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60 lines
2.3 KiB
60 lines
2.3 KiB
% Constructors
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There is exactly one way to create an instance of a user-defined type: name it,
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and initialize all its fields at once:
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```rust
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struct Foo {
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a: u8,
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b: u32,
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c: bool,
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}
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enum Bar {
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X(u32),
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Y(bool),
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}
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struct Unit;
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let foo = Foo { a: 0, b: 1, c: false };
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let bar = Bar::X(0);
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let empty = Unit;
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```
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That's it. Every other way you make an instance of a type is just calling a
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totally vanilla function that does some stuff and eventually bottoms out to The
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One True Constructor.
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Unlike C++, Rust does not come with a slew of built-in kinds of constructor.
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There are no Copy, Default, Assignment, Move, or whatever constructors. The
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reasons for this are varied, but it largely boils down to Rust's philosophy of
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*being explicit*.
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Move constructors are meaningless in Rust because we don't enable types to
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"care" about their location in memory. Every type must be ready for it to be
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blindly memcopied to somewhere else in memory. This means pure on-the-stack-but-
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still-movable intrusive linked lists are simply not happening in Rust (safely).
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Assignment and copy constructors similarly don't exist because move semantics
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are the only semantics in Rust. At most `x = y` just moves the bits of y into
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the x variable. Rust does provide two facilities for providing C++'s copy-
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oriented semantics: `Copy` and `Clone`. Clone is our moral equivalent of a copy
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constructor, but it's never implicitly invoked. You have to explicitly call
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`clone` on an element you want to be cloned. Copy is a special case of Clone
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where the implementation is just "copy the bits". Copy types *are* implicitly
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cloned whenever they're moved, but because of the definition of Copy this just
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means not treating the old copy as uninitialized -- a no-op.
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While Rust provides a `Default` trait for specifying the moral equivalent of a
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default constructor, it's incredibly rare for this trait to be used. This is
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because variables [aren't implicitly initialized][uninit]. Default is basically
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only useful for generic programming. In concrete contexts, a type will provide a
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static `new` method for any kind of "default" constructor. This has no relation
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to `new` in other languages and has no special meaning. It's just a naming
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convention.
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TODO: talk about "placement new"?
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[uninit]: uninitialized.html
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