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:

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. 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"?