Solidity ParserError: Expected identifier but got '='

Question

Why does the code below contain an error (ParserError: Expected identifier but got '=').

contract Test {

    struct Box {
        uint size;
    }

    Box public box;
    box.size = 3;    //<-- error here

    constructor() public {
    }

}

It works if I put the box.size = 3; into the constructor!

contract Test {

    struct Box {
        uint size;
    }

    Box public box;

    constructor() public {
        box.size = 3;
    }

}

Answer 1

The grammar doesn't allow assignments on contract level. But it allows declarations of state variables and these can contain an initializer. Therefore you can initialize it with

Box public box = Box({ size: 3 });

or

Box public box = Box(3);