nomicon/constructors.md

% Constructors

There is exactly one way to create an instance of a user-defined type: name it,
and initialize all its fields at once:

```rust
struct Foo {
    a: u8,
    b: u32,
    c: bool,
}

enum Bar {
    X(u32),
    Y(bool),
}

struct Unit;

let foo = Foo { a: 0, b: 1, c: false };
let bar = Bar::X(0);
let empty = Unit;
```

That's it. Every other way you make an instance of a type is just calling a
totally vanilla function that does some stuff and eventually bottoms out to The
One True Constructor.

Unlike C++, Rust does not come with a slew of built-in kinds of constructor.
There are no Copy, Default, Assignment, Move, or whatever constructors. The
reasons for this are varied, but it largely boils down to Rust's philosophy of
*being explicit*.

Move constructors are meaningless in Rust because we don't enable types to
"care" about their location in memory. Every type must be ready for it to be
blindly memcopied to somewhere else in memory. This means pure on-the-stack-but-
still-movable intrusive linked lists are simply not happening in Rust (safely).

Assignment and copy constructors similarly don't exist because move semantics
are the only semantics in Rust. At most `x = y` just moves the bits of y into
the x variable. Rust does provide two facilities for providing C++'s copy-
oriented semantics: `Copy` and `Clone`. Clone is our moral equivalent of a copy
constructor, but it's never implicitly invoked. You have to explicitly call
`clone` on an element you want to be cloned. Copy is a special case of Clone
where the implementation is just "copy the bits". Copy types *are* implicitly
cloned whenever they're moved, but because of the definition of Copy this just
means not treating the old copy as uninitialized -- a no-op.

While Rust provides a `Default` trait for specifying the moral equivalent of a
default constructor, it's incredibly rare for this trait to be used. This is
because variables [aren't implicitly initialized][uninit]. Default is basically
only useful for generic programming. In concrete contexts, a type will provide a
static `new` method for any kind of "default" constructor. This has no relation
to `new` in other languages and has no special meaning. It's just a naming
convention.

TODO: talk about "placement new"?

[uninit]: uninitialized.html