICE, the type is int, and the value is 1. Indicates that the board position was placed by the player alice
  4. Add BOB, the type is int, and the value is 2. Indicates that the board position was placed by the player bob
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