Rust for CXX programmers

Rust for C++ programmers

Pointers and references

Pointers in C++ are vulnerable to bugs involving dangling or null pointers. In Rust, pointers are non-nullable and the compiler enforces the invariant that pointers are never dangling.

C++	Rust
&	&mut
const&	& (points to immutable data - not just read-only through the reference)
std::unique_ptr	owned pointer: ~
std::shared_ptr	garbage collection: @, @mut (cycles allowed/collected) reference counting: std::rc::{Rc, RcMut} (acyclic, enforced by type bounds)

Borrowed pointers (&) have the same representation as C pointers or C++ references at runtime, but to enforce safety at compile-time they are more restricted and have a concept of lifetimes. See the borrowed pointer tutorial for details. Note that unlike C++ references but like raw pointers, they can be treated as values and stored in containers.

For example, a container can return a reference to an object inside it, and the lifetime will ensure that the container outlives the reference and is not modified in a way that would make it invalid. This concept also extends to any iterator that wraps borrowed pointers, and makes them impossible to invalidate.

Owned pointers are almost identical to std::unique_ptr in C++, and point to memory on a shared heap so they can be moved between tasks.

Managed pointers are similar to std::shared_ptr, but have full garbage collection semantics and point to a task-local heap. By virtue of being on a local heap, they can use a per-task garbage collector.

If managed pointers are avoided, garbage collection is not used (the compiler can enforce this with the managed-heap-memory lint check). Garbage collection will be avoided in the standard library except for concepts like persistent (copy-on-write, with shared substructures) containers that require them.

Strings

C++	Rust
"foo" (const char *)	"foo" (&'static str)
std::string("foo")	~"foo" (~str)
std::make_shared<std::string>("foo") (std::shared_ptr<std::string>)	@"foo" (@str, no indirection)

The &str and &mut str types represent a fixed-size slice (immutable and mutable respectively) of any kind of string.

Vectors

C++	Rust
std::array<int, 3> {{1, 2, 3}}	[1, 2, 3] ([int, ..3])
std::vector<int> {1, 2, 3}	~[1, 2, 3] (~[int])
std::make_shared<std::vector<int>>(std::initializer_list<int>{1, 2, 3}) (std::shared_ptr<std::vector<int>>)	@[1, 2, 3] (@[int], no indirection)

The &[] and &mut [] types represent a fixed-size slice (immutable and mutable respectively) of any kind of vector.

Enums, sum types and option types

Rust's enum is a sum type like boost::variant (tagged union). The match expression is used to pattern match out of it, replacing the usage of a switch with a C++ enum, or a visitor with boost::variant. A match expression is required to cover all possible cases, although a default fallback can be used.

C++	Rust
enum	enum, but can only store the given variants
boost::variant	enum, Either (an enum) for 2 types
boost::optional	Option (an enum)

Option and Either are very common patterns, so they're predefined along with utility methods.

Nominal types

A Rust struct is similar to struct/class in C++, and uses a memory layout compatible with C structs. Members are public by default but can be marked with priv. There is no concept of a friend function or type, because priv applies only at a module level.

Rust has no special syntax or semantics for constructors, and just uses static methods that return the type.

Custom destructors for non-memory resources can be provided by implementing the Drop trait.

Copying and move semantics

Rust has no concept of a copy constructor and only shallow types are implicitly copyable. Assignment or passing by-value will only do an implicit copy or a move. Other types can implement the Clone trait, which provides a clone method.

Rust will implicitly provide the ability to move any type, along with a swap implementation using moves. The compiler enforces the invariant that no value can be read after it was moved from.

Concurrency

Rust does not allow sharing mutable data between threads, so it isn't vulnerable to races for in-memory data (data races). Instead of sharing data, message passing is used instead - either by copying data, or by moving owned pointers between tasks.

There are several scheduling models available, which allows for either mapping tasks N:N with OS threads, or using N:M scheduling like Go and Erlang for lightweight concurrency.

Exceptions

Rust does not include C++-style exceptions that can be caught, only uncatchable unwinding (fail) that can be dealt with at task boundaries. The lack of catch means that exception safety is much less of an issue, since calling a method that fails will prevent that object from ever being used again.

Generally, errors are handled by using a enum of the result type and the error type, and the result module implements related functionality.

There is also a condition system, which allows a failure to be handled at the site of the failure, or otherwise just resorts to fail if it cannot be handled.

Memory safety

In addition to the prevention of null or dangling pointers and data races, the Rust compiler also enforces that data be initialized before being read. Variables can still be declared and then initialized later, but there can never be a code path where they could be read first. All possible code paths in a function or expression must also return a correctly typed value, or fail.

This also means indexing a string/vector will do bounds checking like vector<T>::at() (though there are unsafe functions in vec::raw that allow bypassing the bounds checks)

Methods

C++	Rust
size_t foo() const	fn foo(&self) -> uint
size_t foo()	fn foo(&mut self) -> uint
static size_t foo()	fn foo() -> uint
n/a	fn foo(self), fn foo(~self), fn foo(@self)

Methods in Rust are similar to those in C++, but Rust uses explicit self, so referring to a member/method from within a method is done with self.member. It's also possible to take self by-value, which allows for moving out of self since the object will be consumed.

For an example, the PriorityQueue struct in the standard library has a to_sorted_vec method that takes self by-value, making PriorityQueue::from_vec(xs).to_sorted_vec() an in-place heap sort.

The ~self and @self forms are available for objects allocated as ~ or @ respectively, but you usually want &self.

Parametric polymorphism

Rust generics are more like the proposed concepts extension for C++, not like the templates that exist today. This allows templates to be type-checked at the time you write them, not when you instantiate them.

In Rust, functions and structs can be given generic type annotations (like templated functions and structs), but to actually call a type's methods and use operators, the type annotations must be explicitly bounded by the traits required. Annotating the type bounds for functions is only necessary at the module-level, and not for nested closures where it is inferred.

Despite the different model in the type system, traits are compiled to code similar to what templates would produce.

Metaprogramming

Rust includes macros (not textual ones) and syntax extensions which can be used for many of the other use cases of templates and constexpr. Syntax extensions can also be used in place of user-defined literals.