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    <p><code>Grid::new</code> should initialize <code>Grid::inner</code> with a nested fixed-size array literal, which is written just like the type, but with the value you want to initialize your array with in place of the <code>Cell</code> type:</p> <pre><code>impl Grid { fn new -&gt; Grid { Grid { inner: [[alive, ..GRID_SIZE], ..GRID_SIZE] } } } </code></pre> <p>(You can use <code>Cell::new()</code> instead of <code>alive</code> if you'd rather use the constructor function.)</p> <p>The <code>inner</code> member can then be used in expressions as follows (note that <code>priv</code> only controls visibility to code outside the current module):</p> <pre><code>let grid = Grid::new(); let nested_fixed_sized_array: [[Cell, ..GRID_SIZE], ..GRID_SIZE] = grid.inner; let fixed_sized_array: [Cell, ..GRID_SIZE] = grid.inner[0]; let cell_element: Cell = grid.inner[0][0]; </code></pre> <p>In Rust, nested arrays, as used in this example, are a special case of fixed-size arrays. To see how this works, see the tutorial section on <a href="http://static.rust-lang.org/doc/master/tutorial.html#vectors-and-strings">Vectors and Strings</a>. In particular, unlike vectors (of type <code>~[T]</code>), which are dynamically allocated on the heap and can change their length (if they're mutable), fixed-size arrays have their length embedded in the type (<code>[T, ..LENGTH]</code>), so that they can't change size after being created. <code>T</code> itself must be either a fixed-size type or a pointer. In exchange, however, fixed-size arrays are value types that can be directly allocated on the stack, embedded in <code>struct</code> definitions (like <code>Grid</code>), etc.</p> <p>Since a fixed-size array is itself a fixed-size type, a nested fixed-size array is just a special case where a fixed-size array is the element type of a fixed-size array. In particular, the memory taken up by <code>Grid::inner</code> is exactly <code>GRID_SIZE * GRID_SIZE * sizeof(Cell)</code> (if we ignore alignment). Also useful is a vector of fixed-size arrays <code>~[T, ..LENGTH]</code>, when you know the number of columns in a matrix but not the number of rows.</p> <p>Both vectors and fixed-size arrays can serve as a function argument if the argument is a slice (type <code>&amp;[T]</code>).</p> <p>Some details may change between now and the release of Rust 1.0. If you're curious, searching the <a href="http://reddit.com/r/rust">Rust subreddit</a> for "dynamically sized types" should turn up the proposed changes and the reasoning behind them, or you can always ask around on Reddit or the #rust IRC channel.</p>
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    1. POHow do I initialize this array of arrays within a struct?
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    1. POHow do I initialize this array of arrays within a struct?
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    1. COThanks! Quick q while I attempt to digest this, is having to declare the array size twice, once in the declaration, and once in the constructor (::new()) methods normal? struct Grid { inner: [ [Cell, .. GRID_SIZE], .. GRID_SIZE], } impl Grid { fn new() -> Grid { inner: [[alive, .. GRID_SIZE], .. GRID_SIZE] } }
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    2. COYes, that's how you have to do it. The array size is explicitly part of the type for fixed-size array, so you have to specify it in the declaration of Grid. You then produce a fixed-size array value with [[alive, ..GRID_SIZE], ..GRID_SIZE], where the repeat count ..GRID_SIZE specifies how many elements are there. (In addition, an array literal by itself has type [T, ..LENGTH]. For example, [1, 1, 1] == [1, ..3], and [1, 1, 1] has type [int, ..3]).
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    3. COOk, cool. Since the type contains the length, it makes sense that it must be repeated at both declaration and instantiation. Thanks!
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